In his book Mendel’s Principles of Heredity, William Bateson clarified the importance of Gregor Mendel’s 1866 research findings concerning the inheritance of characteristics in plants.
Before 1900, some investigators of heredity concluded that its mechanism is not a blending of characteristics, as was generally thought, but rather that the characteristics are particulate and unchanged by mixing with others, reappearing unchanged in future generations. That is, inheritance operates in a way similar to the combining of various colored balls rather than to the mixing of different colored liquids.
In 1900, three of these scientists who were studying plant crosses, Hugo de Vries, Carl Erich Correns, and Erich Tschermak von Seysenegg, independently discovered that in 1866 Gregor Mendel had published a paper giving this particulate explanation along with the regular ratios for the appearance of contrasting characteristics in garden peas over several generations. In the first generation of such a cross, only one trait that was termed dominant, of a pair of contrasting ones, was found in all of the offspring. This is usually known as Mendel’s law of dominance. In the second and succeeding generations, the lost trait, termed recessive, would appear in some individuals. In the second inbred generation, it would show up in one-fourth of the offspring of self-pollinated (or selfed) plants. After the second generation, only two-thirds of the dominant-appearing plants carried the recessive traits; the other one-third were pure dominants. Those that carried the recessive traits would have them show up in one-fourth of their offspring when selfed. Those that were pure dominants would breed true and show only that trait in future selfed generations. Further, in crosses in which several traits were followed at the same time, each one of them was inherited independent of the others, and each showed these same ratios. This result is called Mendel’s law of independent assortment.
As early as the eighteenth century, a number of plant hybridizers had noticed the uniformity of the first-generation offspring of a hybrid cross and the lack of uniformity in succeeding generations. None of them noticed that the individual traits were inherited in a systematic way that could be described as percentages or ratios. Mendel was thus unique in keeping careful records of each trait in each generation and in using mathematical ratios to characterize traits. He also noted that it did not matter which trait was examined; all traits behaved in the same manner. In the initial crossing of the two types, it did not matter from which parent the pollen or ovules came.
De Vries, who in 1900 was a professor at Amsterdam University in the Netherlands, realized as early as 1889, when he published a book titled Intracelluläre pangenesis (Intracellular Pangenesis, 1910),
Correns, who in 1900 was teaching at Tübingen University in Germany, had been hybridizing different races of maize and peas. He was particularly interested in a characteristic in maize heredity called xenia, in which the color of the endosperm of the grain is caused by the kind of pollen the plant received from the male parent. Correns, too, found that the “lost” traits in both types of plants reappeared in one-fourth of the offspring in the second inbred generation. Like de Vries, he independently discovered Mendel’s paper sometime before March, 1900, but he did not submit his paper for publication until April, after he had seen de Vries’ paper. He also gave Mendel full credit for the discovery of the ratios and their mechanism. Correns, however, reported cases where the first generation lacked dominance and showed some uniform intermediate state for a particular characteristic. He had questions about the universality of Mendel’s interpretation for all cases. Correns was a highly respected researcher, as was de Vries, and the papers of both men were disseminated widely, encouraging others to do research to test their explanations.
Mendel evaluated the transmission of seven paired traits in his studies of garden peas.
Tschermak von Seysenegg, who had been breeding garden peas and studying their various traits, also noticed the regularity of the traits’ appearance in different generations. He, too, independently found Mendel’s paper when he was preparing to present a lecture on his breeding work as part of his new teaching position at the Land Cultivation University (Hochschule für Bodenkultur) in Vienna. He credited Mendel in a paper submitted for publication in April, 1900. He was a less well known scientist than either de Vries or Correns, and thus his research was less influential.
William Bateson, in England, also had been studying actively discontinuous inheritance of characteristics in plants and animals. In 1894, he published a book titled Materials for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species.
Bateson presented his book on Mendel’s principles as a defense because at that time in England the study of inheritance was dominated by the biometric school founded by Sir Francis Galton. Galton’s explanation of heredity, proposed in 1897, was based on a continuous blending hypothesis in which each ancestor was credited with a certain proportion of the mix. That is, each parent contributed half, each grandparent contributed one-fourth, and so on backward by generations. The Mendelian explanation did not fit the biometricians’ laws of ancestral inheritance. This disagreement led to heated controversy. When one of the biometricians, the zoologist Walter Weldon,
Mendel’s paper was cited in 1881 in a major analytic reference book, W. O. Focke’s Die Pflanzen-Mischlinge (the plant hybrids), which reported that Mendel had found constant ratios for characters in second and succeeding generations of peas. Both Correns and Tschermak von Seysenegg found Mendel’s paper through this reference. De Vries may have first learned about Mendel’s work through the bibliography of a paper by the American horticulturist Liberty Hyde Bailey, who did not see the paper but included the reference from Focke’s book in 1892. It is also possible that de Vries first saw Mendel’s paper when a colleague sent him an offprint of it from his library because he thought it would be useful to de Vries in his work. De Vries was unclear about this when he later tried to recall the event. The literature about the rediscovery of Mendel’s experiments contains some controversy as to whether de Vries gave Mendel enough credit. Correns, for example, believed that de Vries never gave Mendel the credit he deserved. In any case, it is very clear that each of the rediscoverers came to an understanding similar to that of Mendel through his experiments before finding the Mendel paper. Both de Vries and Correns continued to do significant genetic research after 1900. Bateson became the strongest champion of the Mendelian explanation for all types of plant and animal inheritance.
The modern science of genetics began with the rediscovery of Mendel’s explanation for inheritance. After 1900, de Vries, Correns, Tschermak von Seysenegg, and many other scientists began to explain their experimental results in terms of Mendelian ratios or exceptions to those ratios. Particulate inheritance, with genes on chromosomes, replaced the notion of blending inheritance. The appearance of offspring having characteristics of both parents now could be explained by the large number of characteristics involved, each one operating independently in a particulate manner except when linked together with another on the same chromosome.
Within a few years, cytological data connected contrasting traits with sections of chromosomes. This connection, with the use of the term “gene” to describe the controlling unit, provided a physical basis for the particulate explanation of heredity. One early finding was that sexuality is controlled in a Mendelian fashion. Experimental work with various types of organisms showed that the three-to-one dominant-to-recessive Mendelian ratio for the second generation of inbred crosses is the normal situation. When these ratios were not found, further explanations were necessary and were produced, such as lack of dominance; more than two possible contrasting traits for the same chromosomal location, only two of which are present in normal cells; more than one pair of genes controlling the expression of a particular trait; linkage of genes on the same chromosome; and modifier genes that change the expression of a trait.
Cytological studies quickly began to supplement breeding experiments. Genetics became a leading field of biological study and, in applied areas, an important and useful tool. Agriculturists used their understanding of Mendelian genetics to develop better plants and animals. This resulted in the discovery of hybrid vigor in corn and other crops, which significantly increased agricultural production. In medicine, it led to a better understanding of heredity diseases such as sickle-cell anemia. Genetics now plays a major role in the interpretation of data in other fields of biology, such as morphology, embryology, taxonomy, physiology, and evolutionary biology. The search for the nature of the gene led to the development of molecular biology. The twentieth century truly can be said to be the century of genetics in biological sciences.
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Bowler, Peter J. The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Baltimore: The Johns Hopkins University Press, 1989. Documents the importance of Mendelian genetics after 1900 as well as the social and scientific impacts of the rediscovery of Mendel’s work. Provides good background information for an appreciation of Mendelian genetics. -
Carlson, Elof Axel. Mendel’s Legacy: The Origin of Classical Genetics. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2004. Based heavily on early twentieth century sources, this book traces the roots of genetics in breeding analysis and studies of cytology, evolution, and reproductive biology. Highly illustrated. -
Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel. Boston: Houghton Mifflin, 2000. Discusses Mendel’s life, including his work, in the context of his time and also relates the rediscovery of his work to developments in the constantly changing field of genetics. -
Mendel, Gregor J. Experiments in Plant Hybridisation. Translated by William Bateson. Cambridge, Mass.: Harvard University Press, 1948. English translation of Mendel’s 1866 paper, supervised by Bateson and published in his book Mendel’s Principles of Heredity: A Defence in 1902. Provides a clear description of Mendel’s methods, results, and interpretation of those results, as well as background on breeding experiments conducted by other scientists. This work is basic to understanding why those who discovered it thirty-four years later were so impressed with its insights. -
_______, et al. “The Birth of Genetics: Mendel, de Vries-Correns-Tschermak in English Translation.” Genetics, supp. 35 (September, 1950): 1-47. English translations of the original first papers of the three scientists who rediscovered Mendel’s work. Also includes translations of nine letters that Mendel wrote between 1866 and 1873 to Carl Nägeli, professor of botany at the University of Munich, explaining his pea experiments and his later experiments with crossing hawkweeds. -
_______, et al. Fundamenta Genetica: The Revised Edition of Mendel’s Classic Paper with a Collection of Twenty-seven Original Papers Published During the Rediscovery Era, edited by Jaroslav Kříženecky. Oosterhout, Netherlands: Anthropological Publications, 1965. Twenty-eight papers in their original languages published between 1899 and 1904 give the flavor of excitement of the early Mendelian geneticists. A good source to find Bateson’s papers. An introductory essay in English about concepts before Mendel helps to explain why Mendel’s work was so revolutionary and was therefore ignored. Published to celebrate the centenary of the publication of Mendel’s paper. -
Olby, Robert C. Origins of Mendelism. New York: Schocken Books, 1966. A well-balanced discussion of the origins of Mendelism, considerations of other interpretations of heredity before and during Mendel’s life, and detailed explanation of the rediscovery of Mendel’s discovery. Clearly written without extensive quotes from original writings, which appear in a large appendix. -
_______. “William Bateson’s Introduction of Mendelism to England: A Reappraisal.” British Journal of the History of Science 20 (1987): 399-420. Documents the rediscovery of Mendel’s work, but mostly elaborates on Bateson’s role as an early defender of Mendelism. Claims that Bateson took several years to conclude that Mendelian principles were broadly applicable to inheritance in different kinds of organisms. -
Roberts, H. F. Plant Hybridization Before Mendel. 1929. Reprint. New York: Hafner, 1965. Classical presentation of the background of Mendel’s hybridizing experiments, Mendel’s work, and its rediscovery in 1900. Includes comments quoted from de Vries, Correns, and Tschermak von Seysenegg about their recollections of how they found Mendel’s paper. Includes a chapter on Bateson’s contributions. Essential for any serious study of the origins of Mendelism. -
Stern, Curt, and Eva R. Sherwood, eds. The Origin of Genetics: A Mendel Source Book. San Francisco: W. H. Freeman, 1966. Provides a good summary of Mendel’s work and its rediscovery. Includes English translations of Mendel’s papers on plant hybrids and those of de Vries and Correns. Contains a reprint of a paper by the English geneticist R. A. Fisher on the statistics in Mendel’s 1866 paper and why they seem too perfect. Sewall Wright, an American geneticist, responds to Fisher.
McClung Contributes to the Discovery of the Sex Chromosome
Sutton Proposes That Chromosomes Carry Hereditary Traits
Punnett’s Mendelism Includes Diagrams Showing Heredity
Bateson and Punnett Observe Gene Linkage
Hardy and Weinberg Present a Model of Population Genetics
Morgan Develops the Gene-Chromosome Theory
Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”
Sturtevant Produces the First Chromosome Map