mendel's experiments the origin of genetics

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."

QUICK FACTS

• Name: Gregor Mendel
• Birth Year: 1822
• Birth date: July 20, 1822
• Birth City: Heinzendorf
• Birth Country: Austria
• Gender: Male
• 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.
• Science and Medicine
• Astrological Sign: Cancer
• University of Vienna
• University of Olmütz
• Nacionalities
• Death Year: 1884
• Death date: January 6, 1884
• Death City: Brno
• Death Country: Austria

CITATION INFORMATION

• Article Title: Gregor Mendel Biography
• Author: Biography.com Editors
• Website Name: The Biography.com website
• Url: https://www.biography.com/scientists/gregor-mendel
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• Publisher: A&E; Television Networks
• Last Updated: May 21, 2021
• Original Published Date: April 2, 2014
• 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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Who was Gregor Mendel?

Gregor Mendel was an Austrian scientist, teacher, and Augustinian prelate who lived in the 1800s. He experimented on garden pea hybrids while living at a monastery and is known as the father of modern genetics .

Through his careful breeding of garden peas, Gregor Mendel discovered the basic principles of heredity and laid the mathematical foundation of the science of genetics . He formulated several basic genetic laws, including the law of segregation, the law of dominance, and the law of independent assortment, in what became known as Mendelian inheritance .  

Gregor Mendel (born July 20, 1822, Heinzendorf, Silesia, Austrian Empire [now Hynčice, Czech Republic]—died January 6, 1884, Brünn , Austria-Hungary [now Brno, Czech Republic]) was a botanist, teacher, and Augustinian prelate, the first person to lay the mathematical foundation of the science of genetics , in what came to be called Mendelism .

Born to a family with limited means in German-speaking Silesia , Mendel was raised in a rural setting. His academic abilities were recognized by the local priest , who persuaded his parents to send him away to school at the age of 11. His Gymnasium (grammar school) studies completed in 1840, Mendel entered a two-year program in philosophy at the Philosophical Institute of the University of Olmütz (Olomouc, Czech Republic), where he excelled in physics and mathematics , completing his studies in 1843. His initial years away from home were hard, because his family could not sufficiently support him. He tutored other students to make ends meet, and twice he suffered serious depression and had to return home to recover. As his father’s only son, Mendel was expected to take over the small family farm, but he preferred a different solution to his predicament, choosing to enter the Altbrünn monastery as a novitiate of the Augustinian order, where he was given the name Gregor.

The move to the monastery took him to Brünn, the capital of Moravia , where for the first time he was freed from the harsh struggle of former years. He was also introduced to a diverse and intellectual community . As a priest, Mendel found his parish duty to visit the sick and dying so distressing that he again became ill. Abbot Cyril Napp found him a substitute-teaching position at Znaim ( Znojmo , Czech Republic), where he proved very successful. However, in 1850 Mendel failed an exam—introduced through new legislation for teacher certification—and was sent to the University of Vienna for two years to benefit from a new program of scientific instruction. As at Olmütz, Mendel devoted his time at Vienna to physics and mathematics, working under Austrian physicist Christian Doppler and mathematical physicist Andreas von Ettinghausen. He also studied the anatomy and physiology of plants and the use of the microscope under botanist Franz Unger, an enthusiast for the cell theory and a supporter of the developmentalist (pre-Darwinian) view of the evolution of life. Unger’s writings on the latter made him a target for attack by the Roman Catholic press of Vienna shortly before and during Mendel’s time there.

In the summer of 1853, Mendel returned to the monastery in Brünn, and in the following year he was again given a teaching position, this time at the Brünn Realschule (secondary school), where he remained until elected abbot 14 years later. He attempted the teacher exam again in 1856, although the event caused a nervous breakdown and a second failure. However, these years were his greatest in terms of success both as teacher and as consummate experimentalist. Once abbot, his administrative duties came to occupy the majority of his time. Moreover, Mendel’s refusal to permit the monastery to pay the state’s new taxes for a religious fund led to his involvement in a long and bitter dispute with the authorities. Convinced that this tax was unconstitutional, he continued his opposition, refusing to comply even when the state took over the administration of some of the monastery’s estates and directed the profits to the religious fund.

In 1854 Abbot Cyril Napp permitted Mendel to plan a major experimental program in hybridization at the monastery. The aim of this program was to trace the transmission of hereditary characters in successive generations of hybrid progeny. Previous authorities had observed that progeny of fertile hybrids tended to revert to the originating species , and they had therefore concluded that hybridization could not be a mechanism used by nature to multiply species—though in exceptional cases some fertile hybrids did appear not to revert (the so-called “constant hybrids”). On the other hand, plant and animal breeders had long shown that crossbreeding could indeed produce a multitude of new forms. The latter point was of particular interest to landowners, including the abbot of the monastery, who was concerned about the monastery’s future profits from the wool of its Merino sheep, owing to competing wool being supplied from Australia.

Mendel chose to conduct his studies with the edible pea ( Pisum sativum ) because of the numerous distinct varieties, the ease of culture and control of pollination , and the high proportion of successful seed germinations . From 1854 to 1856 he tested 34 varieties for constancy of their traits. In order to trace the transmission of characters, he chose seven traits that were expressed in a distinctive manner, such as plant height (short or tall) and seed colour (green or yellow). He referred to these alternatives as contrasted characters, or character-pairs. He crossed varieties that differed in one trait—for instance, tall crossed with short. The first generation of hybrids (F 1 ) displayed the character of one variety but not that of the other. In Mendel’s terms, one character was dominant and the other recessive . In the numerous progeny that he raised from these hybrids (the second generation, F 2 ), however, the recessive character reappeared, and the proportion of offspring bearing the dominant to offspring bearing the recessive was very close to a 3 to 1 ratio. Study of the descendants (F 3 ) of the dominant group showed that one-third of them were true-breeding and two-thirds were of hybrid constitution. The 3:1 ratio could hence be rewritten as 1:2:1, meaning that 50 percent of the F 2 generation were true-breeding and 50 percent were still hybrid. This was Mendel’s major discovery, and it was unlikely to have been made by his predecessors, since they did not grow statistically significant populations, nor did they follow the individual characters separately to establish their statistical relations.

Mendel’s approach to experimentation came from his training in physics and mathematics , especially combinatorial mathematics . The latter served him ideally to represent his result. If A represents the dominant characteristic and a the recessive, then the 1:2:1 ratio recalls the terms in the expansion of the binomial equation: ( A + a ) 2 = A 2 + 2 A a + a 2 Mendel realized further that he could test his expectation that the seven traits are transmitted independently of one another. Crosses involving first two and then three of his seven traits yielded categories of offspring in proportions following the terms produced from combining two binomial equations, indicating that their transmission was independent of one another. Mendel’s successors have called this conclusion the law of independent assortment .

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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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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.

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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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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Gregor Mendel - Father of Genetics: Biography, Experiments & Facts

Johann Mendel, later known as Gregor Mendel, was born on July 22, 1822, in Heinzendorf bei Odrau, a little village in a part of the Austrian Empire known today as the Czech Republic, or more recently, Czechia.

Mendel is considered the father of modern genetics, but his work was largely ignored until after his death in ­­1884.

He assumed the added name of Gregor upon joining a monastery in 1843, where he tended the monks' gardens and conducted his well-known pea plant experiments.

Gregor Mendel Biography: The Early Years

Johann Mendel was born to peasant farmers, Anton and Rosine Mendel. He grew up in a German-speaking rural area with his parents and two sisters, Veronika and Theresia. Johann attended a prep school called a Gymnasium where his academic promise was recognized by the local priest. At age 11, he was sent away to a school in Troppau.

Being of humble means, his family could not support the boy once he left home. Mendel had to tutor other students to support himself. Throughout his education, he suffered from bouts of depression and returned home periodically to recover, but eventually he graduated.

Mendel then entered a two-year program at the Philosophical Institute of the University of Olmütz, also called Olomouc; this program was required before starting university studies.

Enrollment at the Philosophical Institute

Things did not go so well for Mendel in Olomouc, despite his intelligence and love of learning. He experienced more financial difficulties given the language barrier he faced in the primarily Czech-speaking region.

Once again he experienced severe depression and had to return home to recover.

His younger sister, Theresia, encouraged her brother to finish his education, and even offered to help him with the cost of his schooling. Theresia generously gave Johann her portion of the family estate that she had been planning to use has a dowry.

Years later, Mendel repaid the debt by helping her raise her three sons. Two of them became physicians.

Entering the St. Thomas Monastery

Young Mendel wanted to further his education but could not afford to do so. A professor urged him to join the Abbey of St. Thomas monastery in Brünn (Brno, Czech Republic) and continue his education. Mendel’s inquisitive and analytical mind drew him to the study of math and science. He chose St. Thomas because of the ­­­order’s reputation for progressive thinking inspired by the Age of the Enlightenment.

The monastery operated under the Augustinian credo per scientiam ad sapientiam ("from knowledge to wisdom") and focused on scholarly teaching and research. Upon entering the monastery as a novice in 1843 his name became Gregor Johann Mendel.

His formal schooling and personal experience growing up on a farm made him an asset to the order’s agricultural operations.

Early Life at the St. Thomas Monastery

The Moravian Catholic Church, along with intellectuals and aristocrats, were becoming aware of the importance of science in the 1900s. Gregor Mendel was urged to learn all types of sciences, including plant cultivation. In stark contrast to the rest of his life, Mendel enjoyed the luxury of fine dining.

The monastery was renowned for gastronomy and culinary arts instruction.

Gregor Mendel attended classes at the Brünn Theological College and in 1847, he was ordained a priest. As part of his monastic duties, he worked as a high-school level science teacher. However, he failed a new teacher certification exam in 1850 and examiners recommended that he attend college for two years before taking the test again.

Studies at the University of Vienna

Between 1851-1853, Gregor Mendel enjoyed studying at the University of Vienna under the tutelage of renowned mathematicians and physicists Christian Doppler and Andreas von Ettinghausen. Mendel deepened his understanding of plants when working with botanist Franz Unger.

Mendel’s dissertation explored the origin of rocks , which was a controversial topic at that time.

At the University of Vienna, Mendel learned advanced research technique and scientific methodologies, which he later applied to the systematic cultivation of pea plants. He is called the father of modern genetics because he identified the fundamentals laws of inheritance and calculated their statistical probabilities, a skill that he honed at UV.

Mendel was one of the first scientists to incorporate mathematics into the field of biology.

Where Did Gregor Mendel Work?

Gregor Mendel spent several years of his career teaching high school students at schools in and around Brünn while he resided at St. Thomas monastery. The young monk obtained permission from his superiors to conduct a longitudinal study of plant hybridization in his free time. Mendel was allowed to perform experiments in his own laboratory, which was essentially the monastery greenhouse and 5-acre garden plot.

Later in life, Mendel became abbot of St. Thomas monastery where he lived and worked for the remainder of his days on Earth.

Gregor Mendel’s First Experiments

Mendel’s first genetic experiment started with mice, and then he moved on to garden peas (genus Pisum ). Mendel’s work with mice came to a halt when the bishop learned that Mendel was raising caged mice in his small living quarters. If Mendel had gotten around to crossing pure breeding black and white mice, he would have made an interesting discovery related to codominance and incomplete dominance .

Mendelian genetics – grounded in observations of inherited garden pea traits – would have erroneously predicted all black mice, not gray mice, in the first generation (F 1 ).

Mendel began to plan programs in experimental hybridization of peas at the monastery in 1854. His work was welcomed by abbot Cyril Knapp, who considered the study of traits relevant to international trade that was jeopardizing the monastery’s finances. The monks raised sheep and were concerned about Australian wool imports encroaching on their Merino wool profit margin.

Mendel chose to study garden-variety peas instead of sheep because peas are easy to grow and come in many varieties, and pollination can be controlled.

Gregor Mendel’s Pea Plant Experiments

Between 1854 to 1856 Mendel cultivated and tested 28,000 to 29,000 pea plants. He used statistical models of probability when analyzing the transmission of observable traits. His exhaustive study included tests of 34 varieties of garden peas for trait consistency over several generations.

Mendel’s methodology consisted of crossing varieties of purebred (true breeding) pea plants, and planting the seeds to learn how traits are inherited in the first generation (F 1 ). Mendel recorded stem height, flower color, flower position on the stem, seed shape, pod shape, seed color and pod color. He noted that inherited “factors” (identified as alleles and genes today) were either dominant or recessive for certain traits.

When seeds from cross-pollinated F 1 plants grew, they produced a three-to-one ratio of dominant to recessive traits in the next generation (F 2 ).

Mendel’s findings were not consistent with the ideas of the time, including those of the famous evolutionary biologist Charles Darwin . Like most 19th-century scientists, Darwin thought traits blended, such as a red flower pollinating with a white flower producing pink flowers. Although Darwin noted a a three-to-one ratio of dominant and recessive traits in snapdragons, he didn’t understand the significance.

Ronald Fisher vs. Gregor Mendel: Facts

Statistician Ronald Fisher opined that Mendel’s data and statistical calculations were too perfect to be believable. Other scientists jumped into the fray alleging that research errors, along with Mendel’s conscious or unconscious bias, skewed results. For example, judging phenotypes such as whether a pea is round or wrinkled involves subjectivity.

However, defenders of Mendel’s legacy replicated experiments, ran their own calculations of statistical probability and concluded that Mendel’s findings were valid.

Renewed Interest in Gregor Mendel’s Discovery

In the 1900s, Mendel posthumously rose from obscurity to fame when Carl Correns , Hugo de Vries and Erich Tschermak independently published research findings consistent with Mendel’s results.

The extent to which any of the scientists were familiar with Mendel’s prior hybridization experiments is disputed. The studies corroborated Mendel’s discovery of dominant and recessive traits .

Mendel’s Writing and Scholarship

In addition to being a priest, teacher, gardener and researcher, Mendel was a scholarly writer and lecturer. He published papers describing crop damage by insects.

Mendel also gave lectures on his work at two meetings of the Natural History Society of Brünn in Moravia in 1865. He published his work, "Experiments in Plant Hybridization" in 1866 in Proceedings of the Natural History Society of Brünn .

Gregor Mendel’s Laws

Mendel’s research in a vegetable garden led to Mendel’s theory of heredity and two main findings: the law of segregation and the law of independent assortment .

According to the law of segregation , a pair of hereditary “factors” (alleles) for a given trait separate when haploid eggs and sperm cells form. A fertilized egg has two copies of each allele; one copy inherited from the mother and one copy from the father.

The law of independent assortment states that segregation of an allele pair is generally independent of the actions of other genes, with the exception of linked genes.

Mendel’s insights into the laws of inheritance had little impact initially and were cited about three times over the next 35 years. Mendel died before his contributions to genetics were understood.

The discovery of the deoxyribonucleic acid (DNA) molecule at King's College in London led to advances in genetics, medicine and biotechnology. Geneticists were finally able to identify the vaguely understood hereditary "factors" inferred by Mendel.

Non-Mendelian Genetics

Gregor Mendel's principles of genetics apply to characteristics controlled by a dominant or recessive gene. In the case of pea plants, each of the investigated traits like stem height was determined by one gene with two potential alleles.

Inherited pairs of alleles were either dominant or recessive, and no blending occurred. For instance, the crossing of a tall stem plant with a short stem plant didn’t result in a plant stem of average height.

Non-Mendelian genetics explain more complicated patterns of inheritance. Codominance occurs when both alleles exert their influence. Incomplete dominance happens when the dominant trait is slightly muted, such as pink instead of red coloring. Many types of alleles may be possible for a given trait.

Gregor Mendel’s Later Life

Mendel was promoted to abbot in 1868 and took over the administration of the monastery. He focused on these duties after this point and did not continue experimentation. Acquired data sat on a shelf, and his hand-written notes were burnt by his predecessor.

Mendel died of Bright disease, also known as nephritis, on January 6, 1884. He was remembered as a Catholic priest with a passion for gardening. Even those who admired his intellect and scientific rigor did not realize that their friend and colleague would become legendary in the distant future.

Gregor Mendel Quotes

Mendel’s experiments were motivated by his love of science. No one other than Mendel had an inkling that his work was groundbreaking. Despite his bouts with depression, Mendel remained optimistic that his contributions to science would one day be recognized. He often shared such thoughts with friends:

"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.”

“Even though I have experienced some dark hours during my life time, I am grateful that the beautiful hours have outweighed the dark ones by far.”

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About the Author

Dr. Mary Dowd studied biology in college where she worked as a lab assistant and tutored grateful students who didn't share her love of science. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages. She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university.

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Microbe Notes

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.

excellet ohhh

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Biography of Gregor Mendel, Father of Genetics

Well-Known for His Discovery of Dominant and Recessive Genes

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Gregor Mendel (July 20, 1822 - January 6, 1884), known as the Father of Genetics, is most well-known for his work with breeding and cultivating pea plants, using them to gather data about dominant and recessive genes.

Fast Facts: Gregor Mendel

Known For : Scientist, friar, and abbot of St. Thomas' Abbey who gained posthumous recognition as the founder of the modern science of genetics.

Also Known As : Johann Mendel

Born : July 20, 1822

Died : January 6, 1884

Education : University of Olomouc, University of Vienna

Early Life and Education

Johann Mendel was born in 1822 in the Austrian Empire to Anton Mendel and Rosine Schwirtlich. He was the only boy in the family and worked on the family farm with his older sister Veronica and his younger sister Theresia. Mendel took an interest in gardening and beekeeping as he grew up.

As a young boy, Mendel attended school in Opava. He went on to the University of Olomouc after graduating, where he studied many disciplines, including physics and philosophy . He attended the University from 1840 to 1843 and was forced to take a year off due to illness. In 1843, he followed his calling into the priesthood and entered the Augustinian Abbey of St. Thomas in Brno.

Personal Life

Upon entering the Abbey, Johann took the first name Gregor as a symbol of his religious life. He was sent to study at the University of Vienna in 1851 and returned to the abbey as a teacher of physics. Gregor also cared for the garden and had a set of bees on the abbey grounds. In 1867, Mendel was made an abbot of the abbey.

Gregor Mendel is best known for his work with his pea plants in the abbey gardens. He spent about seven years planting, breeding and cultivating pea plants in an experimental part of the abbey garden that was started by the previous abbot. Through meticulous record-keeping, Mendel's experiments with pea plants became the basis for modern genetics .

Mendel chose pea plants as his experimental plant for many reasons. First of all, pea plants take very little outside care and grow quickly. They also have both male and female reproductive parts, so they can either cross-pollinate or self-pollinate. Perhaps most importantly, pea plants seem to show one of only two variations of many characteristics. This made the data much more clear-cut and easier to work with.

Mendel's first experiments focused on one trait at a time, and on gathering data on the variations present for several generations. These were called monohybrid experiments. He studied a total of seven characteristics. His findings showed that there were some variations that were more likely to show up over the other variations. When he bred purebred peas of differing variations, he found that in the next generation of pea plants one of the variations disappeared. When that generation was left to self-pollinate, the next generation showed a 3 to 1 ratio of the variations. He called the one that seemed to be missing from the first filial generation "recessive" and the other "dominant," since it seemed to hide the other characteristic.

These observations led Mendel to the law of segregation . He proposed that each characteristic was controlled by two alleles, one from the "mother" and one from the "father" plant. The offspring would show the variation it is coded for by the dominance of the alleles. If there is no dominant allele present, then the offspring shows the characteristic of the recessive allele. These alleles are passed down randomly during fertilization.

Link to Evolution

Mendel's work wasn't truly appreciated until the 1900s, long after his death. Mendel had unknowingly provided the Theory of Evolution with a mechanism for the passing down of traits during natural selection . As a man of strong religious conviction, Mendel did not believe in evolution during his life. However, his work has been added together with that of Charles Darwin's to make up the modern synthesis of the Theory of Evolution. Much of Mendel's early work in genetics has paved the way for modern scientists working in the field of microevolution.

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• Review Article
• Published: 12 April 2022

Demystifying the mythical Mendel: a biographical review

• Daniel J. Fairbanks   ORCID: orcid.org/0000-0001-7422-0549 1  

Heredity volume  129 ,  pages 4–11 ( 2022 ) Cite this article

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• Plant sciences

Gregor Mendel is widely recognised as the founder of genetics. His experiments led him to devise an enduring theory, often distilled into what are now known as the principles of segregation and independent assortment. Although he clearly articulated these principles, his theory is considerably richer, encompassing the nature of fertilisation, the role of hybridisation in evolution, and aspects often considered as exceptions or extensions, such as pleiotropy, incomplete dominance, and epistasis. In an admirable attempt to formulate a more expansive theory, he researched hybridisation in at least twenty plant genera, intentionally choosing some species whose inheritance he knew would deviate from the patterns he observed in the garden pea ( Pisum sativum ). Regrettably, he published the results of only a few of these additional experiments; evidence of them is largely confined to letters he wrote to Carl von Nägeli. Because most original documentation is lost or destroyed, scholars have attempted to reconstruct his history and achievements from fragmentary evidence, a situation that has led to unfortunate omissions, errors, and speculations. These range from historical uncertainties, such as what motivated his experiments, to unfounded suppositions regarding his discoveries, including assertions that he never articulated the principles ascribed to him, staunchly opposed Darwinism, fictitiously recounted experiments, and falsified data to better accord with his theory. In this review, I have integrated historical and scientific evidence within a biographical framework to dispel misconceptions and provide a clearer and more complete view of who Mendel was and what he accomplished.

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Introduction.

The year 2022 marks the bicentennial of Gregor Mendel’s birth. He rose from an impoverished childhood in a small village to become a successful teacher, scientist, priest, and ultimately prelate and abbot. His discoveries and interpretations of elegant symmetrical patterns of inheritance in the garden pea ( Pisum sativum L.) led him to develop a theory of inheritance that has endured with little change. No one, including Mendel, recognised the importance of his theory in his day; it languished mostly unnoticed until its dramatic rediscovery founded the science of genetics at the beginning of the twentieth century. The fragmentary evidence of Mendel’s history has left much room for speculation and conjecture. Inevitably, misunderstandings, myths, omissions, and rumours have become part of popular and scholarly accounts of his accomplishments and history. Some mysteries may never be resolved due to the absence of sufficient evidence. In this review, I examine within a biographical framework the scientific and historical evidence to clarify some of the most important Mendelian misconceptions.

Mendel’s youth and early education

Johann Mendel was born in July 1822 in the village of Heinzendorf (Hynčice) Footnote 1 in Austrian Silesia (currently in the Czech Republic); his parents were Rosina Schwirtlich and Anton Mendel. When he became a friar in 1843, he took on the monastic name Gregor. The first of several historical misconceptions is the day of his birth, disputed as July 20 or 22. Some authors, mostly in popular online biographies, have attempted to resolve this discrepancy by speculating that he was born on July 20 and baptised on July 22. However, the evidence contradicts this presumption. The parish birth register lists the date of his birth and baptism as July 20 (Moravian Museum 1965 ). After pointing out several discrepancies in the birth register, Klein and Klein ( 2013 ) noted, “Another peculiarity of the register is that all seven children born in 1822 were baptized on the day of their birth” (p. 123), suggesting that the dates may be incorrect because, at the time, infants were rarely born and baptised on the same day. Mendel himself consistently listed his birthdate as July 22 on all known documents. His nephew, Alois Schindler, wrote that his uncle Gregor and his mother Theresia insisted that the correct birthdate was July 22, the feastday of St. Mary Magdalene. Schindler further reasoned, “Perhaps the parish dates were recorded belatedly and incorrectly” (translated from the original 1902 German in Kříženecký 1965 , p. 80).

Although writers often state that Johann had two siblings, probably based on Iltis ( 1924 , 1966 ), in fact he was the second of five children in the family. Two of his sisters, Veronika and Theresia, lived to adulthood. Two other sisters, both named Rosina, died as children, one as a toddler the other as an infant (Klein and Klein 2013 ). Theresia lived to see her late brother Gregor attain fame as the founder of genetics in the early twentieth century. She provided some of the most important information of his early history based on her recollections and documents she retained. Two of her sons, Alois and Ferdinand Schindler, recorded her reminiscences along with their own (Kříženecký 1965 ).

Johann and his sisters attended classes in a small schoolhouse a short walk from their home. His teachers arranged for him to attend boarding school for gifted children in Leipnik (Lipník) where he studied for one academic year (1833–34). He received admission to the Troppau (Opava) Gymnasium where he would continue his schooling for six academic years, graduating in 1840. He then attended the Olmütz (Olomouc) Philosophical Institute, graduating in 1843. Once while he was in Troppau and again while he was in Olmütz, he suffered episodes of nervous illness so severe that he had to retreat home for months to recuperate, the second time costing him a year of his schooling. Despite these prolonged illnesses, his performance was outstanding at all three schools.

In the time preceding the summer of 1841, he faced a pivotal decision. His father, crippled by an accident three years earlier, could no longer manage the farm. Johann, now at his nineteenth birthday, had to decide whether to take over the family farm or continue his education. A document from the time makes it clear that by the end of that summer he had decided to enter the priesthood. Respecting that decision, his father sold the farmstead to Alois Sturm, Veronika’s husband, with the following provision: “The purchaser shall pay to the son of the seller, Johann by name, if the latter as he now designs should enter the priesthood, or should he in any other way begin to earn an independent livelihood, the sum of 100 fl. … and shall also defray all expenses connected with the first mass” (Iltis 1966 , p. 39). Johann’s physics professor in Olmütz, Friedrich Franz, highly recommended him for admission to the Augustinian order in the St. Thomas monastery in Brünn (Brno), the capital of Moravia. Mendel was officially admitted to the order on October 9, 1843 (Iltis 1966 , p. 43).

Friar, scientist, and teacher

The group of Augustinians Mendel joined in October 1843 was extraordinary. Although often identified as a monk, he was a friar, which is an important distinction. The mendicant orders, including Augustinians, consist of friars in that their members openly serve the community, leading much less cloistered lives than traditional monks. Several of the St. Thomas friars were highly educated, serving as teachers and professors, conducting scholarly research in the sciences, arts, and humanities, and holding prestigious administrative positions in commerce and academic societies. They were especially dedicated to secular academic teaching and research, a situation that often placed them in conflict with their ecclesiastical superiors beyond the monastery.

The abbot, Cyrill Franz Napp, was a highly respected scholar characterised by a fellow friar as “a famous prelate, scientist, secret freethinker, and patriot, and expert in state affairs and economy” (Matalová 1973 , p. 252). Prior to his abbacy, Napp taught at the Brünn Theological Institute. The monastery had large agricultural holdings, and Napp was committed to implementing scientific advances in agriculture. He was an influential member of the Moravian-Silesian Agricultural Society, especially in the society’s sheep breeding and pomological associations. With Napp’s encouragement, Mendel took classes in scientific agriculture at the Brünn Philosophical Institute and was elected to membership in the Agricultural Society in 1851 (Matalová and Matalová 2022 ).

Mendel’s close friend and mentor during these early years was his fellow friar Matouš František Klácel, a philosopher specialising in the writings of Georg Wilhelm Friedrich Hegel and a self-described freethinker who was constantly at odds with church authorities beyond the monastery. Shortly after Mendel arrived, Bishop Anton Ernst Schaffgotsch dismissed Klácel from his teaching position at the Brünn Theological Institute for teaching “pantheism and other heresies related to Hegelianism” (Peaslee and Orel 2007 , p. 152).

Revolutionary sentiment swept much of Europe in 1848 and was especially forceful in Vienna, spilling over into Brünn. The St. Thomas friars supported revolutionary reforms, with Napp’s enthusiastic encouragement. Klácel seized the opportunity to compose a petition demanding greater freedom for friars from religious duties, allowing them to devote themselves more fully to secular research and teaching. The wording of the petition was scathing, its content overflowing with hyperbole. It concluded, “the undersigned professors and pastoral workers in the Order of St. Augustine in Altbrünn take the liberty of appealing to the imperial parliament to grant them constitutional civil rights , and request to be allowed to devote their entire efforts, according to their abilities and past services, to public teaching institutions and to free, united, and indivisible citizenship … [and] make it respectfully their missions to promote science and humanity…” ( underlining in the original , Klein and Klein 2013 , p. 281). Mendel was one of six friars who signed the petition.

Several premature deaths in the 1840s created a shortage of parish priests, leading Napp to recommend Mendel’s ordination at the earliest possible date. Napp assigned Mendel to serve as a parish priest but soon discovered that he was poorly suited to this role. In a letter, Napp informed Schaffgotsch that he had relieved Mendel of his ecclesiastical duties because he was “much less fitted for work as a parish priest, the reason being that he is seized with an unconquerable timidity when he has to visit a sick-bed or to see anyone ill and in pain” (Iltis 1966 , p. 58). Napp, as administrator over Moravian schools, arranged for Mendel to instead assume a teaching position at the Znaim (Znojmo) Gymnasium, southwest of Brünn.

Mendel immediately proved to be an exemplary teacher, loved by his students, and praised by his colleagues. A newly implemented law, however, required that teachers be certified through a gruelling series of examinations. Accordingly, Mendel applied in 1850 to be certified in physics and natural history. He received the first part of the examination, a homework portion that he was to complete by writing two essays in response to questions, one on physics and the other on natural history. His essay on natural history contains his first known allusion to evolution, a part of which reads, “The vegetable and animal life developed more and more richly; its oldest forms disappeared in part to make way for new and more perfect ones” (Fairbanks 2020 ).

The examiner for physics, Andreas von Baumgartner, found Mendel’s essay on this topic to be informed and well written. However, Rudolf Kner, the examiner for natural history, determined that Mendel’s essay on this subject was deficient. Both examiners, however, recommended him for the next part known as the Klausurprüfung , an on-site written examination in a locked room at the University of Vienna with no access to resources. Mendel’s written answers this time were less than favourable. His examiners, nonetheless, allowed him to proceed to the viva voce (oral) portion. Here he faced a commission, among them the famed physicist Christian Doppler, after whom the Doppler effect is named. His physics examiners evaluated him as “unqualified to teach physics….” and Kner wrote that “he is not yet competent to become a teacher” (Iltis 1966 , p. 72). The written report languished in bureaucracy as it bounced from one administrative office to another, finally reaching Napp and Mendel in August 1851, almost a year after the examination in Vienna.

The University of Vienna and the motivation for Mendel’s experiments

By the time the examination report finally arrived, Napp was already arranging for Mendel to study at the University of Vienna in preparation for a teaching career. The wheels of bureaucracy again turned slowly, and when Mendel finally departed for Vienna, he was five weeks late for the beginning of the 1851 fall term. Serendipitously, due to delays in renovation of the physics laboratory, the experimental physics course began at the same time as Mendel’s arrival. This was his only course that fall term, and it was influential, taught by Doppler to thirteen students. For the 1852 spring term, Mendel again enrolled in Doppler’s course, with additional courses in other subjects. Doppler departed that summer for Italy to recuperate from an illness and died soon thereafter, so Mendel was one of his last students. Although Mendel was originally scheduled to spend a year at university, he remained for almost two years, taking advanced courses in physics, mathematics, chemistry, botany, zoology, and palaeontology, and assisting with entomological research in an extracurricular setting.

Some have argued that Mendel was a staunch anti-evolutionist and adherent of the doctrine of special creation (Callender 1988 ; Bishop 1996 ). There is ample evidence, however, to contradict these views, beginning with Mendel’s studies at the University of Vienna. Pre-Darwinian evolutionary theory was prominent at the time, and Mendel studied it in courses on botany, zoology, and palaeontology. One of his most influential professors was Franz Unger, a botanist and palaeontologist. Unger popularised evolution for the public through a series of newspaper articles later compiled as a book (Unger 1852 ). He also published a popular book with hand-tinted lithographs of geological periods dating from the present to hundreds of millions of years ago (Unger 1851 ). Unger’s conception of evolution was remarkably like Darwin’s, even though Origin of Species was still eight years from publication. Gliboff ( 1998 ) thoroughly reviewed Unger’s evolutionary theory, titling it the “theory of universal common descent” (p. 223). Unger’s development of this theory reached its peak while Mendel was studying with him in Vienna.

At the time Mendel was attending Unger’s lectures, he witnessed first-hand a series of anti-evolutionary attacks pitting Catholicism against evolution. Sebastian Brunner was a prominent Catholic priest, a prolific author and orator, purveyor of religious orthodoxy, and anti-Semite, known by the epithet Malleus episcoporum , the bishop’s hammer (Gliboff 1998 ). Brunner publicly singled out Unger in his attacks, which began two days before Mendel’s arrival in Vienna in October of 1851. These attacks persisted unabated until the spring of 1856, approximately a year and a half after Mendel had returned to the monastery. Brunner named Unger in a newspaper headline as “Isis Priest and Philistine” and in another article as “a man who openly denied the creation and the Creator” (Olby 1985 , pp. 202–203). In his most sarcastic article, Brunner wrote that Vienna’s botanists “do everything they can to make themselves into plants of botanical learning that can be smelt from afar—and place themselves voluntarily into the eternally stinking dung-bed of the pantheistic world view, which nevertheless fosters a certain richness of blossoms” (Fairbanks 2020 , p. 265).

By the time Brunner wrote these words, Mendel had been officially appointed as one of these Viennese botanists. In 1853, his professors and colleagues elected him to full membership in the Imperial-Royal Zoological-Botanical Society in Vienna. Some have erroneously surmised that Mendel’s classic 1866 paper was his first scientific publication when, in fact, it was the third of eight (Mendel 1853 , 1854 , 1866 , 1870 , 1871 , 1879a , 1879b , 1882 ). Much of his focus was on physics, which led him to pursue meteorology as one of his principal research activities throughout the remainder of his life. If his published compilations of meteorological data are added to the list, the number of his scientific journal publications totals fourteen. Mendel presented a scientific paper to the Imperial-Royal Zoological-Botanical Society in Vienna in 1853 on lepidopteran predation in radishes. This paper became his first scientific publication when it appeared in the society’s journal (Mendel 1853 ). In 1854, he submitted another paper based on microscopic examination of the pea weevil and its infestation of pea seeds, which Vincenz Kollar, one of his professors, presented to the society in Mendel’s absence. It too was published in the society’s journal (Mendel 1854 ).

Mendel returned from the University of Vienna to the monastery in the summer of 1853. By then, Pope Pius IX had issued an edict that Austrian monasteries be investigated for secularism and neglect of religious piety. Cardinal Schwartzenberg in Prague appointed Bishop Schaffgotsch in Brünn to investigate the St. Thomas monastery. The investigation concluded with a formal visitation in early June 1854. At the time, Mendel had recently accepted a teaching appointment at the Realschule, a school focused on training students in their adolescent years in science, mathematics, and technical subjects. This teaching assignment prompted Schaffgotsch to accuse Mendel of studying “profane sciences at a worldly establishment in Vienna at the expense of the monastery to become a professor of said sciences at a state institution” (Klein and Klein 2013 , p. 295). At the conclusion of his report, Schaffgotsch recommended dissolution of the order, determining that “any hopes that the spirit could be exorcized and the order returned to a conscientious observance of its rules and constitutions must be given up” (Klein and Klein 2013 , p. 295). The report made its way to the Vatican. Although no actions were taken, and Mendel’s monasterial community remained intact, the friars lived under a cloud knowing that dissolution could be imminent.

This threat coincided with Mendel’s earliest known pea experiments (Mendel 1854 , 1866 ; Stern and Sherwood 1966 ; Orel 1996 ; Klein and Klein 2013 ). There is little evidence, however, to indicate the extent to which this threat had any influence on his experimental approach. Some have speculated that this and later threats from ecclesiastical superiors led Mendel to carefully avoid naming controversial evolutionary biologists, such as Darwin and Unger, in his printed publications, but nonetheless showing how his research contributed to evolutionary theory (Klein and Klein 2013 ; Fairbanks 2020 ). Mendel more overtly expressed his Darwinian views in his private correspondence than in his published writings (Iltis 1966 ; Fairbanks 2020 ).

In 1855, Mendel arranged to retake his teacher certification examination. He completed the homework portion at an unknown date then during the first week in May 1856 he travelled to Vienna for the on-site written and oral portions. Fragmentary accounts of what transpired have provoked exaggerated myths regarding Mendel and his motivations for his famous experiments.

In the early part of the twentieth century, Hugo Iltis ( 1924 , 1966 ) interviewed one of Mendel’s school colleagues who recalled that when Mendel returned from the examination, he was “very much out of humour” because “he had a very sharp difference of opinion with the examiner in botany, and had stubbornly maintained his own point of view” (Iltis 1966 , p. 95). This account has morphed into the notion that the unnamed examiner was Eduard Fenzl, one of Mendel’s botany professors. Mendel purportedly insisted during the examination that heredity was biparental whereas Fenzl authoritatively proclaimed that it was purely paternal, the female parent serving merely as a nurse to the pollen (Wunderlich 1982 ; Olby 1985 ; Orel 1996 ; Klein and Klein 2013 ). According to Iltis ( 1966 ), Mendel’s school colleague believed that “this dispute with the examiner led Mendel to begin his experiments” (p. 95).

A letter from Klácel, written immediately after Mendel’s return from the fateful examination, provides a contemporary and much more accurate account of what transpired:

Although he [Mendel] drew easy questions, he fell ill during the first Klausurprüfung and as a consequence was unable to write. He seems to have problems with his nerves generally since he endured several such insidious attacks already and they say that in his youth he suffered from epilepsy. The day passed and nothing was achieved. One has to feel sorry for him, since his homework etc. was graded as excellent. But formalities are formalities; in this case it was not possible to continue. Afraid that further attacks might continue, he returned home without accomplishing anything. (Klein and Klein 2013 , p. 364)

This account makes it clear that Mendel had performed well in the homework portion, but he experienced yet another nervous attack early during the Klausurprüfung (locked-room, written portion) and “was unable to write”. Because he abandoned the examination before the oral portion, he could not have confronted Fenzl. Mendel then rescheduled the examination for August but there is no record that he travelled to Vienna for it.

Further evidence shows that the abandoned examination could not have motivated Mendel’s experiments. Although he began his pea hybridisations that same spring in 1856, he probably planted the parental varieties at least a month earlier. Importantly, he already had his experiments in mind two years earlier, having conducted essential preliminary experiments with the commercial pea varieties during the summers of 1854 and 1855 to ensure that they were true-breeding and to determine which of them were most suitable for his hybridisation experiments.

Although anachronisms dispel the notion that the abandoned examination motivated Mendel’s experiments, an earlier dispute between Unger and Fenzl may have played a role (Olby 1985 ). Cell theory was a rapidly developing discipline at the time, and Unger and Fenzl were two of its leading researchers. They debated the nature of fertilisation, based in part on their interpretations of competing hypotheses of Matthias Jakob Schleiden and Giovanni Battista Amici (Olby 1985 ; Orel 1996 ; Klein and Klein 2013 ). Mendel was undoubtedly familiar with the Unger-Fenzl dispute long before this examination. Several aspects of his experimental design directly addressed this dispute and conclusively resolved it.

Mendel’s experiments and theory

Mendel carried out his hybridisation experiments over eight years (1856–63), then presented them as two lectures in 1865 and published them in his classic paper the following year (Mendel 1866 ). Two recent English translations are freely available online, one by Abbott and Fairbanks ( 2016 ) and the other by Müller-Wille and Hall (Mendel 2016 ). My focus here is on misconceptions, myths, controversies, and omissions shrouding his experiments, discoveries, and theory.

One of Mendel’s most important contributions, often omitted from accounts in textbooks and articles, is his definitive resolution of the Unger-Fenzl dispute. At the time, competing hypotheses regarding fertilisation and inheritance included strict uniparental inheritance, some form of unequal biparental inheritance, or strict biparental equality. Mendel’s definitive resolution of the issue in terms of cell theory is evident in a passage that Sekerák ( 2017 ) highlighted as the place where “Mendel reveals the generally valid essence of the reproduction of living organisms” (p. 65). Here Mendel concluded that “one germ cell and one pollen cell unite into a single cell that is able to develop into an independent organism through the uptake of matter and the formation of new cells. This development takes place according to a constant law that is founded in the material nature and arrangement of the elements” (Abbott and Fairbanks 2016 , p. 420).

To the term “single cell” in this passage, Mendel appended a footnote that unambiguously addressed the dispute between Unger and Fenzl, albeit without naming either:

With Pisum it is shown without doubt that there must be a complete union of the elements of both fertilising cells for the formation of the new embryo. How could one otherwise explain that among the progeny of hybrids both original forms reappear in equal number and with all their peculiarities? If the influence of the germ cell on the pollen cell were only external, if it were given only the role of a nurse, then the result of every artificial fertilisation could be only that the developed hybrid was exclusively like the pollen plant or was very similar to it. In no manner have experiments until now confirmed that. Fundamental evidence for the complete union of the contents of both cells lies in the universally confirmed experience that it is unimportant for the form of the hybrid which of the original forms was the seed or the pollen plant. (Abbott and Fairbanks 2016 , p. 420)

A few years later, in 1869, while reading the chapter on pangenesis in a German translation of Darwin’s Variation of Animals and Plants Under Domestication (Darwin 1868b ), Mendel encountered Darwin’s supposition that fertilisation of a single germ cell requires more than one pollen grain. Mendel annotated a passage (Fairbanks 2020 ), which reads in Darwin’s original English:

The pollen grains of Mirabilis are extraordinarily large, and the ovarium contains only a single ovule; and these circumstances led Naudin to make the following interesting experiments: a flower was fertilised by three grains and succeeded perfectly; twelve flowers were fertilised by two grains, and seventeen flowers by a single grain, and of these one flower alone in each lot perfected its seed; and it deserves especial notice that the plants produced by these two seeds never attained their proper dimensions, and bore flowers of remarkably small size. (Darwin 1868a , p. 364)

This passage compelled Mendel to carry out an experiment, the importance of which is evident in his description of it in an 1870 letter to Carl von Nägeli:

But one experiment seemed to me to be so important that I could not bring myself to postpone it to some later date. It concerns the opinion of Naudin and Darwin that a single pollen grain does not suffice for fertilization of the ovule. I used Mirabilis jalappa for an experimental plant, as Naudin had done; the result of my experiment, however, is completely different. From fertilization with single pollen grains, I obtained 18 well developed seeds, and from these an equal number of plants, of which 10 are already in bloom. … According to Naudin, at least three [pollen grains] are needed! (Stern and Sherwood 1966 , pp. 92–93)

Later observations by microscopists solidified the fundamental concept that two gametes unite at fertilisation to form a zygote. Rarely, however, is Mendel credited with the definitive experimental confirmation of this concept, or the fact that he viewed this discovery as one of his most important achievements.

Of the many misunderstandings and myths obscuring Mendel’s experimental approach are assertions that his description of his experiments was fictitious, that he never articulated the laws of segregation and independent assortment, and that his data were falsified to more closely approximate expectation. Moreover, some phenomena Mendel addressed in his paper are not attributed to him, instead considered as extensions or exceptions to his laws. I will briefly address these issues here. For extensive reviews of them, see Sapp ( 1990 ), Hartl and Orel ( 1992 ), Orel ( 1996 ), Fairbanks and Rytting ( 2001 ), Westerlund and Fairbanks ( 2004 ), Hartl and Fairbanks ( 2007 ), and Franklin et al. ( 2008 ).

The claim that Mendel’s description of his experiments was fictitious dates to Bateson ( 1902 ), who speculated that “it is very unlikely that Mendel could have had seven pairs of varieties such that the members of each pair differed from each other in only one considerable character” (p. 59). Fisher ( 1936 ) quoted Bateson’s claim and dismissed it: “there can, I believe, be no doubt whatever that his report is to be taken entirely literally, and that his experiments were carried out in just the way and in much the order that they are recounted” (p. 132). Corcos and Monaghan ( 1984 ) resurrected Bateson’s claim, then di Trocchio ( 1991 ) amplified it, proposing that Mendel hybridised the 22 parental pea varieties he had chosen as parents in all possible combinations then disaggregated the data into fictitious experiments to make his presentations more understandable. Such assertions, however, directly contradict the words Mendel chose to succinctly describe his monohybrid experiments: “[parental] plants were used that differed in only one essential character” (Abbott and Fairbanks 2016 , p. 412). After examining published characteristics of nineteenth century pea varieties, Fairbanks and Rytting ( 2001 ) determined that “the nature of variation in pea varieties (both old and modern) facilitates, rather than prevents, the construction of monohybrid experiments” (p. 744) and “Mendel’s account describes a well-conceived experimental design that would not have been difficult for him to perform” (p. 745).

Claims that Mendel did not conceive the laws of segregation and independent assortment date at least to Callender ( 1988 ) who referred to “the myth of ‘Mendel’s Law of Segregation’; a law not to be found in either of Mendel’s papers, nor in his scientific correspondence, nor in any statement that can be unambiguously attributed to him” (pp. 41–42), and Monaghan and Corcos ( 1990 ) who contended that “the traditional Mendelian laws of segregation and independent assortment are not given in the paper” (p. 268). Although Mendel did not directly articulate segregation and independent assortment as distinct and separate laws, they are evident in the theory he derived as a “constant law that is founded in the material nature and arrangement of the elements” (Abbott and Fairbanks 2016, p. 420). In a passage appearing shortly after introducing this theory, he lucidly articulated what we can now phrase in modern terms as the pairing of differing alleles of a gene in heterozygotes and their segregation during meiosis:

In relation to those hybrids whose progeny are variable, one might perhaps assume that there is an intervention between the differing elements of the germ and pollen cells so that the formation of a cell as the foundation of the hybrid becomes possible; however, the counterbalance of opposing elements is only temporary and does not extend beyond the life of the hybrid plant. Because no changes are perceptible in the general appearance of the plant throughout the vegetative period, we must further infer that the differing elements succeed in emerging from their compulsory association only during development of the reproductive cells. In the formation of these cells, all existing elements act in a completely free and uniform arrangement in which only the differing ones reciprocally segregate themselves. In this manner the production of as many germ and pollen cells would be allowed as there are combinations of formative elements. (Abbott and Fairbanks 2016 , p. 420)

A key phrase in this passage is “reciprocally segregate themselves” from Mendel’s “ sich gegenseitig ausschliessen ”. This phrase was translated by Müller-Wille and Hall (Mendel 2016 ) as “mutually exclude each other” (p. 42), by Stern and Sherwood ( 1966 ) as “separate from each other” (p. 43), and by Druery and Bateson (Bateson 1902 ) as “mutually separate themselves” (p. 89). Mendel’s explanation of “differing elements” paired in “compulsory association” that “reciprocally segregate themselves” “only during the development of the reproductive cells” clearly reflects the modern concept of paired allelic segregation during meiosis.

Independent assortment, implied by Mendel in the last sentence of this passage, is more fully clarified in other passages, such as the following: “the behaviour of each pair of differing characters in hybrid union is independent of the other differences between the two original plants and, further, that the hybrid produces as many types of germ and pollen cells as there are possible constant combination forms” (Abbott and Fairbanks 2016 , p. 421).

Aspects that Mendel included in his paper, often stated as extensions or exceptions to his laws, include pleiotropy, incomplete dominance, and epistasis. He described a case of pleiotropy for seed coat colour, flower colour, and axillary pigmentation as follows: “The difference in the colour of the seed coat … is either coloured white, a character consistently associated with white flower colour, or it is grey, grey-brown, or leather brown with or without violet spots, in which case the colour of the standard petal appears violet, that of the wings purple, and the stem at the base of the leaf axils is tinged reddish” (Abbott and Fairbanks 2016 , p. 408). As reviewed by Hartl and Fairbanks ( 2007 ), this pleiotropic association clarifies some perplexing questions about Mendel’s experimental design, such as his reason for choosing seed-coat colour as the third character in his trihybrid experiment.

Mendel’s comparison of full and incomplete dominance is evident in the following sentences:

The experiments conducted with ornamental plants in past years already produced evidence that hybrids, as a rule, do not represent the precise intermediate form between the original parents. With individual characters that are particularly noticeable, like those related to the form and size of the leaves and to the pubescence of the individual parts, the intermediate form is in fact almost always apparent; in other cases, however, one of the two original parental characters possesses such an overwhelming dominance that it is difficult or quite impossible to find the other in the hybrid. (Abbott and Fairbanks 2016 , p. 409)

Mendel’s inference of what is now known as epistasis is near the end of his paper in an experiment with flower colour in the common bean ( Phaseolus ). From an interspecific cross between P. nanus L. (with white flowers) and P. multifloris W. (with coloured flowers), he noted partial dominance for flower colour and reduced fertility in the F 1 hybrids. Of the 31 F 2 plants that flowered, one had white flowers, and 30 displayed varying shades of coloured flowers. He attempted to interpret this result in the context of what he had observed in Pisum , speculating that if two “independent characters” (as he put it) influenced flower colour, a 15:1 ratio is expected, whereas if three did so, a 63:1 ratio is expected. He astutely added the caveat, “It must not be forgotten, however, that the explanation proposed here is based only on a mere supposition that has no other support than the very imperfect result of the experiment just discussed” (Abbott and Fairbanks 2016 , p. 418). The ratios he proposed reflect what is now designated as recessive epistasis.

No Mendelian controversy has generated as much debate as the accusation that Mendel’s data were falsified to more closely approximate expectation. Weldon was the first to raise questions, privately writing to Pearson in 1901 that Mendel had “cooked his figures, but that he was substantially right” (Mangello 2004 , p. 23, italics in original). After applying Pearson’s newly developed chi-squared test to Mendel’s data, Weldon ( 1902 ) did not overtly claim in print that Mendel manipulated the data but dangled the possibility in several statements, one of which reads, “the odds against a result as good as this or better are 20 to 1” (p. 235). Fisher, probably influenced by Weldon’s paper, famously stated in a 1911 lecture, “It may just have been luck, or it may be that the worthy German abbot, in his ignorance of probable error, unconsciously placed doubtful plants on the side which favoured his hypothesis” (Norton and Pearson 1976 , p. 160). The controversy, now known as the Mendel-Fisher controversy, is based largely on an article by Fisher ( 1936 ) wherein he famously wrote, “the data of most, if not all, of the experiments have been falsified so as to agree closely with Mendel’s expectations” (p. 132).

This assertion is, in fact, less incriminatory than it may seem when viewed in context of Fisher’s overall paper. Fisher presumed that an assistant, rather than Mendel, must have manipulated the data, and he dedicated only a relatively small part of the paper to evidence of questionable data. Fisher’s admiration for Mendel is evident in the conclusion where he referred to Mendel’s paper as “experimental researches conclusive in their results, faultlessly lucid in presentation, and vital to the understanding not of one problem of current interest, but of many” (Fisher 1936 , p. 137).

After its publication, Fisher’s paper received little attention until the centennial of Mendel’s lectures in 1965 when the controversy began in earnest. It lasted for more than forty years in numerous articles and books whose authors drew a wide range of conclusions based on analyses examining essentially every conceivable aspect of Mendel’s experiments. Allan Franklin’s introductory essay in Franklin et al. ( 2008 ) is the most exhaustive and definitive review of the Mendel-Fisher controversy. After evaluating the complex statistical, historical, and botanical aspects of the many published analyses of Mendel’s data, Franklin concluded that “the experiments that had initially triggered Fisher’s suspicions can be explained without any fraud,” but “the issue of the ‘too-good-to-be-true’ aspect of Mendel’s data found by Fisher still stands”. Finally, he urged, “It is time to end the controversy” (Franklin et al., 2008 , p. 68). Fortunately, most scholars have heeded this plea.

Mendel and Darwin

Mendel became well acquainted with biological evolution from his university studies years before he learned of Darwin. Although Mendel and Darwin were contemporaries, it is unlikely that Mendel learned of Darwin until 1863, the final year of his Pisum experiments. Darwin published Origin of Species in 1859, the fourth year of Mendel’s experiments, but Mendel obtained his German translation of the book in 1863. It contains his hand annotations, published by Fairbanks and Rytting ( 2001 ) as an online supplement. By the time Mendel presented his lectures in 1865, Darwin’s Origin of Species was widely known and popular. In the January 1865 monthly meeting of the Natural Science Society in Brünn, Mendel’s friend and fellow teacher, Alexander Makowsky lectured on Origin of Species, addressing some of the same topics that Mendel addressed in the next two monthly meetings in February and March.

The existing evidence of Mendel’s acquaintance with Darwin’s theory and books, as well as Mendel’s statements referencing Darwin, strongly counter claims that Mendel was “in favor of the orthodox doctrine of special creation” (Bishop 1996 , p. 212) and “an opponent of descent with modification” (Callender 1988 , p. 41). The cumulative evidence suggests that Mendel had strong interest in Darwin’s writings and their relevance to his research, but that he did not become an avid promoter of Darwinism (Fairbanks 2020 ). Those who knew Mendel who lived into the twentieth century to share their recollections, independently confirmed this impression (Iltis 1966 ; Coleman 1967 ).

Although Mendel was thoroughly acquainted with Darwin’s writings, there is no evidence that Darwin knew anything about Mendel. A common rumour purports that Darwin owned an offprint of Mendel’s 1866 paper but that it was uncut. For example, Hennig ( 2000 ) wrote, “Another uncut reprint was found in the library of Charles Darwin, so Mendel must have sent him a copy, too” (p. 143). Despite several similar claims, there is no evidence that Darwin owned an offprint by Mendel (Lorenzano 2011 ). In fact, there is evidence to dispel the common notion that Mendel sent uncut offprints. The offprints contain several typesetter errors, which are hand-corrected in the same places and in the same manner in the offprints Mendel sent, evidence that Mendel made the corrections rather than later readers, which he could do only if the offprints were cut (Müller-Wille and Hall 2016 ; Fairbanks 2022 ).

Darwin owned two books with brief references to Mendel’s experiments. One is a book by Hoffmann ( 1869 ), which contains short and essentially uninformative references, not likely to lead Darwin to seek Mendel’s paper (Olby 1985 ). The other is a book by Focke ( 1881 ), published the year before Darwin’s death, which Darwin loaned to a friend. The pages in this book with references to Mendel remain uncut to this day, possibly the source of the rumour of uncut offprints (Lorenzano 2011 ).

Mendel’s subsequent experiments and letters to Nägeli

Mendel sent an offprint to Carl von Nägeli, a renowned botanist whom Unger often praised, on December 31, 1866 with a detailed accompanying letter. Fortunately, Nägeli retained Mendel’s letters, although at least one is missing, and a page from another may also be missing (van Dijk and Ellis 2016 ). Mendel’s letters to Nägeli provide important and detailed information of his research after 1866. Cautious about drawing sweeping conclusions, Mendel conducted hybridisation experiments in other plant species. These experiments were much more extensive than is often portrayed. Mendel recounted experiments with numerous plant genera, among them Hieracium , Circium , Geum , Linaria , Calceolaria , Zea , Ipomoea , Cheiranthus , Antirrhinum , Tropaeolum , Veronica , Viola , Potentilla , Carex , Verbascum , Mirabilis , Aquilegia , Lychnis , and Matthiola . The letters contain detailed results for several of these genera, especially Hieracium , Circium , Geum , Linaria , Verbascum , Mirabilis , Matthiola , and Zea . Mendel noted that the progeny from hybrids in Matthiola , Zea , and Mirabilis “behave exactly like those of Pisum ” (Stern and Sherwood 1966 , p. 93).

In his classic 1866 paper, Mendel classified hybrids into two types: those that produce variable progeny (as was the case with Pisum and Phaseolus ), and those that produce constant progeny, meaning that all the progeny uniformly and consistently retain the characters of the hybrid parent through repeated generations of self-fertilisation. In his experiments with other plant species, he intentionally included genera that he expected to be variable and others that he expected to be constant. For example, he wrote to Nägeli that Geum “belongs to the few known hybrids that produce nonvariable progeny as long as they remain self-pollinated” (Stern and Sherwood 1966 , pp. 58–59). By researching both types, Mendel hoped to develop a more expansive theory to explain inheritance and speciation in the progeny of hybrids.

Mendel’s choice to research Hieracium is often portrayed as disastrous, as is evident in the following excerpts: “the worst possible choice” (Sturtevant 1965 , p. 11), “shattered the hopes he had entertained of finding a confirmation” (Iltis 1966 , p. 174), “a completely misguided choice” (Hennig 2000 , p. 159), and “the results were a mess” (Mukherjee 2016 , p. 55). However, a detailed examination of Mendel’s Hieracium research in his letters to Nägeli, and in the paper he published on Hieracium (Mendel 1870 ), reveals extensive and productive research. Orel ( 1996 ) characterised Mendel’s choice as “in no way unfortunate”, and “a logical step forward” (p. 184). Disparagement of Mendel’s choice is based on the misguided presumption that all species of Hieracium reproduce exclusively through apomixis, seemingly ensuring uniparental-maternal inheritance and preventing artificial hybridisation. In fact, the genus Hieracium is extraordinarily diverse (one of the reasons Mendel chose it), and its reproductive mechanisms include varying degrees of apomixis, self-fertilisation, self-incompatibility, and cross-fertilisation, as well as a powerful influence of polyploidy on apomixis (Bicknell et al. 2016 ; Mráz and Zdvořák 2019 ; Underwood et al. 2022 ). Mendel’s accounts make it clear that he, like other researchers, obtained true Hieracium hybrids, albeit not without considerable effort. He speculated that the progeny of Hieracium hybrids might remain constant, as in Geum , but he was not initially sure. His decision to choose genera that he suspected would behave differently than Pisum is admirable; it was his intentional attempt to better understand the complexity of hybridisation in nature.

In his brief paper on Hieracium , Mendel ( 1870 ) determined that “we do not possess a complete theory of hybridisation and we may be led into erroneous conclusions if we take rules deduced from observations of certain other hybrids to be Laws of hybridisation and try to apply them to Hieracium without further consideration” (Stern and Sherwood 1966 , p. 52). Mendel observed that the F 1 hybrid plants obtained from apparently true-breeding parents tended to vary among themselves, but that their F 2 progeny from apparent self-fertilisation remained constant. He clearly stated the inevitable conclusion: “In Pisum the hybrids, obtained from the immediate crossing of two forms, have the same type, but their posterity, on the contrary, are variable and follow a definite law in their variations. In Hieracium according to the present experiments exactly the opposite phenomenon seems to be exhibited” (Stern and Sherwood 1966 , p. 55). He then noted that Hieracium was not the only genus to display such behaviour, citing the research of Wichura indicating that Salix behaved similarly.

Mendel’s observations were probably due to natural heterozygosity and polyploidy in the parental plants, which appeared to him to breed true due to apomixis. When he successfully hybridised them, the F 1 progeny displayed variability due to parental heterozygosity and possible variations in ploidy, then the F 2 progeny remained constant, resembling the original F 1 parents, due again to apomixis (Bicknell et al. 2016 ; Mráz and Zdvořák 2019 ). These observations revealed “exactly the opposite” of his observations in Pisum . The fact that he observed concordance with Pisum in several genera and a range of patterns in Hieracium and other genera neither surprised nor misled him. The only true misfortune is that he published only a fraction of what he had discovered.

Mendel’s abbacy and death

After Napp’s death, Mendel was elected abbot in 1868. This change in status did not initially deter him from research; his letters to Nägeli reveal extensive hybridisation research for the next five years (1868–73). However, in his last letter to Nägeli, Mendel lamented that “I am really unhappy about having to neglect my plants and my bees so completely” (Stern and Sherwood 1966 , p. 97). By then, a bitter dispute over monastery taxation was overwhelming him. He sent his Hieracium plants and herbarium specimens to Nägeli, essentially bringing his hybridisation research to a close.

Mendel died on January 6, 1884. Had he published the enormous data he collected on plant hybridisation, his work might have been more broadly known. Why he did not do so has been a matter of speculation. One of the young friars in the monastery, Prior Alphonsus Tkadlec, recalled years later that Mendel “was even attacked and his theory suspected of being contrary to the revealed truths of the Christian religion…. In bitterness he burned everything which reminded him of his previous activity” (Orel 1996 , p. 195). Mendel’s nephew, Ferdinand Schindler, provided a contradictory account: “He often said to us nephews, that we shall find at his heritage, papers for publication, that he could not publish in his life. But we did not receive anything from the cloister, not even a thing for remembrance” (Coleman 1967 , p. 10). Antonín Doupovec, who attended to the aging abbot with his mother, remembered, “thousands of sheets of paper covered with scientific notes and data were found after his death” (Orel, 1971 , p. 270). Another young friar, Pater Clemens Janetschek claimed that most of Mendel’s papers were burned after his death, only the bound books retained (Iltis 1966 , p. 281). It is fortunate that Nägeli and his heirs preserved Mendel’s letters. Otherwise, much of his extensive research after 1866 would have remained unknown.

Mendel’s classic 1866 paper remains one of the finest examples of the nature of science, a detailed and lucid presentation of extensive data exemplifying careful experimental design, hypothesis testing, and the development of an enduring theory of heredity. His paper, as the founding document for the science of genetics, is much enhanced when viewed in the context of his life, his choices, and those who influenced him at one the most extraordinary times in the history of science. In this review, I have attempted to demystify key events in his history and scientific approach to hopefully provide a clearer view of who he was and what he accomplished as we commemorate the bicentennial of his birth.

When naming cities and places, I have used the Anglicised name if it is available (for example Moravia instead of Mähren or Morava). Because many of the places associated with Mendel are now in the Czech Republic and bear Czech names, but were known by both their German and Czech names in his day, and he typically used their German names, I have included the German name first in each instance, followed by the Czech name in parentheses, and used only the German name for each subsequent use.

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Acknowledgements

Some of the historical information in this article is based on research for a recently completed book-length biography of Mendel (Fairbanks 2022 ). I am much indebted to Jiří Sekerák, Anna Matalová, Eva Matalová, and Eva Janečková for generously sharing documentary information and offering critique, and to Peter van Dijk for sharing recent discoveries on apomixis in Hieracium . I am grateful to Barbara Mable whose editorial recommendations substantially improved the manuscript, and to three anonymous referees for their helpful and constructive comments.

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Fairbanks, D.J. Demystifying the mythical Mendel: a biographical review. Heredity 129 , 4–11 (2022). https://doi.org/10.1038/s41437-022-00526-0

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