In his book Mendelism, Reginald Crundall Punnett introduced diagrams to explain how traits are inherited and thus helped to popularize Mendel’s work.
In 1905, Reginald Crundall Punnett’s landmark book Mendelism was published. It was the first textbook on the subject and was so popular that the Westminster Gazette listed it along with a novel by Marie Corelli as the best seller of the week. The small book stated clearly the principles of heredity espoused by Austrian monk Gregor Mendel
When Mendelism first appeared, the subject was unknown to most investigators. Mendel’s original paper of 1866, long ignored because it had been published in an obscure journal, had been rediscovered independently in 1900 by three botanists: Hugo de Vries, Carl Erich Correns, and Erich Tschermak von Seysenegg. Although Mendel’s ideas were intriguing, early twentieth century biologists did not agree on the universality of his “laws” of heredity. They searched for an understanding of the significance of the Mendelian patterns. Punnett’s book became an excellent learning tool for the rediscovered Mendelism.
When white-flowered sweet pea plants were crossed, the first-generation progeny (F1 ) all had purple flowers. When these plants were self-fertilized, the second-generation progeny (F2 ) revealed a ratio of nine purple to seven white. This result can be explained by the presence of two genes for flower pigmentation, P (dominant) and p (recessive) and C or c. Both dominant forms, P and C, must be present in order to produce purple flowers.
Punnett formed a fruitful partnership with William Bateson, whose rejection of traditional Darwinism caused him to be out of favor with the academic establishment. In 1910, Bateson resigned from the inadequately funded, temporary chair of biology at the University of Cambridge. The chair became adequately endowed in 1912 and was given to Punnett, who became the first Arthur Balfour Professor of Genetics at the University of Cambridge.
Undoubtedly, Punnett’s work on the Mendelian explanation of sex determination, sex linkage, complementary factors and factor interaction, autosomal linkage, and mimicry resulted in important scientific accomplishments. His practical achievement during World War I, when he devised a scheme to distinguish sex in very young chickens, was ingenious. (By looking for sex-linked plumage-color factors that would appear only in male chicks, researchers could distinguish and destroy unwanted males so that food would not be wasted on them.) Also important, but in a different way, was Bateson and Punnett’s joint founding of the Journal of Genetics
Mendel used the pea plant, Pisum, to investigate the inheritance of individual characteristics such as seed pod color, height, flower color, and pod shape. In one set of experiments, he investigated only the heights of the plants. When he crossed pure tall plants (one to two meters high) and pure dwarf plants (half a meter to one meter high), all of the offspring (Fl generation) were tall. Mendel applied the term “dominant” to the tall plants and “recessive” to the dwarf plants. Next, he collected the seeds from the Fl plants. When he sowed them the following year, both tall and dwarf plants appeared in the offspring (F2 generation), but each individual was either tall or dwarf, with no intermediates. In one set of experiments, out of a total of 1,064 of these F2 plants, 787 were tall and 277 were dwarf, approximately a ratio of 3:1. The next year, Mendel planted the seeds of the F2 generation. He found that although the seeds from the dwarf plants produced only dwarfs, the tall plants produced two different kinds of seed, some giving rise to tall plants only and others producing both talls and dwarfs in a 3:1 proportion. By breeding subsequent generations, Mendel demonstrated that pure dominants and recessives breed true, but that impure dominants always produce dominants and recessives in a constant proportion of 3:1. Because pure dominants are only half as numerous as impure dominants, when impure dominants are crossed with each other they produce pure dominants, impure dominants, and recessives in the ratio of 1:2:1.
The case considered above involves only a single set of characteristics (in this example, height) and is known as a monohybrid cross. Mendel ascertained that when the original parents differ in two pairs of characters, such as seed color and height (a dihybrid cross), the two traits are assorted independent of each other. When a tall yellow-seeded pea is crossed with a dwarf green-seeded plant, the Fl plants all exhibit the dominant character of each pair and are tall yellows. In the F2 generation, talls and dwarfs appear in the ratio of 3:1, and yellows and greens also appear in a 3:1 ratio. Consequently, out of the total sixteen plants, twelve will be tall and the other four dwarf. Out of every four talls, three will be yellows and the remaining one green. Out of the twelve talls, therefore, nine will be yellow and three will be green. Out of the four dwarfs, three will be yellow and one will be green. Consequently, the F2 generation arising from the cross will result in nine yellow talls, three green talls, three yellow dwarfs, and one green dwarf. For every sixteen plants, nine will show both dominants, two classes of three each will show the dominant character of one pair and the recessive of the other, and one plant will show both recessive characters. This principle holds true for three sets of characters and can be extended indefinitely for any number of pairs of characteristics.
After describing the crossing procedure and results, Punnett showed his simple graphic way of viewing the probabilities from these crosses. In Punnett’s square, he used Aa and Bb to represent two sets of unit characteristics (in the above example, height and seed color). The capital letters represent dominant characteristics, and the lowercase represent recessive characteristics.
A = dominantB = dominant
a = recessive b = recessive
If a parent pure (homozygous) for the dominant A characteristic is crossed with a parent pure (homozygous) for the recessive a character (AA × aa), all of the offspring (Fl generation) resemble the dominant A parent but carry the recessive a (Aa). If two of these Fl individuals are crossed (Aa × Aa), the ratio of their offspring (F2 generation) will be 1 (AA), 2 (Aa), and 1 (aa). To demonstrate this 1:2:1 ratio, Punnett drew a large square divided into four cells. One A and one a is assumed to appear across the horizontal axis, and one A and one a down the vertical axis (Punnett did not explicitly complete this step). When the A and the a from the horizontal axis are multiplied by the A and the a from the vertical axis, the four cells of the large square indicate one individual with AA, two with Aa, and one with aa, as Mendel predicted.
If a second set of characteristics Bb × Bb is considered, the result will be the predicted 1:2:1. To demonstrate the disposition of the B and b factors, Punnett divided each of the four original cells into four again. In each of the new cells, one individual is BB, two are Bb, and one bb—again, the predicted 1:2:1 ratio for a monohybrid cross.
A Punnett square can also be used to illustrate a dihybrid cross, in which two sets of characters are considered simultaneously. If in the first parental cross (P1) one parent is pure (homozygous) for both dominants (AABB) and the other parent is pure (homozygous) for both recessives (aabb), the Fl generation will be heterozygous, having both A and a and B and b.
P1 crossAABB × aabb
F1 generation AaBb
F1 cross AaBb × AaBb
Of the sixteen squares necessary to illustrate this cross, nine contain both A and B, three contain A but not B, three contain B but not A, and one contains neither A nor B. The ratio illustrated by the diagram is the 9:3:3:1 expected from a dihybrid cross.
Although this method can be used to consider three or more characteristics, the numbers of squares required in such cases makes it cumbersome. Illustrating a trihybrid cross would require 64 squares, and considering four different characteristics would require 256 squares.
Punnett’s book Mendelism, in which he created a simple way to illustrate Mendel’s ratios visually, played a vital role in popularizing Mendel’s work. Through this book, Punnett launched an aggressive propaganda campaign for Mendel’s ideas. Even after 1900, when Mendelism was rediscovered, it was by no means universally known or, where known, universally accepted. Rules that worked for the inheritance of color in pea plants did not seem to apply to the inheritance of comb types in chickens or to apparent cases of blending inheritance in certain traits. Using his clear diagrams, Punnett engaged in a crusade for Mendelism, showing how apparent exceptions could be explained within the framework of Mendel’s conclusions.
Punnett’s emphasis on the importance of Mendelism inspired a biological revolution, but one that he never joined. During the latter part of the nineteenth century, cytologists (those who study cells) observed the peculiar way in which chromosomes are distributed during ordinary body cell division (mitosis) and in the special divisions that produce egg and sperm. They proceeded in their microscopic studies, totally unaware of the significance of Mendel’s work for their own observations. In the meantime, botanists such as Correns and de Vries, cast in the Mendelian mold of plant hybridizers, continued with their breeding experiments equally unaware of the studies on chromosomes that were being undertaken at the same time.
Although some late nineteenth century cytologists suspected that chromosomes might play a part in heredity, they had no evidence as to how traits were distributed among offspring. Mendel, of course, had already determined some of these relationships, but his ideas were unavailable to cytologists until after 1900.
In the period before Mendel’s laws were rediscovered, many competing theories of heredity were developed. No one theory could be taken as the paradigm, or the accepted “establishment” point of view. Mendel’s theory on its own was not sufficient to represent a complete paradigm until it was fused with the cytological ideas that placed Mendel’s “factors” on the chromosomes. Between 1900 and 1910, evidence mounted to indicate that chromosomes represented the physical basis for Mendel’s observed ratios.
Punnett worked only on one side of this problem, the Mendelian. Throughout his career, he remained a Mendelian and was not affected by the development of the theory of the gene and of cytogenetics. Nevertheless, his brilliant work on the Mendelian aspects of heredity, including his clear ways of presenting his data (through his diagrammatic approach), made possible the fusion between Mendelian genetics and cytology and the establishment of the new paradigm that occurred around 1910.
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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. -
Dunn, L. C. A Short History of Genetics: The Development of Some of the Main Lines of Thought, 1864-1939. 1965. Reprint. Ames: Iowa State University Press, 1991. Summarizes the general history of theories of heredity and places Bateson’s and Punnett’s work in context. Includes a glossary and a bibliography of both primary and secondary sources. -
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. -
Olby, Robert C. The Origins of Mendelism. 2d ed. Chicago: University of Chicago Press, 1985. Evaluates Mendel’s work and considers the consequences of its rediscovery for early twentieth century biology. -
Punnett, Reginald Crundall. “Early Days of Genetics.” Heredity 4 (April, 1950): 1-10. In this paper—an address delivered at the one hundredth meeting of the Genetical Society in Cambridge, England, on June 30, 1949—Punnett provides an entertaining look at the early development of Mendelian genetics in England. -
_______. “Reginald Crundall Punnett.” Biographical Memoirs of Fellows of the Royal Society 13 (1967): 323-326. This memoir contains the most complete biographical treatment of Punnett as well as a complete bibliography of his writings. -
Sturtevant, A. H. A History of Genetics. 1965. Reprint. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2001. Summarizes general theories of heredity and helps put Punnett’s work into context.
Bateson Publishes Mendel’s Principles of Heredity
McClung Contributes to the Discovery of the Sex Chromosome
Sutton Proposes That Chromosomes Carry Hereditary Traits
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