Gregor Mendel Discovers the Basic Laws of Heredity while Breeding Pea Plants (1866)

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Gregor Mendel Discovers the Basic Laws of Heredity while Breeding Pea Plants (1866)

Overview

In a monastery garden rather far removed from the rest of the scientific community, Gregor Mendel studied the transmission of physical characteristics from one generation of pea plants to the next, thereby deciphering the basic principles governing heredity. Mendel was not the first person to study heredity, but he was the first to carefully study the inheritance of traits with planned experiments, carefully recorded data, and statistical analysis of results. His quantitative approach allowed him to translate his findings into a coherent and reproducible theory of how traits are passed from one generation to the next. Mendel's contribution was not appreciated during his lifetime but became the foundation for our understanding of genetics in the twentieth century.

Background

Gregor Mendel (1822-1884) was not the first scientist to question how physical characteristics are transmitted from one generation to the next. Centuries before Mendel began breeding pea plants, humans grasped the idea of inheritance despite having no idea how it worked. Throughout history, inheritance of "familial" traits in humans has been important in social organization. That children often resemble parents or grandparents was noted far back in history. The protruding bottom lip of the Hapsburg family (ca. thirteenth century), for example, was a distinct physical characteristic that helped define members of this royal clan. Historically, inheritance played a large role in agriculture as well. Farmers learned that by crossbreeding and inbreeding animals (and plants) with different traits, they could improve on nature and create hybrids with desirable characteristics.

Recorded theories about inheritance date back to the days of Aristotle (384-322 b.c.). For example, Aristotle suggested that a mixture of semen from a male and menstrual fluid from a female combine to generate offspring. When Anton van Leeuwenhoek (1632-1723) developed the microscope and discovered sperm in 1677, people believed that they saw in the sperm cell a miniature person (a homunculus) that was ready to be incubated in the female womb. Others, called ovists, believed it was the egg that harbored the next generation. The concept of spontaneous generation, in which simple life forms could take shape from substances such as ooze and mud, was another very popular theory until Louis Pasteur (1822-1895) disproved it in 1864. The theory of inheritance of acquired characteristics, which states that changes (through use and disuse) in an organism's body that occur during its lifetime are passed to offspring, was incorporated into Jean Baptiste Lamarck's theory of evolution in 1809 and was widely accepted through most of the 1800s.

In 1856, when Mendel started breeding pea plants to try to understand heredity, he knew of some of these theories and was interested in them. He knew of gametes and fertilization, but mitosis, meiosis, chromosomes, genes, and DNA had yet to be discovered. Mendel also was well aware of the history of breeding plants and animals to create hybrids. He knew that hybrids from the same kinds of plants looked similar but that when those hybrids mated they could produce offspring with traits different from those of their parents.

Mendel had good scientific instinct. His approach to the study of heredity was new and different (and unappreciated during his lifetime). He was successful because he framed his question in a scientific manner, he chose a good model (the pea plant), and he analyzed his data quantitatively. The pea plant was easy to grow and it self-pollinated, which meant that he could control reproduction between individuals. He began by spending two years breeding more than thirty varieties of pea plants to make sure that they bred true (meaning that offspring had the same physical characteristics as the parents) and to define distinct physical characteristics. Mendel then chose seven traits for further study and carefully designed experiments in which he crossed parents with different traits. He began by crossing parents with different variations of a particular trait (e.g. height) and counting the number of offspring bearing each form (e.g. short and tall). He later crossed those offspring to get a second generation, again counting individuals that were tall or short. Later still, Mendel crossed parents with two and then three distinct traits.

A mathematical approach to biology was unprecedented in the 1800s, but Mendel took this approach and analyzed his data statistically. His analysis of the numbers of offspring with particular traits that came from crosses of parents with particular traits illustrated that the results of breeding could be predicted based on mathematical probabilities. Based on his data, Mendel deciphered what came to be called Mendel's law of segregation: hereditary units that determine a particular trait occur in pairs that separate during gamete formation so that a gamete receives half of the pair. This means that two units (now known to be genes), one from each parent, combine to determine which form of a trait an organism will have, and the factors can be distributed in different ways in each generation. These factors are discrete (meaning that they do not blend), and one factor is dominant over (or masks) the other. Mendel's examination of multiple traits led to the law of independent assortment, which says that members of each pair of genes are distributed independently when gametes are formed.

In the twentieth century scientists have questioned Mendel's methods because his data seemed to match statistical probabilities too closely. Historians of science wonder if, perhaps, Mendel already knew what to expect prior to actually analyzing the data or if he already had his theories in mind when he did his experiments. Regardless of his methods, Mendel's pea plants and the write-up of his results form the foundation for modern genetics.

Impact

Mendel's discoveries had virtually no impact on the scientific community or on life in general in the nineteenth century. He reported his results at a meeting of the Brünn Natural History Society in 1865, but no one was interested in his work. He published the results in a manuscript, titled Experiments with Plant Hybrids, in the Proceedings of the Brünn Natural History Society in 1866. The journal was distributed to 120 societies and libraries throughout Europe and America, but it was cited in only four papers before it was found and recognized as an explanation for inheritance in 1900.

During the years between Mendel's publication and its rediscovery in 1900, several others proposed theories to explain inheritance (although none have stood the test of time). For example, Charles Darwin (1809-1882) developed the theory of pangenesis to explain how variation arises in a population. He proposed that each tissue in the adult buds off particles (gemmules or pangenes) that concentrate in the reproductive organs; thus, an acquired trait (such as enlarged muscle resulting from weightlifting) could be passed to offspring because it left a lasting imprint on the cells of the body. Like Lamarck, Darwin thought that use and disuse would affect heredity and provide the source of variation on which natural selection would act. Likewise, Ernst Haeckel (1834-1919) proposed that the basic units of living matter have a memory. Blending inheritance, in which the information for a trait donated by each parent would blend together to create an intermediate trait, was another popular theory. August Weismann (1834-1914), on the other hand, rejected pangenesis and instead proposed "continuity of the germplasm" (gameteproducing cells) in which he recognized the importance of gametes and the cell nucleus in heredity. He believed that the body was simply a host for the germ cells; this theory explained how cells could be reproduced generation after generation and remain unchanged, thus supporting the popular notion of the fixity of species.

While these theories were being disputed in scientific circles, Mendel gave up trying to push his theory of heredity. However, it was his work (if not all of his laws) that stood the test of time. The first golden age of genetics was kicked off in 1900 when botanists Hugo De Vries (1848-1935) in Holland, Carl Correns in Germany, and Erich von Tschermak in Austria independently found Mendel's paper and realized that it explained much of their own research and described the mechanism for heredity. At this point in time, the old notions about heredity were being disproved and the world was ready to gradually accept Mendel's work.

William Bateson (1861-1926), who is considered one of the creators of modern genetics, championed Mendel's work and coined the terms "Mendelism" and "genetics." A host of other scientists took an interest in heredity, and advances in understanding came quickly. Cytology, or the study of cells, also had burgeoned in the late 1800s and provided an understanding of how cells divide. Together, Mendel's theories and research at the cellular level could explain inheritance. For example, in the early 1900s Mendel's two laws were explained by the finding that chromosomes carry genes, that chromosomes segregate at meiosis, and that there are genes on different chromosomes.

Mendel's work also impacted Darwin's theory of evolution. One of Darwin's problems in explaining inheritance was that he thought inheritance and variation were opposite processes that could not coexist. In the early 1900s, however, De Vries fixed this problem when he proposed that mutations (changes) in genes produce variation. Scientists in the 1930s then generated the synthetic theory of evolution that incorporated natural selection, the principles of genetics, the concept of mutation, and the idea that populations are the units on which natural selection acts.

By 1910 Mendel's work had been amplified, and it was clear that inheritance often was not as straightforward as predicted by Mendelian genetics. However, Mendel's basic laws provided the framework that allowed researchers to determine how exceptions were produced. For example, while the law of segregation always held true, scientists often did not observe dominance and found that blending (codominance) seemed to occur. Independent assortment was found to be limited to genes on different chromosomes. In addition, researchers found that multiple genes influence many traits, genes can be linked, traits can be linked to sex chromosomes, lethal genes exist, and many times a trait has multiple states rather than just two (e.g. tall and short).

From 1910 to the turn of the twenty-first century, our understanding of genetics has advanced quickly and tremendously. Classical genetics gave rise to molecular biology and biotechnology. We are in the midst of a second golden age of genetics, in which biotechnology offers the promise of better crops, fixing damaged genes during gestation, and curing disease with "gene therapy." Mendel's basic findings discovered in a monastery garden, however, resonate throughout modern genetics. His work has positively affected the agriculture and livestock industries. His laws are used daily in genetic counseling to help parents determine the risk of having babies with various diseases and birth defects. And Mendel's work led to genetic terms sprinkled throughout every basic biology and genetics textbook currently published.

LYNN M.L. LAUERMAN