The Rediscovery of Mendel's Laws of Heredity

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The Rediscovery of Mendel's Laws of Heredity

Overview

In the 1860s, in an Augustinian monastery garden, Gregor Mendel (1822-1884) carried out a systematic experimental analysis of plant hybridization and inheritance patterns. Although Mendel published an account of his work and attempted to communicate with leading naturalists of his day, his work was essentially ignored for over 30 years. At the beginning of the twentieth century, however, Mendel and his laws were "rediscovered" by Hugo Marie de Vries, Karl Franz Joseph Correns, and Erich Tschermak von Seysenegg, firmly attaching Mendel's name to the basic laws of genetics. William Bateson, who came close to rediscovering Mendel's laws through his own experiments, became one of the leading advocates of Mendelian genetics.

Background

After exploring various animal and plant systems, Mendel conducted studies of 34 different strains of peas and selected 22 kinds for further experiments. He chose to study traits that were distinct and discontinuous and exhibited clear patterns of dominance and recessiveness. The "law of segregation," also known as Mendel's first law, refers to Mendel's proof that recessive traits reappear in predictable patterns. Crosses of peas that differed in one trait produced the now-famous 3:1 ratios. Complex studies that followed the variations of two or three traits led to the patterns of recombination now known as Mendel's second law, or the "law of independent assortment."

Mendel discussed his results at a meeting of the Brno Society for Natural History in March 1865 and published his paper "Research on Plant Hybrids" in the 1866 issue of the Society's Proceedings. He also sent reprints of his article to prominent scientists but received little attention and virtually no understanding.

Contemporaries tended to dismiss Mendel's "numbers and ratios" as merely empirical and devoid of a respectable theoretical framework. Sir Ronald A. Fisher (1890-1962), however, argued that the experimental design reported in Mendel's classic paper was so elegant that the experiments had to have been a confirmation, or demonstration, of a theory Mendel had previously formulated. Furthermore, Fisher claimed that Mendel's ratios are closer to the theoretical expectation than sampling theory would predict and he insisted that such results could not be obtained without an "absolute miracle of chance." Although Mendel was discouraged by the lack of response from the scientific community, he remained convinced of the fundamental value and universality of his work.

Impact

Although Gregor Mendel's name is now attached to the fundamental laws of genetics, his work was essentially ignored and misunderstood for over thirty years. Classical genetics, therefore, began not with the publication of Mendel's papers in the 1860s but at the beginning of the twentieth century with the independent rediscovery of the laws of inheritance by three botanists—Hugo de Vries (1848-1935), Carl Correns (1864-1935), and Erik Tschermak (1871-1962). Between the 1860s and 1900 developments in the study of cell division, fertilization, and the behavior of subcellular structures had established a new framework capable of accommodating Mendel's "ratios and numbers." A revival of interest in discontinuous, or "saltative," evolution was probably a significant factor in the approach the rediscoverers brought to the study of heredity; if the sudden appearance of new character traits led to the formation of new species, studies of the transmission of these traits using Mendelian breeding experiments might furnish the key to a new science of heredity and evolution.

Historians of science suggest that de Vries, Correns, and Tschermak attempted to emphasize their own creativity by claiming to have discovered the laws of inheritance before finding Mendel's paper. Citing Mendel's earlier work also helped them avoid a priority battle. During the 1890s de Vries had observed the 3:1 ratio from his own F2 hybrids, the reappearance of recessive traits, and independent assortment. By 1900 he had demonstrated his law of segregation in hybrids of 15 different species. Later, de Vries complained that it was unfair for Mendel to be honored as the founder of genetics.

Correns and Tschermak, however, were more generous towards Mendel than de Vries. They even suggested the use of the terms "Mendelism" and "Mendel's laws." Their work, however, had been anticipated by de Vries and they did not have as much at stake as he in any potential priority war. Correns admitted that the task of discovering Mendel's laws in 1900 was much simpler than it had been in the 1860s. Growing hybrids of maize and peas for several generations and analyzing new developments in cytology apparently had led Correns to think about the transmission of paired characters. A literature search led him to Mendel's paper and the realization that Mendel had anticipated his work by 35 years. Correns soon found out that Hugo de Vries was about to claim priority. While Correns praised Mendel, he was very critical of de Vries and often implied that de Vries was trying to suppress references to their predecessor. On the other hand, Tschermak often complained that his contributions to genetics had been slighted. Tschermak's interest in the question of hybrid vigor led to a series of experiments on the effects of foreign pollen. When he analyzed the results of these experiments, Tschermak observed the 3:1 ratio; after conducting a literature search, Tschermak also discovered Mendel's paper. Tschermak's work, however, was not as extensive as Mendel's. In March 1900 Tschermak received a reprint of de Vries's paper, "On the Law of Segregation of Hybrids," followed by a copy of Corren's paper, "Gregor Mendel's Law." Tschermak quickly prepared an abstract of his own work and sent copies of his article on artificial hybridization to de Vries and Correns to establish himself as a participant in the rediscovery of Mendel's laws.

The apparently simultaneous discovery of Mendel's laws by de Vries, Correns, and Tschermak suggests that by 1900 rediscovery had become inevitable. Indeed, others such as William Bateson (1861-1926) were also working along similar lines. In the 1890s Bateson had begun to analyze the inheritance patterns of discontinuous variations in the offspring of experimental hybrids; therefore, when Bateson read Mendel's paper, he was already thinking about the inheritance of discrete units. A reprint of de Vries's paper led Bateson to the 1865 Brno Proceedings. Bateson soon became a dedicated defender of Mendelian genetics. He coined and popularized many of the terms now used by geneticists, including the words "genetics" (from the Greek word for descent), "allelomorph" (allele), "zygote," "homozygote," and "heterozygote." The Danish botanist Wilhelm L. Johannsen (1857-1927) introduced the term "gene" to replace such older terms as "factor," "trait," and "character" and coined the terms "phenotype" and "genotype." In 1902 Bateson published Mendel's Principles of Heredity. The text included a translation of Mendel's paper and Bateson's assertion that Mendel's laws would prove to be universally valid. Indeed, further studies of the patterns of inheritance proved that Mendel's laws were applicable to animals as well as plants.

LOIS N. MAGNER