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Mendel’s experiments.

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Mendel is known as the father of genetics because of his ground-breaking work on inheritance in pea plants 150 years ago.

Gregor Johann Mendel was a monk and teacher with interests in astronomy and plant breeding. He was born in 1822, and at 21, he joined a monastery in Brünn (now in the Czech Republic). The monastery had a botanical garden and library and was a centre for science, religion and culture . In 1856, Mendel began a series of experiments at the monastery to find out how traits are passed from generation to generation. At the time, it was thought that parents’ traits were blended together in their progeny .

Studying traits in peas

Mendel studied inheritance in peas ( Pisum sativum ). He chose peas because they had been used for similar studies, are easy to grow and can be sown each year. Pea flowers contain both male and female parts, called stamen and stigma , and usually self-pollinate. Self-pollination happens before the flowers open, so progeny are produced from a single plant.

Peas can also be cross-pollinated by hand, simply by opening the flower buds to remove their pollen-producing stamen (and prevent self-pollination) and dusting pollen from one plant onto the stigma of another.

Traits in pea plants

Mendel followed the inheritance of 7 traits in pea plants, and each trait had 2 forms. He identified pure-breeding pea plants that consistently showed 1 form of a trait after generations of self-pollination.

Mendel then crossed these pure-breeding lines of plants and recorded the traits of the hybrid progeny. He found that all of the first-generation (F1) hybrids looked like 1 of the parent plants. For example, all the progeny of a purple and white flower cross were purple (not pink, as blending would have predicted). However, when he allowed the hybrid plants to self-pollinate, the hidden traits would reappear in the second-generation (F2) hybrid plants.

Dominant and recessive traits

Mendel described each of the trait variants as dominant or recessive Dominant traits, like purple flower colour, appeared in the F1 hybrids, whereas recessive traits, like white flower colour, did not.

Mendel did thousands of cross-breeding experiments. His key finding was that there were 3 times as many dominant as recessive traits in F2 pea plants (3:1 ratio).

Traits are inherited independently

Mendel also experimented to see what would happen if plants with 2 or more pure-bred traits were cross-bred. He found that each trait was inherited independently of the other and produced its own 3:1 ratio. This is the principle of independent assortment.

Find out more about Mendel’s principles of inheritance .

The next generations

Mendel didn’t stop there – he continued to allow the peas to self-pollinate over several years whilst meticulously recording the characteristics of the progeny. He may have grown as many as 30,000 pea plants over 7 years.

Mendel’s findings were ignored

In 1866, Mendel published the paper Experiments in plant hybridisation ( Versuche über plflanzenhybriden ). In it, he proposed that heredity is the result of each parent passing along 1 factor for every trait. If the factor is dominant , it will be expressed in the progeny. If the factor is recessive, it will not show up but will continue to be passed along to the next generation. Each factor works independently from the others, and they do not blend.

The science community ignored the paper, possibly because it was ahead of the ideas of heredity and variation accepted at the time. In the early 1900s, 3 plant biologists finally acknowledged Mendel’s work. Unfortunately, Mendel was not around to receive the recognition as he had died in 1884.

Useful links

Download a translated version of Mendel’s 1866 paper Experiments in plant hybridisation from Electronic Scholarly Publishing.

This apple cross-pollination video shows scientists at Plant & Food Research cross-pollinating apple plants.

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21 Mendel’s Experiments

By the end of this section, you will be able to:

• Explain the scientific reasons for the success of Mendel’s experimental work
• Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles

Johann Gregor Mendel (1822–1884) (Figure 1) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics that is used to study a specific biological phenomenon to gain understanding to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural history society. He demonstrated that traits are transmitted faithfully from parents to offspring in specific patterns. In 1866, he published his work, Experiments in Plant Hybridization, 1 in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community, which incorrectly believed that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring. This hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation is the range of small differences we see among individuals in a characteristic like human height. It does appear that offspring are a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. Mendel worked instead with traits that show discontinuous variation . Discontinuous variation is the variation seen among individuals when each individual shows one of two—or a very few—easily distinguishable traits, such as violet or white flowers. Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring as would have been expected at the time, but that they were inherited as distinct traits. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime; in fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Crosses

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, meaning that pollen encounters ova within the same flower. The flower petals remain sealed tightly until pollination is completed to prevent the pollination of other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true-breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendel performed hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.

Plants used in first-generation crosses were called P , or parental generation, plants (Figure 2). Mendel collected the seeds produced by the P plants that resulted from each cross and grew them the following season. These offspring were called the F 1 , or the first filial (filial = daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 generation, F 4 generation, and so on, but it was the ratio of characteristics in the P, F 1 , and F 2 generations that were the most intriguing and became the basis of Mendel’s postulates.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea-pod size, pea-pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants and reported results from thousands of F 2 plants.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he was using plants that bred true for white or violet flower color. Irrespective of the number of generations that Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical. This was an important check to make sure that the two varieties of pea plants only differed with respect to one trait, flower color.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait had completely disappeared in the F 1 generation.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that 705 plants in the F 2 generation had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which parent—male or female—contributed which trait. This is called a reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics that Mendel examined, the F 1 and F 2 generations behaved in the same way that they behaved for flower color. One of the two traits would disappear completely from the F 1 generation, only to reappear in the F 2 generation at a ratio of roughly 3:1 (Figure 3).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these dominant and recessive traits, respectively. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-colored flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (and were not blended) in the plants of the F 1 generation. Mendel proposed that this was because the plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of their two copies to their offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic, or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

CONCEPTS IN ACTION

For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of inheritance, visit the Mendel’s Peas web lab .

Also, check out the following video as review

• Johann Gregor Mendel, “Versuche über Pflanzenhybriden.” Verhandlungen des naturforschenden Vereines in Brünn , Bd. IV für das Jahr, 1865 Abhandlungen (1866):3–47. [for English translation, see http://www.mendelweb.org/Mendel.plain.html]

Introductory Biology: Evolutionary and Ecological Perspectives Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Mendel's Experiments: The Study of Pea Plants & Inheritance

Gregor Mendel was a 19th-century pioneer of genetics who today is remembered almost entirely for two things: being a monk and relentlessly studying different traits of pea plants. Born in 1822 in Austria, Mendel was raised on a farm and attended the University of Vienna in Austria's capital city.

There, he studied science and math, a pairing that would prove invaluable to his future endeavors, which he conducted over an eight-year period entirely at the monastery where he lived.

In addition to formally studying the natural sciences in college, Mendel worked as a gardener in his youth and published research papers on the subject of crop damage by insects before taking up his now-famous work with Pisum sativum, the common pea plant. He maintained the monastery greenhouses and was familiar with the artificial fertilization techniques required to create limitless numbers of hybrid offspring.

An interesting historical footnote: While Mendel's experiments and those of the visionary biologist Charles Darwin both overlapped to a great extent, the latter never learned of Mendel's experiments.

Darwin formulated his ideas about inheritance without knowledge of Mendel's thoroughly detailed propositions about the mechanisms involved. Those propositions continue to inform the field of biological inheritance in the 21st century.

Understanding of Inheritance in the Mid-1800s

From the standpoint of basic qualifications, Mendel was perfectly positioned to make a major breakthrough in the then-all-but-nonexistent field of genetics, and he was blessed with both the environment and the patience to get done what he needed to do. Mendel would end up growing and studying nearly 29,000 pea plants between 1856 and 1863.

When Mendel first began his work with pea plants, the scientific concept of heredity was rooted in the concept of blended inheritance, which held that parental traits were somehow mixed into offspring in the manner of different-colored paints, producing a result that was not quite the mother and not quite the father every time, but that clearly resembled both.

Mendel was intuitively aware from his informal observation of plants that if there was any merit to this idea, it certainly didn't apply to the botanical world.

Mendel was not interested in the appearance of his pea plants per se. He examined them in order to understand which characteristics could be passed on to future generations and exactly how this occurred at a functional level, even if he didn't have the literal tools to see what was occurring at the molecular level.

Pea Plant Characteristics Studied

Mendel focused on the different traits, or characters, that he noticed pea plants exhibiting in a binary manner. That is, an individual plant could show either version A of a given trait or version B of that trait, but nothing in between. For example, some plants had "inflated" pea pods, whereas others looked "pinched," with no ambiguity as to which category a given plant's pods belonged in.

The seven traits Mendel identified as being useful to his aims and their different manifestations were:

• Flower color:  Purple or white.
• Flower position:  Axial (along the side of the stem) or terminal (at the end of the stem).
• Stem length:  Long or short.
• Pod shape:  Inflated or pinched.
• Pod color:  Green or yellow.
• Seed shape:  Round or wrinkled.
• Seed color:  Green or yellow.

Pea Plant Pollination

Pea plants can self-pollinate with no help from people. As useful as this is to plants, it introduced a complication into Mendel's work. He needed to prevent this from happening and allow only cross-pollination (pollination between different plants), since self-pollination in a plant that does not vary for a given trait does not provide helpful information.

In other words, he needed to control what characteristics could show up in the plants he bred, even if he didn't know in advance precisely which ones would manifest themselves and in what proportions.

Mendel's First Experiment

When Mendel began to formulate specific ideas about what he hoped to test and identify, he asked himself a number of basic questions. For example, what would happen when plants that were true-breeding for different versions of the same trait were cross-pollinated?

"True-breeding" means capable of producing one and only one type of offspring, such as when all daughter plants are round-seeded or axial-flowered. A true line shows no variation for the trait in question throughout a theoretically infinite number of generations, and also when any two selected plants in the scheme are bred with each other.

• To be certain his plant lines were true, Mendel spent two years creating them.

If the idea of blended inheritance were valid, blending a line of, say, tall-stemmed plants with a line of short-stemmed plants should result in some tall plants, some short plants and plants along the height spectrum in between, rather like humans. Mendel learned, however, that this did not happen at all. This was both confounding and exciting.

Mendel's Generational Assessment: P, F1, F2

Once Mendel had two sets of plants that differed only at a single trait, he performed a multigenerational assessment in an effort to try to follow the transmission of traits through multiple generations. First, some terminology:

• The parent generation was the P generation , and it included a P1 plant whose members all displayed one version of a trait and a P2 plant whose members all displayed the other version.
• The hybrid offspring of the P generation was the F1 (filial) generation .
• The offspring of the F1 generation was the  F2 generation  (the "grandchildren" of the P generation).

This is called a monohybrid cross : "mono" because only one trait varied, and "hybrid" because offspring represented a mixture, or hybridization, of plants, as one parent has one version of the trait while one had the other version.

For the present example, this trait will be seed shape (round vs. wrinkled). One could also use flower color (white vs. purpl) or seed color (green or yellow).

Mendel's Results (First Experiment)

Mendel assessed genetic crosses from the three generations to assess the heritability of characteristics across generations. When he looked at each generation, he discovered that for all seven of his chosen traits, a predictable pattern emerged.

For example, when he bred true-breeding round-seeded plants (P1) with true-breeding wrinkled-seeded plants (P2):

• All of the plants in the F1 generation had round seeds . This seemed to suggest that the wrinkled trait had been obliterated by the round trait. 
• However, he also found that, while about three-fourths of the plants in the F2 generation has round seeds, about one-fourth of these plants had wrinkled seeds . Clearly, the wrinkled trait had somehow "hidden" in the F1 generation and re-emerged in the F2 generation.

This led to the concept of dominant traits (here, round seeds) and recessive traits (in this case, wrinkled seeds).

This implied that the plants' phenotype (what the plants actually looked like) was not a strict reflection of their genotype (the information that was actually somehow coded into the plants and passed along to subsequent generations).

Mendel then produced some formal ideas to explain this phenomenon, both the mechanism of heritability and the mathematical ratio of a dominant trait to a recessive trait in any circumstance where the composition of allele pairs is known.

Mendel's Theory of Heredity

Mendel crafted a theory of heredity that consisted of four hypotheses:

• Genes  (a gene being the chemical code for a given trait) can come in different types.
• For each characteristic, an organism inherits one  allele  (version of a gene) from each parent.
• When two different alleles are inherited, one may be expressed while the other is not.
• When gametes (sex cells, which in humans are sperm cells and egg cells) are formed, the two alleles of each gene are separated.

The last of these represents the law of segregation , stipulating that the alleles for each trait separate randomly into the gametes.

Today, scientists recognize that the P plants that Mendel had "bred true" were homozygous for the trait he was studying: They had two copies of the same allele at the gene in question.

Since round was clearly dominant over wrinkled, this can be represented by RR and rr, as capital letters signify dominance and lowercase letters indicate recessive traits. When both alleles are present, the trait of the dominant allele was manifested in its phenotype.

The Monohybrid Cross Results Explained

Based on the foregoing, a plant with a genotype RR at the seed-shape gene can only have round seeds, and the same is true of the Rr genotype, as the "r" allele is masked. Only plants with an rr genotype can have wrinkled seeds.

And sure enough, the four possible combinations of genotypes (RR, rR, Rr and rr) yield a 3:1 phenotypic ratio, with about three plants with round seeds for every one plant with wrinkled seeds.

Because all of the P plants were homozygous, RR for the round-seed plants and rr for the wrinkled-seed plants, all of the F1 plants could only have the genotype Rr. This meant that while all of them had round seeds, they were all carriers of the recessive allele, which could therefore appear in subsequent generations thanks to the law of segregation.

This is precisely what happened. Given F1 plants that all had an Rr genotype, their offspring (the F2 plants) could have any of the four genotypes listed above. The ratios were not exactly 3:1 owing to the randomness of the gamete pairings in fertilization, but the more offspring that were produced, the closer the ratio came to being exactly 3:1.

Mendel's Second Experiment

Next, Mendel created dihybrid crosses , wherein he looked at two traits at once rather than just one. The parents were still true-breeding for both traits, for example, round seeds with green pods and wrinkled seeds with yellow pods, with green dominant over yellow. The corresponding genotypes were therefore RRGG and rrgg.

As before, the F1 plants all looked like the parent with both dominant traits. The ratios of the four possible phenotypes in the F2 generation (round-green, round-yellow, wrinkled-green, wrinkled-yellow) turned out to be 9:3:3:1

This bore out Mendel's suspicion that different traits were inherited independently of one another, leading him to posit the law of independent assortment . This principle explains why you might have the same eye color as one of your siblings, but a different hair color; each trait is fed into the system in a manner that is blind to all of the others.

Linked Genes on Chromosomes

Today, we know the real picture is a little more complicated, because in fact, genes that happen to be physically close to each other on chromosomes can be inherited together thanks to chromosome exchange during gamete formation.

In the real world, if you looked at limited geographical areas of the U.S., you would expect to find more New York Yankees and Boston Red Sox fans in close proximity than either Yankees-Los Angeles Dodgers fans or Red Sox-Dodgers fans in the same area, because Boston and New York are close together and both are close to 3,000 miles from Los Angeles.

Mendelian Inheritance

As it happens, not all traits obey this pattern of inheritance. But those that do are called Mendelian traits . Returning to the dihybrid cross mentioned above, there are sixteen possible genotypes:

RRGG, RRgG, RRGg, RRgg, RrGG, RrgG, RrGg, Rrgg, rRGG, rRgG, rRGg, rRgg, rrGG, rrGg, rrgG, rrgg

When you work out the phenotypes, you see that the probability ratio of

round green, round yellow, wrinkled green, wrinkled yellow

turns out to be 9:3:3:1. Mendel's painstaking counting of his different plant types revealed that the ratios were close enough to this prediction for him to conclude that his hypotheses were correct.

• Note: A genotype of rR is functionally equivalent to Rr. The only difference is which parent contributes which allele to the mix.

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Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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Gregor Mendel

(1822-1884)

Who Was Gregor Mendel?

Gregor Mendel, known as the "father of modern genetics," was born in Austria in 1822. A monk, Mendel discovered the basic principles of heredity through experiments in his monastery's garden. His experiments showed that the inheritance of certain traits in pea plants follows particular patterns, subsequently becoming the foundation of modern genetics and leading to the study of heredity.

Gregor Johann Mendel was born Johann Mendel on July 20, 1822, to Anton and Rosine Mendel, on his family’s farm, in what was then Heinzendorf, Austria. He spent his early youth in that rural setting, until age 11, when a local schoolmaster who was impressed with his aptitude for learning recommended that he be sent to secondary school in Troppau to continue his education. The move was a financial strain on his family, and often a difficult experience for Mendel, but he excelled in his studies, and in 1840, he graduated from the school with honors.

Following his graduation, Mendel enrolled in a two-year program at the Philosophical Institute of the University of Olmütz. There, he again distinguished himself academically, particularly in the subjects of physics and math, and tutored in his spare time to make ends meet. Despite suffering from deep bouts of depression that, more than once, caused him to temporarily abandon his studies, Mendel graduated from the program in 1843.

That same year, against the wishes of his father, who expected him to take over the family farm, Mendel began studying to be a monk: He joined the Augustinian order at the St. Thomas Monastery in Brno, and was given the name Gregor. At that time, the monastery was a cultural center for the region, and Mendel was immediately exposed to the research and teaching of its members, and also gained access to the monastery’s extensive library and experimental facilities.

In 1849, when his work in the community in Brno exhausted him to the point of illness, Mendel was sent to fill a temporary teaching position in Znaim. However, he failed a teaching-certification exam the following year, and in 1851, he was sent to the University of Vienna, at the monastery’s expense, to continue his studies in the sciences. While there, Mendel studied mathematics and physics under Christian Doppler, after whom the Doppler effect of wave frequency is named; he studied botany under Franz Unger, who had begun using a microscope in his studies, and who was a proponent of a pre-Darwinian version of evolutionary theory.

In 1853, upon completing his studies at the University of Vienna, Mendel returned to the monastery in Brno and was given a teaching position at a secondary school, where he would stay for more than a decade. It was during this time that he began the experiments for which he is best known.

Experiments and Theories

Around 1854, Mendel began to research the transmission of hereditary traits in plant hybrids. At the time of Mendel’s studies, it was a generally accepted fact that the hereditary traits of the offspring of any species were merely the diluted blending of whatever traits were present in the “parents.” It was also commonly accepted that, over generations, a hybrid would revert to its original form, the implication of which suggested that a hybrid could not create new forms. However, the results of such studies were often skewed by the relatively short period of time during which the experiments were conducted, whereas Mendel’s research continued over as many as eight years (between 1856 and 1863), and involved tens of thousands of individual plants.

Mendel chose to use peas for his experiments due to their many distinct varieties, and because offspring could be quickly and easily produced. He cross-fertilized pea plants that had clearly opposite characteristics—tall with short, smooth with wrinkled, those containing green seeds with those containing yellow seeds, etc.—and, after analyzing his results, reached two of his most important conclusions: the Law of Segregation, which established that there are dominant and recessive traits passed on randomly from parents to offspring (and provided an alternative to blending inheritance, the dominant theory of the time), and the Law of Independent Assortment, which established that traits were passed on independently of other traits from parent to offspring. He also proposed that this heredity followed basic statistical laws. Though Mendel’s experiments had been conducted with pea plants, he put forth the theory that all living things had such traits.

In 1865, Mendel delivered two lectures on his findings to the Natural Science Society in Brno, who published the results of his studies in their journal the following year, under the title Experiments on Plant Hybrids . Mendel did little to promote his work, however, and the few references to his work from that time period indicated that much of it had been misunderstood. It was generally thought that Mendel had shown only what was already commonly known at the time—that hybrids eventually revert to their original form. The importance of variability and its evolutionary implications were largely overlooked. Furthermore, Mendel's findings were not viewed as being generally applicable, even by Mendel himself, who surmised that they only applied to certain species or types of traits. Of course, his system eventually proved to be of general application and is one of the foundational principles of biology.

Later Life, Death and Legacy

In 1868, Mendel was elected abbot of the school where he had been teaching for the previous 14 years, and both his resulting administrative duties and his gradually failing eyesight kept him from continuing any extensive scientific work. He traveled little during this time and was further isolated from his contemporaries as the result of his public opposition to an 1874 taxation law that increased the tax on the monasteries to cover Church expenses.

Gregor Mendel died on January 6, 1884, at the age of 61. He was laid to rest in the monastery’s burial plot and his funeral was well attended. His work, however, was still largely unknown.

It was not until decades later, when Mendel’s research informed the work of several noted geneticists, botanists and biologists conducting research on heredity, that its significance was more fully appreciated, and his studies began to be referred to as Mendel’s Laws. Hugo de Vries, Carl Correns and Erich von Tschermak-Seysenegg each independently duplicated Mendel's experiments and results in 1900, finding out after the fact, allegedly, that both the data and the general theory had been published in 1866 by Mendel. Questions arose about the validity of the claims that the trio of botanists were not aware of Mendel's previous results, but they soon did credit Mendel with priority. Even then, however, his work was often marginalized by Darwinians, who claimed that his findings were irrelevant to a theory of evolution. As genetic theory continued to develop, the relevance of Mendel’s work fell in and out of favor, but his research and theories are considered fundamental to any understanding of the field, and he is thus considered the "father of modern genetics."

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• Name: Gregor Mendel
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• Best Known For: Gregor Mendel was an Austrian monk who discovered the basic principles of heredity through experiments in his garden. Mendel's observations became the foundation of modern genetics and the study of heredity, and he is widely considered a pioneer in the field of genetics.
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• My scientific studies have afforded me great gratification; and I am convinced that it will not be long before the whole world acknowledges the results of my work.

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Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)

• In the 1860s, an Austrian monk named Gregor Mendel introduced a new theory of inheritance based on his experimental work with pea plants.
• Mendel believed that heredity is the result of discrete units of inheritance, and every single unit (or gene) was independent in its actions in an individual’s genome.
• According to this Mendelian concept, the inheritance of a trait depended on the passing-on of these units.
• For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes. We now understand the alternate forms of these units as ‘alleles’.
• If the two alleles that form the pair for a trait are identical, then the individual is said to be homozygous and if the two genes are different, then the individual is heterozygous for the trait.
• The breeding experiments of the monk in the mid‐1800s laid the groundwork for the science of genetics.
• He studied peas plant for 7 years and published his results in 1866 which was ignored until 1900 when three separate botanists, who also were theorizing about heredity in plants, independently cited the work.
• In appreciation of his work he was considered as the “Father of Genetics”.
• A new stream of genetics was established after his name as Mendelian genetics which involves the study of heredity of both qualitative (monogenic) and quantitative (polygenic) traits and the influence of environment on their expressions.
• Mendelian inheritance while is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900.

Table of Contents

Interesting Science Videos

Mendel’s Experiment

Mendel carried out breeding experiments in his monastery’s garden to test inheritance patterns. He selectively cross-bred common pea plants ( Pisum sativum ) with selected traits over several generations.  After crossing two plants which differed in a single trait (tall stems vs. short stems, round peas vs. wrinkled peas, purple flowers vs. white flowers, etc), Mendel discovered that the next generation, the “F1” (first filial generation), was comprised entirely of individuals exhibiting only one of the traits.  However, when this generation was interbred, its offspring, the “F2” (second filial generation), showed a 3:1 ratio- three individuals had the same trait as one parent and one individual had the other parent’s trait.

Mendel’s Laws

I. Mendel’s Law of Segregation of genes (the “First Law”)

Image Source:  Encyclopædia Britannica .

• The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles.
• An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent.
• Hence, according to the law, two members of a gene pair segregate from each other during meiosis; each gamete has an equal probability of obtaining either member of the gene.

II. Mendel’s Law of Independent Assortment (the “Second Law”)

• Mendel’s second law. The law of independent assortment; unlinked or distantly linked segregating genes pairs behave independently.
• The Law of Independent Assortment states that alleles for separate traits are passed independently of one another.
• That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait.
• Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios.
• This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.

III. Mendel’s Law of Dominance (the “Third Law”)

• The genotype of an individual is made up of the many alleles it possesses.
• An individual’s physical appearance, or phenotype, is determined by its alleles as well as by its environment.
• The presence of an allele does not mean that the trait will be expressed in the individual that possesses it.
• If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism’s appearance and is called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive allele.
• Thus, the dominant allele will hide the phenotypic effects of the recessive allele.
• This is known as the Law of Dominance but it is not a transmission law: it concerns the expression of the genotype.
• The upper case letters are used to represent dominant alleles whereas the lowercase letters are used to represent recessive alleles.
• Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
• Gardner, E. J., Simmons, M. J., & Snustad, D. P. (1991). Principles of genetics. New York: J. Wiley.
• https://www.cliffsnotes.com/study-guides/biology/plant-biology/genetics/mendelian-genetics
• http://kmbiology.weebly.com/mendel-and-genetics—notes.html
• http://knowgenetics.org/mendelian-genetics/
• https://en.wikipedia.org/wiki/Mendelian_inheritance
• https://www.acpsd.net/site/handlers/filedownload.ashx?moduleinstanceid=40851&dataid=33888&FileName=Mendelian%20Genetics.pdf

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Sagar Aryal

2 thoughts on “Mendel’s 3 Laws (Segregation, Independent Assortment, Dominance)”

Good to know when one works with plants like me.

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1865: Mendel's Peas

1865: mendel's peas.

From earliest time, people noticed the resemblance between parents and offspring, among animals and plants as well as in human families. Gregor Johann Mendel turned the study of heredity into a science.

Mendel was a monk in the Augustinian order, long interested in botany. He studied mathematics and science at the University of Vienna to become a science teacher. For eight years, starting in 1857, he studied the peas he grew in the garden of his monastery. He carefully pollinated the plants, saved seeds to plant separately, and analyzed the succeeding generations.

He self-pollinated plants until they bred true - giving rise to similar characteristics generation after generation. He studied easily distinguishable characteristics like the color and texture of the peas, the color of the pea pods and flowers, and the height of the plants.

When he crossed true-breeding lines with each other, he noticed that the characteristics of the offspring consistently showed a three to one ratio in the second generation. For example, for approximately every three tall plants, one would be short; for about every three plants with yellow peas, one would have green peas. Further breeding showed that some traits are dominant (like tall or yellow) and others recessive (like short or green). In other words, some traits can mask others. But the traits don't blend: they are inherited from the parents as discrete units and remain distinct. Furthermore, different traits - like height and seed color - are inherited independently of each other.

More Information

References:.

Mendel read his paper, "Experiments in Plant Hybridization" at meetings on February 8 and March 8, 1865. He published papers in 1865 and 1869 in the Transactions of the Brunn Natural History Society .

Some Biographies of Mendel:

Iltis, Hugo, Life of Mendel . Eden and Cedar Paul, trans. London: George Allen & Unwin Ltd. 1932. From the German publication, "Gregor Johann Mendel, Leben, Werk, und Wirkung", Berlin: Julius Springer, 1924.

Orel, Vitezslav, Gregor Mendel: The First Geneticist . Oxford & London: Oxford University Press, 1996.

In the following paper, scientists explained, in molecular detail, the cause of the wrinkled seed trait that Mendel had observed in his peas:

Bhattacharyya M.K., Smith A.M., Ellis T.H., Hedley C., and Martin C.. The wrinkled-seed character of pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branding enzyme. Cell , 60: 115-122, 1990.

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• 19 July 2022

The true legacy of Gregor Mendel: careful, rigorous and humble science

You have full access to this article via your institution.

Mendel showed that flower colour in pea plants can be inherited. The flower in the centre is a cross between the pink flower and the white flower. Credit: Oxford Science Archive/Print Collector/Getty

Genetics is fiendishly complex. We know this from decades of molecular biology, from the resulting studies on the sequencing and analysis of genomes and from our increasing knowledge of how genes interact with the environment. So how did the Augustinian friar, teacher and citizen scientist Gregor Mendel manage to describe principles of inheritance that still stand today — from work he performed alone in his monastery garden in the 1850s and 1860s?

Many of the details have been lost to history, because notes of Mendel’s experiments, including his interim observations and his working methods, were burnt after his death, as Kim Nasmyth at the University of Oxford, UK, describes in a Perspective article in Nature Reviews Genetics 1 .

But from his published works, as well as historical sources that have recently come to light, it’s clear that Mendel was a careful scientist; cautious, patient and committed to data. These qualities allowed him to make discoveries that have stood the test of time. The 200th anniversary of his birth on 22 July 1822 provides an opportunity to celebrate and recognize a giant in science. “Viewed in the light of what was known of cells in the mid-nineteenth century, Mendel was decades ahead of his time,” write Peter van Dijk at KeyGene in Wageningen, the Netherlands, and his colleagues in a Perspective article in Nature Genetics 2 .

Model communication

Although Mendel had no knowledge of genes, chromosomes or genomes, he laid the foundations for genetics in a paper, ‘Experiments on plant hybrids’, which he presented to the Natural History Society of Brno (now in the Czech Republic) in 1865 3 . Starting with 22 plants of the garden pea, Pisum sativum , and using manual pollination, Mendel crossbred these specimens and their progeny multiple times, producing more than 10,000 plants over 8 years. Plants from each pollination cycle were classified according to various characteristics, such as the colour and shape of the seeds and the position of flowers. By analysing these data, Mendel discovered that certain traits — shape and colour, for example — can be passed down from one generation to the next.

How did Mendel arrive at his discoveries?

The paper is a model for research communication. It describes, in accessible language, how Mendel established controls and protected the integrity of his experiments (such as taking steps to reduce the risk of wind-blown or insect pollination). He is generous in crediting others’ work on the subject. The final part of the manuscript includes a discussion of caveats and potential sources of error. “The validity of the set of laws suggested for Pisum requires additional confirmation and thus a repetition of at least the more important experiments would be desirable,” Mendel writes in the conclusion.

Although in his paper he did coin the terms ‘dominant’ and ‘recessive’ — which remain fundamental concepts in genetics today — Mendel’s caution in interpreting his results proved well-founded. Generations of geneticists and molecular and structural biologists have since demonstrated that observable characteristics do not result from genes alone. By working with model organisms and studying familial diseases and human populations, scientists have shown time and again that characteristics are influenced by an intricate interplay between a host of factors. These include RNA, epigenetics (chemical alterations to DNA bases that don’t change the DNA sequence), the position of a gene within both the genome and the nucleus of a cell, and how all of the above interact with environmental factors.

A statue of Gregor Mendel in the Abbey of St Thomas in Brno, Czech Republic, where Mendel was abbot. Credit: Alamy

And yet, as has been well documented, Mendel’s name was wrongly and irresponsibly appropriated to give weight to eugenics, the scientifically inaccurate idea that humans can be improved through selective breeding. Just a few decades after his death in 1884, his work began to be discussed and cited by scientists advocating theories of racial superiority. That shadow of scientific racism — in which research and evidence are distorted to cause harm — still stalks science today.

Genetics, along with palaeontology, has gone on to provide extraordinarily precise tools for understanding human origins. Genetics has also revealed that there is more genetic variation between people in the same racial category than there is between people from different races, illustrating that there is no biological basis for what we call race. Genetics still holds many secrets, including the role of genes in human behaviour. But we now know that genes are not destiny, four words that bear repeating loudly and frequently.

In laying the foundations of genetics, Mendel set an example in his patient and comprehensive approach to collecting data. In science’s current age of hyper-competitiveness, it is worth pausing for just a moment to celebrate his absolute commitment to careful observation, rigour in analysis and humility in interpreting the results.

Nature 607 , 421-422 (2022)

doi: https://doi.org/10.1038/d41586-022-01953-z

Editor’s note: There is scholarly debate on Mendel’s date of birth. According to historians, Mendel and his family celebrated on 22 July, and many surviving documents also point to this date. However, there is also evidence for 20 July, which is the date for the official 200th anniversary commemoration.

Nasmyth, K. Nature Rev. Genet. 23 , 447–452 (2022).

Article   PubMed   Google Scholar  

van Dijk, P. J., Jessop, A. P. & Ellis, T. H. N. Nature Genet. 54 , 926–933 (2022).

Mendel, G. Verh. Ver. Brünn 4, 3–47 (1866).

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Understanding Evolution

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The History of Evolutionary Thought

Discrete genes are inherited: gregor mendel.

Throughout the nineteenth century, heredity remained a puzzle to scientists. How was it that children ended up looking similar to, but not exactly like, their parents? These questions fascinated and frustrated  Charles Darwin  deeply. After all, heredity lies at the heart of evolution. The variations in each generation are the raw material for  natural selection , while the continuity from one generation to the next allows the changes wrought by natural selection to have long-term effects. Darwin himself proposed that each cell in an animal’s body released tiny particles that streamed to the sexual organs, where they combined into eggs or sperm. They would then blend together when the animal mated. But “pangenesis,” as Darwin called it, didn’t hold up to scrutiny.

Ironically, it was just as Darwin was publishing the Origin of Species  that someone got the first real glimpse of the biological machinery behind heredity. In a secluded monastery in what is now the Czech Republic, a monk named Gregor Mendel was studying heredity in a garden of peas. Mendel, the son of a farmer, had always been interested in plants, and while at the University of Vienna he had been trained in mathematics and learned how to design experiments and analyze data. In the 1850s, he decided to run an experiment to better understand what kept species distinct and what made it possible for hybrids to form. He bred thousands of pea plants and recorded how traits were passed on from one generation to the next.

Trait inheritance

Mendel proposed that the peas were not blending their “wrinkled” and “smooth” traits together. Each hybrid pea inherited both traits, but only the smooth trait became visible. In the next generation, the traits were handed down again, and a quarter of the new peas inherited two “wrinkled” traits, which made them wrinkled. Mendel had discovered what later scientists called “dominant” and “recessive”  alleles .

Mendel’s work goes unrecognized

Mendel tried to drum up interest in his results but to no avail. Part of the problem was that botanists of Mendel’s time were not accustomed to statistics being applied to natural history, and so they couldn’t recognize the importance of Mendel’s discovery. And when Mendel tried to replicate his results with hawkweed, he failed — not because his original insights were wrong, but because the genetics of hawkweed are very peculiar. Nevertheless, the patterns that Mendel saw did apply to many organisms and were there in nature for anyone to see. Darwin himself noted a three-to-one ratio in the colors of snapdragons. But for all his genius, Darwin didn’t realize the importance of that ratio.

Mendel abandoned his experiments in the 1860s and turned his attentions to running his monastery. When he died in 1884, he was remembered as a puttering monk with a skill for breeding plants. It was only some 15 years after his death that scientists realized that Mendel had revealed the answer to one of life’s greatest mysteries.

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Gregor Mendel

By Sam Wong

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Gregor Mendel discovered the basic principles of heredity through experiments with pea plants, long before the discovery of DNA and genes. Mendel was an Augustinian monk at St Thomas’s Abbey near Brünn (now Brno, in the Czech Republic). He studied natural sciences and mathematics at the University of Vienna, Austria, but twice failed to obtain a teaching certificate, instead becoming a part-time assistant teacher and carrying out research in plant breeding.

His most famous experiments were done between 1857 and 1864, during which time he grew some 10,000 pea plants. Pea plants are hermaphroditic, meaning they have both male and female sex cells and usually fertilise themselves. Mendel was able to cross-breed the plants by transferring pollen with a paintbrush. He meticulously recorded a range of characteristics for each plant, including its height, pod shape, pea shape and pea colour. When plants self-fertilised, these characteristics remained consistent in the offspring.

At the time, it was widely believed that heredity worked by blending the characteristics of parents, producing offspring that were in some way diluted. Mendel showed that when two varieties of purebred plants cross-breed, the offspring resembled one or other of the parents, not a blend of the two. He found that some traits are dominant and would always be expressed in a first generation cross, while others are recessive and would not appear in this generation. However, these recessive traits re-appear in the next generation if these first-generation plants self-fertilise.

Mendel hypothesised that parents contribute some particulate substance to the offspring which determine its heritable characteristics. We now know that these particles correspond to genes made of DNA. Without any knowledge of the molecules involved, Mendel was able to infer that heritable particles are separated into gametes – eggs and sperm – and that offspring inherit one particle from each parent.

Mendel was far ahead of his time, and his work was largely ignored for the next 35 years. In 1868 he was appointed as an abbot and, overwhelmed with administrative duties, had little time left to continue his research. Late in his career, he wrote: “My scientific work brought me such satisfaction, and I am convinced the entire world will recognise the results of these studies.” He died in 1884, aged 62.

In 1900, three scientists independently confirmed his work, but it was another 30 years before his conclusions were widely accepted. Then evolutionary biologists such as Ronald Fisher realised that Mendel’s laws of inheritance could explain how natural selection could make beneficial traits become more prevalent and eliminate negative ones. His work formed part of “the modern synthesis”, a reformulation of Darwin’s ideas based on the new understanding of genetics .

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Mendel went on to relate his results to the cell theory of fertilization , according to which a new organism is generated from the fusion of two cells. In order for pure breeding forms of both the dominant and the recessive type to be brought into the hybrid, there had to be some temporary accommodation of the two differing characters in the hybrid as well as a separation process in the formation of the pollen cells and the egg cells. In other words, the hybrid must form germ cells bearing the potential to yield either the one characteristic or the other. This has since been described as the law of segregation , or the doctrine of the purity of the germ cells. Since one pollen cell fuses with one egg cell, all possible combinations of the differing pollen and egg cells would yield just the results suggested by Mendel’s combinatorial theory.

Mendel first presented his results in two separate lectures in 1865 to the Natural Science Society in Brünn . His paper “ Experiments on Plant Hybrids ” was published in the society’s journal, Verhandlungen des naturforschenden Vereines in Brünn , the following year. It attracted little attention, although many libraries received it and reprints were sent out. The tendency of those who read it was to conclude that Mendel had simply demonstrated more accurately what was already widely assumed—namely, that hybrid progeny revert to their originating forms. They overlooked the potential for variability and the evolutionary implications that his demonstration of the recombination of traits made possible. Most notably, Swiss botanist Karl Wilhelm von Nägeli actually corresponded with Mendel, despite remaining skeptical as to the significance of his results and doubting that the germ cells in hybrids could be pure.

Mendel appears to have made no effort to publicize his work, and it is not known how many reprints of his paper he distributed. He had ordered 40 reprints, the whereabouts of only eight of which are known. Other than the journal that published his paper, 15 sources are known from the 19th century in which Mendel is mentioned in the context of plant hybridization. Few of these provide a clear picture of his achievement, and most are very brief.

By 1871 Mendel had only enough time to continue his meteorological and apicultural work. He traveled little, and his only visit to England was to see the Industrial Exhibition in 1862. Bright disease made his last years painful, and he died at the age of 61. Mendel’s funeral was attended by many mourners and proceeded from the monastery to the monastery’s burial plot in the town’s central cemetery, where his grave can be seen today. He was survived by two sisters and three nephews.

In 1900 Dutch botanist and geneticist Hugo de Vries , German botanist and geneticist Carl Erich Correns , and Austrian botanist Erich Tschermak von Seysenegg independently reported results of hybridization experiments similar to Mendel’s, though each later claimed not to have known of Mendel’s work while doing their own experiments. However, both de Vries and Correns had read Mendel earlier—Correns even made detailed notes on the subject—but had forgotten. De Vries had a diversity of results in 1899, but it was not until he reread Mendel in 1900 that he was able to select and organize his data into a rational system. Tschermak had not read Mendel before obtaining his results, and his first account of his data offers an interpretation in terms of hereditary potency. He described the 3:1 ratio as an “unequal valancy” ( Wertigkeit ). In subsequent papers he incorporated the Mendelian theory of segregation and the purity of the germ cells into his text.

In Great Britain, biologist William Bateson became the leading proponent of Mendel’s theory. Around him gathered an enthusiastic band of followers. However, Darwinian evolution was assumed to be based chiefly on the selection of small, blending variations, whereas Mendel worked with clearly nonblending variations. Bateson soon found that championing Mendel aroused opposition from Darwinians. He and his supporters were called Mendelians, and their work was considered irrelevant to evolution. It took some three decades before the Mendelian theory was sufficiently developed to find its rightful place in evolutionary theory.

The distinction between a characteristic and its determinant was not consistently made by Mendel or by his successors, the early Mendelians . In 1909 Danish botanist and geneticist Wilhelm Johannsen clarified this point and named the determinants genes . Four years later American zoologist and geneticist Thomas Hunt Morgan located the genes on the chromosomes , and the popular picture of them as beads on a string emerged. This discovery had implications for Mendel’s claim of an independent transmission of traits, for genes close together on the same chromosome are not transmitted independently. Moreover, as genetic studies pushed the analysis down to smaller and smaller dimensions, the Mendelian gene appeared to fragment. Molecular genetics has thus challenged any attempts to achieve a unified conception of the gene as the elementary unit of heredity . Today the gene is defined in several ways, depending upon the nature of the investigation. Genetic material can be synthesized, manipulated, and hybridized with genetic material from other species , but to fully understand its functions in the whole organism, an understanding of Mendelian inheritance is necessary. As the architect of genetic experimental and statistical analysis, Mendel remains the acknowledged father of genetics.

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show/hide words to know

Chromosome: a long, thread-like molecule made of the chemical called DNA (deoxyribonucleic acid) that is held together with special proteins and is visible (with strong microscopes) during cell division...  more

Cultural generation: all of the individuals born at about the same time.

Discrete: distinct and separate units.

Gene: a region of DNA that instructs the cell on how to build protein(s). As a human, you usually get a set of instructions from your mom and another set from your dad... more

Genetics: the field of biology that studies how genes control the appearance of living things and how genes are passed down from parent to offspring...  more

Mated: putting together male and female reproductive cells to create offspring.

Probability: a number, usually in percentages, that tells you the likelihood that an event will happen.

Trait: a characteristic of an organism that can be the result of genes and/or influenced by the environment. Traits can be physical like hair color or the shape and size of a plant leaf. Traits can also be behaviors such as nest building behavior in birds.

Has anyone ever told you that you have your mother’s dimples, or your father’s nose? Have you ever wondered why you are a particular height, have curly hair, or maybe green eyes? All of these questions can be answered with one word – genetics.

Gregor Mendel (1822-1884)

For almost 200 years scientists have been learning about genes and how traits,like the freckles on your face, are passed along from parent to child. Before that time, farmers knew that if they mated two animals or plants with a desired trait, the offspring was likely to have that trait. What the farmers did not know was how this was happening. It was a mystery that would remain until Gregor Mendel began studying the traits of peas.

Born on July 20, 1822, Mendel was the only son of a peasant family in what is now called the Czech Republic. Even at an early age Mendel liked to ask a lot of questions about the living world. He also had a lot of interests including physics, botany, mathematics, astronomy, and beekeeping. By the age of 23 he graduated from the Philosophical Institute in Olomouc. It was while studying at the Philosophical Institute his physics teacher recommended he join the Augustinian Abbey of St. Thomas in Brno.

Life in the Monastery

Mendel's Garden *

Once at the Abbey, Mendel followed his interest in science and also teaching. He designed an extensive experiment using peas. It would be these experiments that would help solve the mystery of traits and how they were passed from parent to offspring. With the support of the chief friar and his fellow friars, Mendel used a section of land next to the monastery to carry out experiments in his garden. Using pea plants, he would spend years experimenting to find out how traits were passed from parent plants to their offspring.

At the time many scientists thought traits from both parents mixed together to become a new, completely blended trait in the offspring. This was called blended inheritance and was not unlike combining two colors of paint. When the colors are mixed they make a new color that can no longer be separated into the two original colors. The problem with blended inheritance is it could not explain certain things that could be observed, such as traits that sometimes skipped a generation, or how two people of medium height could have a child who grew up to be much taller than they were.

A New Model of Inheritance

Illustration of the common type of pea plants ( Pisum sativum ) Mendel used in his experiments. Wikimedia: Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz

Mendel’s experiments with peas were able to disprove blended inheritance and show that genes are actually discreet units that keep their separate identities when passed from generation to generation. One of the reasons for the success of Mendel’s experiments was that they were very carefully designed and controlled. This was possible due to his strong understanding of the natural world and the life cycle of plants. Mendel also kept detailed notes of everything that he did and what he observed. In addition, Mendel was familiar with both mathematics and probability. This knowledge is what allowed him to see patterns in the outcome of his experiments and realize what those patterns meant. 

The entire set of pea experiments took eight years to complete (1856-1863). In 1865, Mendel published his findings in a paper called Experiments on Plant Hybridization , which was mostly ignored at the time due to a number of reasons. First, Mendel was not well known in the scientific community. Second, his theory ran against the popular model of blended inheritance. His work also used mathematics and probability, which was a very unusual way to approach a scientific problem at the time and difficult for many people to understand.

It was more than thirty years after Mendel’s paper was published until the importance of his work was truly appreciated. Mendel's experiments are a good example that scientific discoveries are sometimes slow to be added to the collection of scientific knowledge. It took time for the community to fully understand his work and the methods he used to unlock one of the early mysteries of genetics. It is also interesting to know that while Mendel was a great thinker and scientist, he also failed two of his major exams needed to become a teacher. Many believe he had terrible test anxiety when taking exams. You could be someone that has similar problems when facing a big test. Just knowing that there have been and still are people that have the same problem might be helpful when you take your next exam.

References :

Klug, W.S., Cummings, M.R., Spencer, C. (2005) Concepts of Genetics, 8th Edition. Menlo Park, CA: Benjamin Cummings

Heller, H.C., Orians, G.H., Purves, W.K., Sadava, D. (2003) Life: The Science of Biology, 7th Edition. Sunderland, MA: Sinauer Associates, Inc. & W. H. Freeman and Company

Henig, R. M. (2001) The Lost and Found Genius of Gregor Mendel, the Father of Genetics. New York, NY: Houghton Mifflin. Retrieved from http://books.google.com/books?id=NEO2bQ-k-nMC

* Courtesy of American Philosophical Society, Curt Stern Papers - Cold Spring Harbor Laboratory .

Additional images and illustrations from Wikimedia Commons. Pea photo by Rasbak.

Read more about: Solving a Genetic Mystery

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• Article: Solving a Genetic Mystery
• Author(s): Sabine Deviche
• Publisher: Arizona State University School of Life Sciences Ask A Biologist
• Site name: ASU - Ask A Biologist
• Date published: July 20, 2010
• Date accessed: September 10, 2024
• Link: https://askabiologist.asu.edu/explore/solving-genetic-mystery

Sabine Deviche. (2010, July 20). Solving a Genetic Mystery. ASU - Ask A Biologist. Retrieved September 10, 2024 from https://askabiologist.asu.edu/explore/solving-genetic-mystery

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Mendel used the common pea plant for his studies of genetics.

Solving a Genetic Mystery

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12.1 Mendel’s Experiments and the Laws of Probability

Learning objectives.

In this section, you will explore the following questions:

• Why was Mendel’s experimental work so successful?
• How do the sum and product rules of probability predict the outcomes of monohybrid crosses involving dominant and recessive alleles?

Connection for AP ® Courses

Genetics is the science of heredity. Austrian monk Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Working with garden peas, Mendel found that crosses between true-breeding parents (P) that differed in one trait (e.g., color: green peas versus yellow peas) produced first generation (F1) offspring that all expressed the trait of one parent (e.g., all green or all yellow). Mendel used the term dominant to refer to the trait that was observed, and recessive to denote that non-expressed trait, or the trait that had “disappeared” in this first generation. When the F1 offspring were crossed with each other, the F2 offspring exhibited both traits in a 3:1 ratio. Other crosses (e.g., height: tall plants versus short plants) generated the same 3:1 ratio (in this example, tall to short) in the F2 offspring. By mathematically examining sample sizes, Mendel showed that genetic crosses behaved according to the laws of probability, and that the traits were inherited as independent events. In other words, Mendel used statistical methods to build his model of inheritance.

As you have likely noticed, the AP Biology course emphasizes the application of mathematics. Two rules of probability can be used to find the expected proportions of different traits in offspring from different crosses. To find the probability of two or more independent events (events where the outcome of one event has no influence on the outcome of the other event) occurring together, apply the product rule and multiply the probabilities of the individual events. To find the probability that one of two or more events occur, apply the sum rule and add their probabilities together.

The content presented in this section supports the learning objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The AP ® learning objectives merge essential knowledge content with one or more of the seven science practices. These objectives provide a transparent foundation for the AP ® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP ® exam questions.

Teacher Support

Two rules of probability are used in solving genetics problems: the rule of multiplication and the rule of addition. The probability that independent events will occur simultaneously is the product of their individual probabilities. If two dices are tossed, what is the probability of landing two ones? A die has 6 faces, and assuming the die is not loaded, each face has the same probability of outcome. The probability of obtaining the number 1 is equal to the number on the die divided by the total number of sides: 1 6 1 6 . The probability of rolling two ones is equal to 1 6   ×   1 6   =   1 36 1 6   ×   1 6   =   1 36 .

The probability that any one of a set of mutually exclusive events will occur is the sum of their individual probabilities. The probability of rolling a 1 or a 2 is equal to 1 6   +   1 6   =   1 3 1 6   +   1 6   =   1 3 because the two outcomes are mutually exclusive. If we roll a 1, it cannot be a 2.

Tell students that Gregor Mendel was a monk who had received a solid scientific education and had excelled at mathematics. He brought this knowledge of science into his experiments with peas.

Engage students in describing what makes a good organism to study genetics. One approach is to ask the class if they would use elephants to study genetics. The disadvantages of using elephants actually highlight the advantages of using peas, corn, fruit flies, or mice for genetics studies: short life cycle, easy to maintain and handle, large number of offspring for statistical analysis, etc.

The concepts of statistics are not intuitive. Practice with dice and coins. Explain that the probability ratios are achieved with large numbers of trials.

Dominant traits are the ones expressed in a dominant/recessive situation. They do not usually repress the recessive trait. A dominant trait is not necessarily the most common trait in a population. For example, type O blood is a recessive trait, but it is the most frequent blood group in many ethnic groups. A dominant trait can be lethal. A dominant allele is not better than the recessive allele. Whether a trait is beneficial depends on the environment. Give the example of wing color in moths. Dark pigmentation is beneficial in a polluted environment where predators would not pick up the moths on dark tree barks. For example, the population peppered moths in 19th century London shifted so that their wing colors were darker to blend in with the soot of the Industrial Revolution. After pollution levels dropped, light pigmentation became more prevalent because it helped the moths to escape notice.

Johann Gregor Mendel (1822–1884) ( Figure 12.2 ) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, 1 in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community that believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring; this hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation results from the action of many genes to determine a characteristic like human height. Offspring appear to be a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by the blending in the offspring, but we now know that this is not the case. Mendel was the first researcher to see it. Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes (specifically, violet versus white flowers); this is referred to as discontinuous variation . Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were they absorbed, but rather that they kept their distinctness and could be passed on. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel’s Model System

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum , to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendelian Crosses

Mendel performed hybridizations , which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant’s flowers before they had a chance to mature.

Plants used in first-generation crosses were called P 0 , or parental generation one, plants ( Figure 12.3 ). Mendel collected the seeds belonging to the P 0 plants that resulted from each cross and grew them the following season. These offspring were called the F 1 , or the first filial ( filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendel’s experiments extended beyond the F 2 generation to the F 3 and F 4 generations, and so on, but it was the ratio of characteristics in the P 0 −F 1 −F 2 generations that were the most intriguing and became the basis for Mendel’s postulates.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants, reporting results from 19,959 F 2 plants alone. His findings were consistent.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F 1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait in the F 1 generation had completely disappeared.

Importantly, Mendel did not stop his experimentation there. He allowed the F 1 plants to self-fertilize and found that, of F 2 -generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait. This is called a reciprocal cross —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics Mendel examined, the F 1 and F 2 generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F 1 generation only to reappear in the F 2 generation at a ratio of approximately 3:1 ( Table 12.1 ).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate (not blended) in the plants of the F 1 generation. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

So why did Mendel repeatedly obtain 3:1 ratios in his crosses? To understand how Mendel deduced the basic mechanisms of inheritance that lead to such ratios, we must first review the laws of probability.

Science Practice Connection for AP® Courses

Think about it.

Students are performing a cross involving seed color in garden pea plants. Yellow seed color is dominant to green seed color. What F1 offspring would be expected when cross true-breeding plants with green seeds with true-breading plants with yellow seeds? Express the answer(s) as percentage.

This question is an application of Learning Objectives 3.14 and Science Practice 2.2 because students are applying a mathematical routine (probability) to determine a Mendelian pattern of inheritance.

Possible answer:

Probability basics.

Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur. Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur. An example of a genetic event is a round seed produced by a pea plant. In his experiment, Mendel demonstrated that the probability of the event “round seed” occurring was one in the F 1 offspring of true-breeding parents, one of which has round seeds and one of which has wrinkled seeds. When the F 1 plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was now three out of four. In other words, in a large population of F 2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds. Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses.

The Product Rule and Sum Rule

Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring. As will be discussed, Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses. For instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds still produced offspring that had a 3:1 ratio of green:yellow seeds (ignoring seed texture) and a 3:1 ratio of round:wrinkled seeds (ignoring seed color). The characteristics of color and texture did not influence each other.

The product rule of probability can be applied to this phenomenon of the independent transmission of characteristics. The product rule states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone. To demonstrate the product rule, imagine that you are rolling a six-sided die (D) and flipping a penny (P) at the same time. The die may roll any number from 1–6 (D # ), whereas the penny may turn up heads (P H ) or tails (P T ). The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this action ( Table 12.2 ), and each event is expected to occur with equal probability.

Of the 12 possible outcomes, the die has a 2/12 (or 1/6) probability of rolling a two, and the penny has a 6/12 (or 1/2) probability of coming up heads. By the product rule, the probability that you will obtain the combined outcome 2 and heads is: (D 2 ) x (P H ) = (1/6) x (1/2) or 1/12 ( Table 12.3 ). Notice the word “and” in the description of the probability. The “and” is a signal to apply the product rule. For example, consider how the product rule is applied to the dihybrid cross: the probability of having both dominant traits (for example, yellow and round) in the F 2 progeny is the product of the probabilities of having the dominant trait for each characteristic, as shown here:

On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes that can come about by more than one pathway. The sum rule states that the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities. Notice the word “or” in the description of the probability. The “or” indicates that you should apply the sum rule. In this case, let’s imagine you are flipping a penny (P) and a quarter (Q). What is the probability of one coin coming up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads (P H ) and the quarter may be tails (Q T ), or the quarter may be heads (Q H ) and the penny may be tails (P T ). Either case fulfills the outcome. By the sum rule, we calculate the probability of obtaining one head and one tail as [(P H ) × (Q T )] + [(Q H ) × (P T )] = [(1/2) × (1/2)] + [(1/2) × (1/2)] = 1/2 ( Table 12.3 ). You should also notice that we used the product rule to calculate the probability of P H and Q T , and also the probability of P T and Q H , before we summed them. Again, the sum rule can be applied to show the probability of having exactly one dominant trait in the F 2 generation of a dihybrid cross:

To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him to calculate the probabilities of the traits appearing in his F 2 generation. As you will learn, this discovery meant that when parental traits were known, the offspring’s traits could be predicted accurately even before fertilization.

• 1 Johann Gregor Mendel, Versuche über Pflanzenhybriden Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr , 1865 Abhandlungen, 3–47. [go here for the English translation here ]

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Science News

How we got from gregor mendel’s pea plants to modern genetics.

Philosopher Yafeng Shan explains how today's understanding of inheritance emerged from a muddle of ideas

In 1900, Gregor Mendel’s experiments on pea plants were introduced into the study of heredity.

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By Elizabeth Quill

February 7, 2022 at 11:00 am

The year was 1900. Three European botanists — one Dutch, one German and one Austrian — all reported results from breeding experiments in plants. Each claimed that they had independently discovered some remarkable patterns in inheritance that had been noticed by Gregor Mendel decades earlier and reported in “Versuche über Pflanzen-Hybriden,” or “Experiments in Plant Hybridization.” All three relied on or built upon the work of the Austrian monk, whose experiments in pea plants are famous today as the foundation of genetics.

Yet at the time, “there was no such discipline as genetics, nor was there a concept of the gene,” says Yafeng Shan, a philosopher of science at the University of Kent in England. Instead, there were many theories of how traits were inherited, including Charles Darwin’s theory of pangenesis, which described particles of inheritance called “gemmules” thought to be given off by all cells in the body and to collect in the reproductive organs.

From the muddle of ideas, Shan says, those three reports at the dawn of the 20th century helped introduce Mendel’s work to other scientists in the fledgling field of heredity. That set the stage for the development of Mendelian genetics as we know it today, and no doubt played into a century’s worth of developments in molecular biology, from the discovery of the structure of DNA to the sequencing of the human genome and the rise of genetic engineering.

To celebrate our 100th anniversary, we’re highlighting some of the biggest advances in science over the last century. To see more from the series, visit Century of Science .

But the path to our current understanding of the inheritance and variation at the heart of modern biology has been far more winding than most biology textbooks reveal. In the conversation that follows, Elizabeth Quill, special projects editor at Science News , talks with Shan about the origins of genetics and what progress over the past century tells us about the nature of science.

Quill: Our understanding of genetics has emerged nearly entirely in the last century. Can you take us back? What did scientists know at the beginning of the century?

Shan: The term genetics was coined to describe the study of heredity in 1905 by the English biologist William Bateson in a letter to his friend. The term gene was introduced later, in 1909, by the Danish biologist Wilhelm Johannsen to refer to the unit of hereditary material.

That said, there were at least 30 different theories of heredity or inheritance at the beginning of the 20th century. So to borrow Charles Dickens’ phrase: It was the best of times, and it was the worst of times for the study of heredity. There were many different theories, methods and lines of inquiry available, but there was no consensus on the mechanism and patterns of inheritance, nor was there any consensus on a reliable way to study them.

Quill: In biology classes, we learn that Gregor Mendel’s experiments breeding pea plants in the mid-19th century taught us that inherited traits are delivered to offspring on pairs of genes, one from each parent, and that there are dominant and recessive forms of genes. But if the concept of gene wasn’t fully developed in Mendel’s day, what did his work actually reveal?

Shan: If you walk into any university library and pick up a copy of a genetics textbook today, you may find the following narrative: Mendel developed a theory of inheritance, but unfortunately, the theory was neglected or overlooked for over three decades, and only rediscovered in 1900.

Actually, there are mistakes in that: Mendel’s theory was not a theory of inheritance. He never used the German word for heredity — Vererbung . His concern was instead about the development of hybrids. In other words, Mendel did propose a theory for patterns of characteristics in plant hybrids, but it is not a theory of inheritance. And Mendel’s theory was not neglected or overlooked. There were more than a dozen citations to his paper before 1900. That’s not a lot, but definitely not overlooked.

Some fascinating things did happen in 1900, though. Mendel’s work was introduced to the study of heredity by Hugo de Vries, Carl Correns and Erich von Tschermak. All of them renewed Mendel’s work for different purposes. That being said, none of these three became a pioneer of Mendelism as we know it today.

Quill: Who was that pioneer?

Shan: After the introduction of Mendel’s work to the study of heredity, one important pioneer was William Bateson, an English biologist. Originally, he was not interested in the problem of heredity. So, to some extent, he was an outsider. He was studying evolution, but he found Mendel’s work useful. Based on Mendel’s findings, he said, we can develop a new theory that is the correct way to study heredity and will further shed light on the nature of evolution. He was one of the most prominent figures in the movement, which at first was resisted by many people.

To cut the story short, Mendelism won the victory — though in the early days, it was quite different from the Mendelian genetics of today, which was mainly established and developed by T.H. Morgan and his students and team at Columbia.

Quill: Thomas H. Morgan isn’t as widely known as Mendel or Darwin, for example. Why was his work so important and what made it different from what came before?

Shan: He may not have become a household name, but Morgan is considered one of the most influential geneticists ever. He actually began his career as a zoologist and had diverse interests in morphology, regeneration, embryology, et cetera. He was using fruit flies as experimental organisms to test the Darwinian theory of evolution. Darwin believed evolution happened through a series of minor and gradual changes. Others, including de Vries, believed species evolved through mutations: radical, sudden change. Morgan bought that argument.

Initially, his work was not very successful, in his own words. He started his experiment in 1908 and found nothing at all until 1910. He mentioned to an office friend that it was two years’ time, just wasted. But sometimes magical things just happen. After two years, he was surprised to find a mutation.

But he was puzzled. This mutation that he observed could not be explained by de Vries’ theory of mutation. Rather, it could be better accounted for by the Mendelian approach. So here is where Morgan and his team began developing a Mendelian approach.

What Morgan did differently from early Mendelians, say Bateson, was that he and his team incorporated Mendelism with another important line of inquiry in the field, the chromosome theory of inheritance, which was developed primarily by American geneticist Walter Sutton and German zoologist Theodor Boveri. They came up with the idea that hereditary material must be somewhere within the chromosomes. That provided a physical basis for hereditary material.

Quill: And that must have proved successful?

Shan: Combining Mendelism and the chromosome theory of inheritance leads to one of the most remarkable achievements of Morgan and his colleagues: They produced the chromosome map for the fruit fly. They located different genes at different locations on the chromosome. With that map, you can calculate the frequency of recombination of genes in the following generations. With that single map, you can identify not only the position of the genes on the chromosomes, but also predict the phenomenon of inheritance.

Quill: We haven’t yet talked about DNA. Were geneticists interested in DNA at that time?

Shan: The study of DNA was part of the job of biochemists. DNA was first identified in the mid-19th century, roughly the same time as when Mendel was working on his peas. Swiss chemist Friedrich Miescher was looking for the most fundamental constituents of life. He identified some substance coming from the nucleus of the cell and named it “nuclein.” That is what we now call DNA.

After his great discovery, the importance of and implications of nuclein, or DNA, were debated for decades. By the turn of the 20th century, nuclein was identified as a nucleic acid, and the five bases of nucleic acids — G, A, C, T and U — were also identified. In the 1920s and ’30s, biochemists came to know that the nucleic acid present in chromosomes is DNA.

But the makeup of DNA was only being pursued by biochemists. Those who studied the problem of heredity did not pay serious attention to DNA until the 1940s.

Quill: How did DNA get incorporated into the study of heredity?

Shan: That is the process of merging of the two lines of inquiry — the line of inquiry in genetics and the line of inquiry in biochemistry. For geneticists, their main concern was about a pattern and mechanism of inheritance and how a particular trait is transmitted from generation to generation. And on the other hand, biochemists were looking for the physical foundations of life.

With the success of T.H. Morgan and his colleagues, geneticists had a better capacity to predict and explain the patterns of inheritance. Then an immediate question arose: So, what are genes?

According to the Morgan school of classical genetics, a gene is just a segment of the chromosome. That’s very easy. There was very popular analogy in which they described genes as beads on the string. But it was still quite unclear what the physical basis was.  

Oswald Avery and his colleagues reported evidence in 1944 that DNA, rather than protein, carries hereditary information. Even though Avery’s experiment was not actually the first — it was confirming work done by others in 1939 — his work was better received and better known within the community. People often refer to Avery’s great experiment, though at the time some skepticism remained.

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Quill: That background helps explain why the discovery of the double-helix structure of DNA, from James Watson and Francis Crick, along with Rosalind Franklin and Maurice Wilkins, was so monumental. By knowing the structure of DNA, people could think about how the physical process of inheritance might work. Is that right?

Shan: Today we say, ‘Ah, so the process of inheritance is quite straightforward: Basically, DNA can be transcribed to RNA, and RNA can be translated into protein, and protein is responsible for phenotypic traits.’ Roughly speaking, it is like that.

That double-helix model provided a very reliable and useful framework to study DNA replication, and transcription. That’s crucially important for the later work in molecular genetics. At the time, in 1953, when Watson and Crick proposed that model, their work was not immediately well-received. It was not cited a lot — just like Mendel’s paper — until the end of the 1950s, when other work confirmed that the structure of DNA provides a mechanism of controlling protein synthesis.

There are quite a lot of important discoveries that followed. It’s probably unfair, but from my point of view, the others aren’t as exciting as the discovery of the double helix. If I can borrow a phrase from American philosopher Thomas Kuhn, we are now in the period of “normal science,” or what he calls “mopping up.” It took another 40 or 50 years to get where we are now, but in terms of milestones in the history of genetics, if you ask me if there’s anything as important as the introduction of Mendel’s work and the discovery of the double helix, I would say I’m afraid nothing else is as fascinating.

Quill: Looking back at the history of genetics, are there lessons to take away in how we think about science and scientific progress?

Shan: When we look back, we see that genetics developed through multiple parallel lines from the very beginning. We’ve got Darwin. We’ve got de Vries developing Darwin’s approach. We’ve got Francis Galton and his biometric approach, developed further by Karl Pearson and Raphael Weldon — which we didn’t even get to discuss. We’ve got Bateson borrowing ideas from Mendel. And there is also the important line of inquiry, the chromosome theory, independently developed primarily by Sutton and Boveri.  

Across the century, we start from classical genetics, then molecular genetics and now epigenetics (which studies changes in an organism that result from how genes are turned on and off, rather than alterations to the DNA sequence). That’s three historical episodes. One popular interpretation is that these three historical episodes or paradigms can be viewed as three scientific revolutions. But these paradigms are interactive with each other, not destructive or revolutionary. For instance, molecular genetics arises from the need to better understand the physical basis of heredity in classical genetics. Even today, the methods of classical genetics are still used in some problems.

I think there are lessons here about the nature and the aim of science. Science seems to be often characterized as an enterprise in explaining or understanding the phenomena of the world. It’s right to say scientists do make efforts to explain and understand. But there is another essential feature of science, namely exploratory or investigative. From the very beginning, none of the geneticists of the past century probably had a very clear idea of what a good explanation, what a good theory, what a good experiment would look like.

Our understanding of inheritance improved with the development of investigative or exploratory research. Ultimately, some of science’s most important features cannot be simply captured by concepts like truth or knowledge or understanding.

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• Proc Natl Acad Sci U S A
• v.119(30); 2022 Jul 26

Gregor Johann Mendel and Modern Evolutionary Biology

Behavioral genetics and genomics: mendel’s peas, mice, and bees, hopi e. hoekstra.

a Department of Organismic & Evolutionary Biology, Harvard University, Cambridge, MA 02138;

b Department of Molecular & Cellular Biology, Harvard University, Cambridge, MA 02138;

c Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138;

d HHMI, Harvard University, Cambridge, MA 02138;

Gene E. Robinson

e Department of Entomology, University of Illinois at Urbana–Champaign, Urbana, IL 61801;

f Neuroscience Program, University of Illinois at Urbana–Champaign, Urbana, IL 61801;

g Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana–Champaign, Urbana, IL 61801

Author contributions: H.E.H. and G.E.R. wrote the paper.

Associated Data

There are no data underlying this work.

The question of the heritability of behavior has been of long fascination to scientists and the broader public. It is now widely accepted that most behavioral variation has a genetic component, although the degree of genetic influence differs widely across behaviors. Starting with Mendel’s remarkable discovery of “inheritance factors,” it has become increasingly clear that specific genetic variants that influence behavior can be identified. This goal is not without its challenges: Unlike pea morphology, most natural behavioral variation has a complex genetic architecture. However, we can now apply powerful genome-wide approaches to connect variation in DNA to variation in behavior as well as analyses of behaviorally related variation in brain gene expression, which together have provided insights into both the genetic mechanisms underlying behavior and the dynamic relationship between genes and behavior, respectively, in a wide range of species and for a diversity of behaviors. Here, we focus on two systems to illustrate both of these approaches: the genetic basis of burrowing in deer mice and transcriptomic analyses of division of labor in honey bees. Finally, we discuss the troubled relationship between the field of behavioral genetics and eugenics, which reminds us that we must be cautious about how we discuss and contextualize the connections between genes and behavior, especially in humans.

Gregor Mendel’s greatest contribution—now known as Mendel’s laws of inheritance—stemmed from experiments he conducted with garden pea plants ( Pisum sativum ; 1 ). He chose peas, in part, because they are easy to grow, can be sown each year, and can conveniently be cross-pollinated by hand. In addition, peas have visible polymorphisms: Mendel focused on plants that varied in seed color (green or yellow) and seed morphology (wrinkled or smooth). Each trait had only two distinct forms, which could easily be scored in the offspring of his crosses. By calculating the ratios of each trait’s form across several generations, Mendel could identify consistent patterns, based on dominance and recessivity, of inherited “factors” (now called “genes”). It was the simplicity in the patterns of trait inheritance—single genetic variants that cause simple and specific phenotypic differences—that allowed Mendel to gain key insights into genetic inheritance, laying the foundation for modern-day genetics.

Built on this foundation, we now have a wealth of examples linking genetic variants to, for example, genetic diseases in humans and morphological polymorphisms in a wide diversity of species. Indeed, the genetic lesions for the vast majority of common Mendelian diseases have been identified ( 2 ), and we have a rich and growing database of major-effect genes contributing to variants in model organisms ( 3 ), domestication traits ( 4 ), and adaptations in wild populations ( 5 ).

However, it is also clear that not all inheritance patterns follow Mendel’s laws, not all traits have a “simple” genetic basis, and not all are unaffected by environmental conditions. The study of the genetic basis of behavior is faced with all of these challenges—in particular, behavioral variation often (although not always) has complex patterns of inheritance, involving the action and interactions of many genes, and is often strongly influenced by environmental variation. Nonetheless, even Darwin (Chapter 7 in ref. 6 ) recognized that behavior, like morphological variation, could evolve in the same way—through change over evolutionary time—leading some to suggest that Darwin laid the foundation for behavioral (and hence, behavioral genetic) research for the next century ( 7 ).

The first attempt to dissect the genetic basis of behavior was led by Seymour Benzer, who famously used forward-genetic screens to localize chromosomal regions (and ultimately genes) responsible for behavioral differences in mutagenized Drosophila melanogaster , for example, the period locus that affects circadian rhythm ( 8 ; reviewed in ref. 9 ). Today, approaches that rely on ever more powerful and cheaper genome sequencing technologies enable us to efficiently interrogate all of an organism’s genes and the natural variation therein, which means we can identify DNA variation associated with inherited differences in behavior, as a first step in elucidating the pathway from genotype to phenotype through the nervous system.

New sequencing technologies also opened an additional avenue of study: genome-wide studies of behaviorally related gene expression. Behavioral transcriptomics differs from behavioral genetics in that it does not necessarily focus on inherited differences in behavior. Instead, these studies, especially when followed by functional manipulation, are useful in identifying the neural and/or endocrine mechanisms underlying particular behaviors and understanding how these mechanisms are affected by the environment ( 10 ). Thus, the behavioral-transcriptomic approach complements the behavioral genetics approach. Behavioral transcriptomics provides insight into the more dynamic aspects of the relationship between genes and behavior and, together with the more deterministic aspects revealed by discoveries of causal behavior genes, provides a more comprehensive understanding of the relationships between genes and behavior.

To provide an overview of our current understanding of the relationship between genes and behavior, here we review some of the progress that has been made with both approaches: a focus on DNA variation associated with variation in burrowing behavior in deer mice ( Peromyscus sp.) and studies of brain gene expression and gene regulatory networks (GRNs) as they relate to hormonally regulated division of labor in honey bees ( Apis mellifera ). Readers interested in broader surveys of the field can consult other recent reviews ( 11 – 14 ). Because the study of genes and behavior is a complicated and societally fraught subject, we conclude with a brief discussion of the broader implications of how the legacy of Mendel relates to our current understanding of the relationship between genes and behavior.

Mendel’s Other Interests: Mice and Bees

In 1843, when Mendel joined a monastery in Brno at the age of 21, he was drawn to both experimentation and the natural world. Specifically, he was interested in understanding how variation—in what we now recognize as simple traits—is inherited from generation to generation to give rise to differences between their offspring ( Fig. 1 ). He initially thought to conduct these experiments in mice, which were popular among mouse fanciers at the time and have a wide range of coat colors that could be easily tracked across generations. In fact, he started to cross wild-type and albino mice in his monastery room ( 15 ). However, he never saw the outcome, as once the bishop caught wind of his idea to propagate mice (which, of course, involved sexual reproduction) in the abbey, Mendel’s plans were quickly shut down ( 15 , 16 ).

Mendel’s diverse interests. Other organisms, including house mice and honey bees, piqued Mendel’s interest but, for both biological and nonbiological reasons, his ability to perform genetic crosses on those species was limited. Instead, it is from his elegant study of the progeny of pea plants that he derived his laws of inheritance.

In the early 1900s, when Mendel’s laws were first rediscovered ( 17 ) and scientists scrambled to confirm his findings in other organisms, Lucien Cuénot, a French biologist, carried out the experiment that Mendel had started nearly a half century earlier. Specifically, Cuénot crossed mice of different coat colors but surprisingly, in one cross, was unable to replicate simple Mendelian inheritance ( 18 ). It was later determined that the pigment allele responsible for the color difference between the focal strains was a recessive lethal ( 19 ), resulting in a 2:1 ratio of color morphs in offspring derived from a monohybrid cross (rather than an expected Mendelian 3:1 ratio). The pigment allele segregating in what had come to be known as “Cuénot’s odd mice” has since been identified—the Ay allele of the Agouti signaling protein —and its molecular mechanism revealed ( 20 ). Thus, the bishop’s directive likely had a profound effect on the nascent field that would later be known as “genetics,” a term coined by W.B. Bateson, the chief popularizer of Mendel’s ideas following their rediscovery.

Mendel also was an avid beekeeper and tried to study inheritance in honey bees ( 21 ). Mendel kept colonies of honey bees at the monastery and was interested in several topics of practical importance, including improving beehive designs and honey production. He also likely performed the first genetic studies of honey bees ( 22 ), crossing Cyprian and Carniolan bees to observe effects on a variety of behavioral and physiological traits. It is not exactly clear how he performed the cross, however, because reproduction in honey bees is much more difficult to control than in pea plants. Honey bee queens and drones mate outside in flight; many have tried and failed to coax them to mate in captivity, including Mendel (prompting the development of instrumental insemination of queen bees in the mid-20th century). Moreover, queen honey bees mate with multiple drones, making it impossible for Mendel to have determined the pedigree of the resulting worker bees. Nevertheless, Mendel reported that the first-generation hybrid worker bees were extremely industrious, and the queens were extremely fecund. Given the difficulties, Mendel wisely did not follow up on this early discovery of hybrid vigor, which still today is not well understood at the molecular level, in any species (e.g., 23 ). However, with the help of sophisticated breeding paradigms, instrumental insemination, and one of the first sequenced insect genomes ( 24 – 26 ), honey bee genetics has made strong advances over the past three decades. Notable discoveries include identifying genomic regions contributing to variation in the propensity to forage for nectar or pollen ( 27 ) and the identification and functional validation of the csd gene, which underlies haplodiploid sex determination in honey bees ( 28 ).

Mendel did not only dodge the complexities of mouse coat color and honey bee reproductive biology, but there are many additional complexities, from a genetic perspective, that can obscure a simple connection between the inheritance of a particular allele and a trait of interest. Such deviations can include peculiarities of specific alleles/genes (e.g., codominance, incomplete dominance), the effect of multiple loci and their interaction (e.g., epistasis, which can lead to incomplete penetrance and/or variable expressivity), or the interaction between an allele and the environment (i.e., environmental effects) or gene-by-environment interactions, to name a few. Most trait variation, including behavior, is anything but simple.

Behavioral Genetics

The founding of behavior genetics—a field broadly interested in the question of how genetic (and environmental) differences influence behavioral differences—has been attributed largely to Francis Galton, a cousin of Charles Darwin, who in the nineteenth century studied, among other things, the inheritance patterns of “social and intellectual achievement” in a large human pedigree of wealthy British families, concluding there was statistical evidence for a hereditary contribution to differences in achievement among individuals ( 29 ). By acknowledging a role for the environment, he also reignited the ongoing “nature versus nurture” debate. The field and Galton’s contributions, however, were later undermined by his leading role in eugenics, a term he coined to describe the idea that selective breeding combined with knowledge about the inheritance of behavior could improve the human species ( 30 , 31 ). In addition to the deep racial prejudices associated with eugenics, the idea is based on an overly simplistic and erroneous understanding of the inheritance of behavioral variation.

It was not until the mid-20th century that behavioral genetics reemerged as a respectable scientific field. It now is widely accepted in the scientific community that variation in many, if not most, behaviors in animals (including humans) has some genetic influence, although the size of the genetic effect for any particular trait can differ widely. At the time of this rebirth, the genetic contribution to behavioral variation was most often explored either through twin or family studies in humans or in a handful of model animals, which could be studied in the laboratory. In recent years, with the advent of (affordable) genomic approaches, it is increasingly possible to connect genes and their expression to a wide diversity of behaviors in a wide diversity of (even nonmodel) species, allowing for exciting new insights into how behavior evolves in the wild.

Approaches to Connecting Genetic Variation and Behavioral Variation

One goal of behavioral genetics is to identify the precise genetic contributors to differences in natural behavior. Over the last several decades, behavioral geneticists have been estimating the number and location of genomic regions (those harboring causal mutations) associated with behavioral differences (e.g., 32 , 33 ; reviewed in ref. 34 ), whereas others focused on the specific roles of known genes, such as neuropeptides, on behavior (e.g., 35 , 36 ). Both approaches—forward genetics, which focuses on determining the genetic basis of a given phenotype, and reverse genetics, which involves genetic manipulations to elucidate gene function by examining changes to phenotypes—have contributed to our understanding of the link between genes and behavior. Now, the field is well poised to combine quantitative genetics and molecular genetics to interrogate the entire genome (using unbiased approaches) to identify specific genes that contribute to complex behavioral variation from a molecular perspective.

The ability to easily score millions of genetic markers or sequence entire genomes across hundreds or thousands of individuals in almost any species (e.g., genotype-by-sequencing approaches) has enabled researchers to more efficiently narrow in on causal genes through a variety of forward-genetic approaches, such as quantitative trait locus (QTL) mapping, genome-wide association studies, or comparisons of genomic differentiation between behaviorally distinct populations (reviewed in ref. 13 ). For example, using QTL mapping, Ding and colleagues ( 37 ) identified an insertion of a retroelement in the regulatory region of an ion channel in the slowpoke locus that affects a specific aspect of courtship song (sine song frequency) in Drosophila . By generating resequencing data from sweat bees from social and solitary populations, Kocher and colleagues ( 38 ) implicate a single genetic variant in the cis -regulation of syntaxin-1 , a gene that mediates synaptic vesicle release, contributing to a social-behavior polymorphism. Or a genome-wide scan of genetic differentiation between two warbler populations pointed to a strong candidate gene, Vacuolar protein sorting 13A ( Vps13A ), at which allelic variation may contribute to a binary choice in winter migration path ( 39 ). Clearly, new genome-wide genotyping and sequencing approaches have enabled the identification of (candidate) genes, and in some cases mutations, contributing to behavioral variation segregating within and between species, adding to a set of classic examples of the genetics of behavioral polymorphisms [e.g., colony structure in fire ants ( 40 ); foraging behavior in Drosophila ( 41 )]. These approaches are also applicable to the dissection of even more complex behavioral variation.

Behavioral Genetics in Deer Mice

While house mice (genus Mus ), like those that Mendel once considered studying, have long served as a premier model for mammalian behavioral genetics, the study of additional rodent species, which either express more extreme differences in behavior or even behaviors not performed by Mus , can serve as a complementary model for the genetic dissection of complex behavioral variation. For example, deer mice (genus Peromyscus ) have well documented variation in behavior (and other traits), often associated with their local environment ( 42 ). This natural behavioral variation combined with the ability to maintain these mice in the laboratory allows for both estimates of the heritability as well as the ability to conduct forward genetics, starting with genetic crosses to document inheritance of behavioral differences. In some cases, large behavioral differences observed in the wild, such as differences in aggression between island and mainland deer mice ( 43 ), show no heritable component when offspring of wild mice were reared in a common laboratory environment ( 44 ). However, other behaviors have a strong genetic component, such as interspecific differences in parental care: forward-genetic dissection of the parental-care behavior between two Peromyscus species, combined with a high-resolution whole-genome sequence and RNA-sequencing data from the hypothalamus, resulted in the localization of a specific gene, arginine vasopressin ( Avp ), whose difference in allele-specific expression level leads to differences in parental nest building behavior ( 45 ). Still other heritable behavior differences in deer mice have more lessons to share—specifically the evolution of burrowing behavior ( 46 )—thus providing promising opportunities to apply modern molecular genetics to understanding the genetic basis of behavior evolution.

Genetic Architecture of Burrowing

Many organisms build structures in their environment (e.g., bird nests, spider webs, beaver dams), which requires a suite of coordinated behaviors. Burrows represent one such architecture, which can provide safety from predators, buffering from environmental fluctuations, and/or a place to store resources ( 47 ). All Peromyscus use rhythmic head and limb movements to dig their burrows, yet different species consistently produce burrows of a particular size and shape ( 48 , 49 ). One species in particular, the oldfield mouse ( Peromyscus polionotus ), has an extremely long and stereotyped burrow architecture, which includes an entrance tunnel, nest chamber, and a secondary tunnel, which radiates up toward the ground’s surface but does not penetrate the surface and is used as an escape tunnel ( 50 ). It has been hypothesized that this burrow shape may be key to their survival: these mice live almost exclusively in an open habitat (e.g., oldfields, abandoned agricultural fields, and coastal dunes), with little vegetative cover as protection from predators or thermal fluctuations ( 51 ). This burrow shape contrasts strikingly with the burrows constructed by other species, which either do not build burrows at all or build small, simple burrows (e.g., the deer mouse, Peromyscus maniculatus ), with one exception: the Aztec mouse ( Peromyscus aztecus ), which constructs a long burrow ( 49 ). Importantly, however, all lack an escape tunnel. Because Peromyscus burrowing behavior can be recapitulated in the laboratory in a common soil environment, and some sister-species pairs are interfertile and thus can be intercrossed, this provides an opportunity to determine, as Mendel did with his peas, the inheritance patterns of specific burrow traits across hybrid generations ( Fig. 2 ).

Simple versus complex inheritance. ( A ) Pea seed color, as studied by Mendel, exhibits a simple genetic basis. When pea plants that generate yellow- and green-colored seeds are crossed (F0), the seeds of all progeny are green (F1). When F1 progeny are crossed, the resulting F2 progeny produce either yellow seeds or green seeds, fully recapitulating the F0 parental phenotypes with no intermediates. These results can be explained by a single gene for seed coat color, identified as staygreen ( sgr ; ref. 90 ), with a dominant green and recessive yellow allele. ( B ) Burrow architecture in Peromyscus mice exhibits a more complex genetic basis. When oldfield mice are crossed with deer mice (F0), the resulting offspring dig burrows like oldfield mice (F1). However, when F1 mice are backcrossed with deer mice, BC1 offspring display a continuous distribution of burrow length that spans parental species phenotypes. These results are consistent with multiple, dominant loci from oldfield mice contributing to longer burrows.

The burrows built by oldfield mice are remarkable, both for their clever design and for the consistency with which they are constructed. Several lines of evidence support a strong genetic component for this particular behavior. First, mice born in the laboratory, despite never witnessing burrow construction or even experiencing soil, produce a species-typical burrow ( 49 , 52 ). Oldfield mice will even do so when cross-fostered by short burrowing deer mouse parents ( 53 ). Even more remarkable, young oldfield mice, starting at 19 d of age (note that pups are weaned at 24 d of age) produce burrows with species-typical shape, only the overall burrow size is smaller; this contrasts with deer mice that do not start building their simple burrows until much later, at 27 d of age ( 53 ). In fact, the burrow size and shape constructed by an individual mouse is highly repeatable across trials ( 49 , 54 ). Both males and females dig burrows, with no differences in burrow architecture between the sexes ( 55 ). While it was originally proposed that pregnant females may be most motivated to dig natal burrows ( 52 ), there is no evidence for differences in burrow construction between virgin, mated, or pregnant females ( 56 ). Burrow length is almost invariant even in the wild, even if burrow depth varies with soil type (i.e., burrows built in heavy loam are shallower than those built in loose sand; ref. 55 ). Together these data are consistent with the complex burrowing behavior of the oldfield mouse being largely innate.

The burrows constructed by the oldfield mouse and its sister species, the deer mouse ( P. maniculatus ), differ dramatically—both in size and shape ( Fig. 2 B ). In the laboratory, oldfield mice produce burrows that are, on average, 40 cm in total length with a 20-cm entrance tunnel; by contrast, deer mice burrows have only a short entrance tunnel of ∼7 cm ( 49 , 55 ). These two species are interfertile, which offers the opportunity to dissect the genetic basis of these differences in burrowing behavior. Indeed, first-generation (F1) hybrids produce burrows that are statistically indistinguishable from the oldfield mice—long and including an escape tunnel—suggesting the behaviors, and the underlying causal alleles, act in a dominant fashion ( Fig. 2 B ; 52 , 55 ). Crossing these F1 hybrids to deer mice produced a generation of backcross (BC1) hybrids, for which 25% of alleles are inherited from oldfield and 75% from deer mice, that resulted in a wide range of burrow architectures, including burrows that matched the parental burrows (i.e., long with an escape tunnel, or short without an escape tunnel) as well as those that differed from the parents (i.e., long without an escape, and short with an escape; Fig. 2 B ). These BC1 burrow phenotypes suggest that, like Mendel’s peas, in which seed color and seed morphology seemed to segregate independently, the major genes affecting burrow length and burrow shape may be unlinked ( 55 ). In other words, the complex burrow seems to be subserved by distinct genetic modules.

The genetic architecture of Peromyscus burrow variation also showed some clear differences from the simple inheritance patterns that Mendel observed in his peas ( Fig. 2 ). For example, in the BC1 hybrids, burrow length did not have only two discrete states (e.g., long or short), but instead was continuously distributed. This pattern suggested that allelic variation at more than just one gene likely drives the difference in burrow length. Indeed, by genotyping the BC1 population with thousands of markers across the genome, genomic regions were localized in which particular genotypes were predictive of burrow phenotype. There were three regions on three separate chromosomes that correlated with burrow length, and notably a separate fourth chromosome was associated with the presence and/or absence of escape tunnel ( 55 ). Together, the three regions accounted for ∼15% of the variation in burrow length. Based on the repeatability of burrowing behavior in the oldfield mice, ∼24% of variation in burrowing can be explained by genetics, suggesting that the three regions account for a majority of the genetic effects; nevertheless, these results also suggest that additional loci and/or environmental effects likely also influence burrowing. Indeed, a recent study intersected alleles that were differentially expressed in the brain both between species and between burrowing and nonburrowing animals with these mapping results to implicate a role for additional small-effect loci that contribute to burrowing difference ( 57 ).

These discoveries have been facilitated by high-resolution linkage maps, enabled by efficient and cost-effective genotyping approaches, and, more recently, genome assemblies with gene annotations for these species. Genome assemblies improve the precision of genetic mapping and downstream investigations of the possible causal genes and mutations. Moreover, long-read sequencing now allows us to move beyond surveying only single nucleotide mutations and discover larger structural genetic changes that have been associated with multiple adaptive traits as in Peromyscus ( 58 ) as well as behavioral polymorphisms [e.g., mating behavior in ruffs ( 59 , 60 ), reproductive behavior in white-crowned sparrows ( 61 ), and social organization in fire ants ( 62 ); reviewed in Wellenreuther and Bernatchez ( 63 )].

Simple versus Complex Genetics

Mendel’s experiments were both elegant and powerful because of their simplicity—the traits were simple and the underlying genetics was simple. A single gene with just two alleles fully explained the inheritance of an easily measured trait (e.g., color of a pea seed) ( Fig. 2 A ). Most trait variation, however, is more complex, as is often the case for behavior. Nonetheless, sometimes what is most striking is the contrast between the degree of perceived complexity at the phenotypic versus genotypic level. In the case of burrowing, one can argue that the evolution from a short, simple burrow to a long burrow with intricate engineering (i.e., a well-designed escape hatch) is likely to be complex. Nonetheless, the ability to map a handful of regions in a relatively small mapping population is remarkable, and the possibility of a small number of discrete genetic modules (e.g., relatively large-effect loci that each affect a different aspect of burrow size/shape) suggests a relatively simple genetic architecture. However, this result also does not preclude the contribution of allelic variation at a few or even many small-effect loci to variation in burrowing behavior. Much work remains (and is indeed ongoing) to narrow in on causal genes within these regions, which is arguably the most challenging and time-consuming step of forward-genetic approaches, especially for behavioral traits for which candidate genes may be obscure. In the meantime, it is interesting to imagine what types of genetic changes could lead to a longer burrow—perhaps genetic changes leading to faster-acting or longer-lasting muscle movements, changes in locomotory sequence, or a change in motivation to dig, for example. In any case, whatever the allele(s) may be, it is unlikely that there is a gene or genes “for burrowing.” Most genes are pleiotropic and play multiple roles in organisms, at different timepoints and/or in different tissues. Thus, even for very tractable behaviors, the situation is not as simple as one gene–one trait. But the power of genomics is now letting us investigate the genetic basis of the vast majority of heritable traits, behavior included, that are not as simple as traits in Mendel’s peas.

Behavioral Genomics

The advent of genomics, now almost 30 y ago, also enabled the development of a new approach to study the connection between genes and behavior, which complements the powerful forward-genetic approaches described earlier. This involves measuring gene expression (most often in the brain) associated with differences in behavior. Behaviorally related gene expression analyses certainly were performed prior to the genomic era, but then they were usually restricted to one or a few genes at a time. Genomics enables entire transcriptomes to be interrogated—from whole brains, brain regions, or individual brain cells—to provide unbiased gene discovery ( 64 , 65 ). Differences in gene expression associated with differences in behavior result in lists of usually hundreds to thousands of differentially expressed genes (DEGs). Sometimes these DEG lists provide candidate genes for functional studies to go beyond correlation: for example, RNA sequencing of mosquitoes that are attracted to humans versus other mammals pointed to a candidate olfactory receptor later shown with additional experiments to cause difference in host preference ( 66 ). However, it is important to note that using transcriptomics alone to find causal genes is often difficult because even a single casual genetic variant can affect the expression of thousands of genes. Nonetheless, annotation tools such as Gene Ontology can be used to provide suggestive insights into the pathways or mechanistic themes that might characterize the differences in behavior. In addition, transcriptomics also can serve as the basis for studies of mechanisms of gene regulation via the modeling of GRNs, an approach discussed in detail later in this work, and can be particularly powerful in understanding the influence of the environment. Thus, without needing to perform crosses, the traditional Mendelian starting point, genomics has dramatically expanded the scope of the study of genes and behavior, enabling many naturally occurring and complex behaviors to be studied (especially those not amenable to laboratory experiments) in many different species at the molecular level.

Division of Labor in Honey Bee Colonies

The social behaviors of honey bees gave Charles Darwin pause ( 67 ). Darwin struggled—for his theory of evolution by natural selection to hold true, he needed to convince himself that what he referred to as the “wonderful instincts” of social insects could evolve through the accumulation of small changes over time (Chapter 7 in ref. 6 ). Indeed, the inheritance of such complex social behavior of honey bees also fascinated Mendel, as discussed earlier. Both scholars yearned to understand how complex, innate social organization could be inherited over generations, a question we are only now just beginning to unravel from a genetic perspective.

One of the most important components of the intricate organization of the honey bee colony is a division of labor among worker bees, which is based on a process of individual behavioral maturation ( Fig. 3 A ; ref. 68 ). Adult worker honey bees typically live for 4 to 7 wk during the active season; they work in the hive sequentially performing a series of tasks for the first 2 to 3 wk including brood care (“nursing”) and honey processing, and then spend the remainder of their lives defending the hive and searching for nectar and pollen (“foraging”). In addition, honey bee colonies cope with changes in health, age demography, resource availability, and other factors through flexible changes in division of labor based on worker bees accelerating, delaying, or even reversing their behavioral maturation to serve the needs of their colony. This is usually determined by measuring the age at onset of foraging because the transition from working in the hive to foraging outside the hive is particularly sharp.

Gene expression networks underlie behavioral plasticity. ( A ) A honey bee worker will begin its adult life working inside the hive as a nurse and then will transition through a middle-age phase of tasks before a shift to working outside the hive as a forager. ( B ) The behavioral changes associated with this labor transition are tied to changes in brain gene expression in thousands of genes. The coordinated changes in brain gene expression result in markedly different gene expression networks between nurse and forager worker bees.

So far, about a dozen social, nutritional, hormonal, neurochemical, and molecular determinants of age at onset of foraging have been identified, including several genes ( 69 ). Each determinant either speeds up or slows down behavioral maturation to cause a precocious or delayed onset of foraging, respectively; the focus here will be on endocrine-related determinants. There also are inherited differences in rate of behavioral maturation ( 70 ); while no specific DNA variants have yet been identified, forward-genetic mapping studies, like those described earlier, suggest that the genetic architecture underlying variation in rate of behavioral maturation is highly polygenic ( 27 ).

In an early study of division of labor-related brain gene expression ( 71 ), and one of the first genome-wide surveys of behaviorally related gene expression in any species, nursing and foraging bees were compared. In addition to their well-known differences in behavior, extensive research has revealed that nurses and foragers differ the most in physiology, brain structure, and brain chemistry relative to all other groups of task specialists in honey bee colonies ( 72 ). Given these differences, it was expected that at least some genes will show differences in expression between nurses and foragers. Nonetheless, it was surprising to learn that almost 40% of the roughly 5,500 genes (with the early technology available at the time) with measurable activity in the brain were either up-regulated or down-regulated in nurses compared to foragers. Though only correlative, the differences measured in bees were so robust that it was possible to computationally predict whether a bee was a nurse or forager solely by its brain gene expression profile. Since that report, many studies conducted in a wide range of species report behavioral differences associated with brain expression differences in hundreds or more often thousands of genes.

Genes Involved in Hormonal Regulation of Division of Labor

Many of the genes differentially expressed in the brain of nurses and foragers are related to hormonal regulation of division of labor ( 73 ). Division of labor among worker honey bees has evolved by co-opting elements of the basic insect reproductive system, especially juvenile hormone (JH) and vitellogenin (Vg) ( 74 ). In most insect species, an increase in the blood levels of JH is associated with the onset of reproduction, but in honey bees it is associated with the onset of foraging. Hormone treatments that mimic this increase cause precocious foraging, while removal of the JH-producing corpora allata delays it. In addition, Vg inhibits JH production, JH inhibits Vg production, and the two of them together form a double repressor feedback loop to control age at onset of foraging ( 75 ).

So far, four JH-related genes have been identified that differ in expression between nurses and foragers and also cause changes in behavioral maturation (however, no specific DNA variants in behaviorally related genes have yet been identified in honey bees). The four are vg , ultraspiracle ( usp ), broad , and fushi tarazu transcription factor 1 ( ftz-f1 ). The vg gene encodes Vg, a lipoprotein that is one of the most abundant yolk proteins in insect eggs. In honey bees, Amdam, Page, and colleagues ( 27 ) have demonstrated that vg has evolved novel roles related to immune function, longevity, nutrition, and behavior; knockdown of vg in the abdomen by RNA interference (RNAi) increases blood levels of JH and leads to precocious foraging ( 75 , 76 ). Usp is involved in orchestrating the transcriptional response to JH; it also is part of a gene family known to encode proteins regulating metabolism and nutritional physiology in many species. This gene was of particular interest because changes in nutritional physiology are linked to honey bee behavioral maturation; for example, starvation is a factor that induces precocious foraging. Moreover, usp expression is up-regulated in the fat body following JH treatment, and knockdown of usp by RNAi causes a delay in the onset of foraging ( 77 ). Broad and ftz-f1 also have been linked to JH transcriptional effects: broad encodes a protein that integrates signals from diverse endocrine pathways including JH and Vg, while the expression of ftz-f1 itself is regulated by broad . Knockdowns of the expression of broad or ftz-f1 in the brain by RNAi also cause delays in the onset of foraging ( 78 ). These four genes represent only a small fraction of the molecular machinery underlying hormonal regulation of division of labor, but studies about their regulation already have yielded important insights into the dynamic relationship between genes and behavior.

GRNs and Division of Labor

Genes and neurons work together to make brain function possible, and both function within networks. GRNs are located inside each cell in the brain (and other tissues) and coordinate gene expression. Neuronal networks (NNs) coordinate the activity of neural circuits that transmit electrochemical signals from one neuron to another. We are only just beginning to understand how GRNs and NNs integrate brain activity to control behavior ( 79 ).

It is assumed that the honey bee genome contains a comparable number of transcription factors (TFs) to Drosophila , roughly 700. How many TFs regulate the expression of each gene, and how many genes are regulated by each TF? There is not yet enough information for firm numbers, but it is thought that each gene is regulated by about 5 to 100 TFs ( 80 ), and each TF regulates the expression of tens to hundreds of genes, sometimes including other TFs. It is thus very clear that gene regulation occurs in hierarchical, cascading, networks, with tiers of TFs working in combination to regulate the expression of their target genes.

Usp , broad , and ftz-f1 code for TFs, and as expected, each of these TFs regulates many genes ( Fig. 3 B ; refs. 69 and 81 ). vg RNAi abdomen knockdown results in changes in the expression of thousands of genes, in both the fat body and brain ( 76 ). Vg, however, is not a TF; these effects are thus indirect, due to interactions between Vg and associated TFs. A computational analysis of the genomes of 10 different species varying in level of social complexity also has implicated usp and broad in the evolution of social life in bees ( 82 ).

Computational analyses enable genome-scale analyses to find the location of all copies of DNA binding motifs associated with each TF to identify the genes predicted to be regulated by each TF. This approach has yielded two important insights thus far. First, regulation of gene expression by TFs involves complex and variable combinatorial rules. Recall that honey bee behavioral maturation is affected by a variety of social, nutritional, hormonal, neurochemical, and molecular determinants, only some of which were discussed earlier. A study of 11 of these determinants showed that they collectively influence the expression of overlapping sets of genes in the brain ( 73 ). Using newly created bioinformatic tools, Ament and colleagues ( 77 ) showed that these overlapping genes are in turn regulated by overlapping sets of TFs. But the precise combination of TFs varied for the different determinants, and the ways that TFs are involved appear to follow different forms of computational logic (e.g., “and, “or,” and “not” gates). For example, two cis -regulatory motifs together predict gene expression responses to the 11 determinants of behavioral maturation but follow different Boolean combinatorial rules. These findings hint at a highly complex regulatory code that governs how the same genes regulate different behaviors ( Fig. 3 B ).

Second, regulatory relationships between TFs and target genes are context dependent. Based on a meta-analysis of brain gene expression profiles from 853 individual adult worker honey bees exhibiting 48 distinct behavioral states associated with division of labor, the manner in which a TF and its target genes are coexpressed in the brain is predicted to vary with behavioral state (e.g., whether the measurements were made from a bee engaged in nursing or foraging; ref. 80 ). Focusing on broad and ftz-f1 , Hamilton and colleagues ( 78 ) found results consistent with this prediction: patterns of coexpression between broad and ftz-f1 and their respective targets indeed varied with behavioral state, and also with experimentally induced changes in broad and ftz-f1 brain expression due to RNAi. For example, a brain GRN predicts a positive correlation between the expression of broad and the target gene GB15608 for bees in an aggressive behavioral state associated with hive defense, but a negative correlation for bees engaged in foraging ( 78 ). Changes in regulatory relationships between TFs and target genes have already been discovered to occur at evolutionary time scales ( 83 ). These new results now extend this concept to shorter time scales associated with neurobiology and behavior.

Behavioral Genetics and Genomics

A complete understanding of the relationship between genes and behavior requires probing the consequences of genetic variation at multiple levels: from DNA sequences to brain gene expression, to brain connectivity, to neuronal firing dynamics, to behavioral variation. Behavioral genetics provides information on causal genes and alleles, whereas behavioral genomics can help identify mechanistic roles of these alleles through transcriptomic and GRN analyses; both approaches are necessary to gain a complete picture of how genes contribute to behavior ( Fig. 4 ). While there are a growing number of examples in which allelic variation at genes is associated with behavior, there are many fewer cases in which we also have a clear understanding of how those genes act in or on neural circuits to cause differences in behavior, especially in the context of evolutionary changes. We do know that experience-dependent changes in brain gene expression can help prepare individuals to adjust their behavior to new environmental conditions ( 72 , 84 ). The use of transcriptomics to discover evolutionarily conserved “genetic toolkits” for behavior is yet another approach to facilitate a comprehensive understanding of the relationship between genes and behavior ( 85 ).

Behavioral genetics and behavioral genomics as complementary approaches. Both behavioral genetics and behavioral genomics investigate the link between genes and behavioral variation. Behavioral genetics, as exemplified by Peromyscus burrowing, seeks to connect inherited genetic variation to behavioral variation. Changes in genetic sequence are represented by different colors of chromosomes (pink vs. green). Behavioral genomics, as exemplified by honey bee division of labor, seeks to connect changes in gene expression to behavioral plasticity. Behavioral plasticity can be caused by changes in the environment (external or internal), which can lead to, for example, epigenetic modifications (yellow circles) without changing DNA sequence (purple). Both changes in DNA sequence and the environment result in changes in gene regulation, and, ultimately, behavior.

Reconsidering the Relationship between Genes and Behavior

Mendel’s pioneering studies provided the foundation for the science of genetics. His choice of traits with simple inheritance eventually led others to adopt a “genes for” shorthand for an expectation of a direct, specific, and causal relationship between a gene and a trait. For behavior, it is important to acknowledge that this perspective has been all too easily misused to promote inequities, racism, and genocide ( 86 ). The studies reviewed here highlight that the relationship between genes and behavior is more complicated than suggested by the “genes for” shorthand. To be sure, there are indeed excellent examples of individual genes that have DNA variants with specific and causal relationships to a behavior, some of which affect the expression of many other genes (e.g., 35 , 45 , 66 , 87 , 88 ; reviewed in ref. 13 ). But the studies described in depth earlier serve as examples of how “genes for” thinking is misguided. There are no genes that specify behavior; rather, genetic variation acts to change biochemical and cellular pathways that alter neuronal circuits to result in behavioral variation and, ultimately, behavioral evolution. Because most genes are pleiotropic, changes in any one gene often affect many traits. Moreover, even the most fundamental elements of gene regulation are not fixed, and we have as yet little knowledge of the dynamic and context-dependent codes that govern the operation of behaviorally related GRNs. In addition, human genome-wide association studies reveal that associations between specific alleles and behavioral variation are heavily dependent on the populations that are studied (i.e., the genetic background in which alleles are found), providing another example of “context-dependent” connections between genes and behavior ( 89 ). Given that behaviors are the product of both nature and nurture, it is clear that both inherited and environmental influences affect the precise and ever-changing relationships between genes and behavior ( Fig. 4 ).

Understanding the complex relationship between genes and behavior—and how these connections may vary among individuals, populations, and species—is a grand challenge for both science and society. Studies such as those reviewed here are pushing scientists to move past the outdated “genes for” paradigm, and it is critically important for scientists to work hard to enable the public to recognize and understand the underlying science. In this way, we can honor the legacy of Mendel’s pioneering discoveries, highlight the scientific and societal progress we have made since, and identify the important work that still must be done.

Acknowledgments

H.E.H.’s research was supported by the Howard Hughes Medical Institute. G.E.R.’s research was supported by the NSF, NIH, Burroughs Wellcome Foundation, and the Christopher Foundation. We thank N. C. Bedford, A. M. Bell, M. R. Berenbaum, A. R. Hamilton, O. S. Harringmeyer, C. K. Hu, N. Jourjine, H. C. Metz, D. J. Robinson, S. Sinha, M. R. Sokolowski, M. L. Woolfolk, and two anonymous reviewers for comments that improved this manuscript. C. K. Hu designed and illustrated all the figures.

The authors declare no competing interest.

This article is a PNAS Direct Submission.

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