The Concept of Heredity
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We need to understand in appropriate technical detail what the phrase "children resemble their parents" means. Gregor Johann Mendel (1822 July 20 Ė 1884 Jan 06) took the first substantive steps toward giving us such an understanding. Between 1856 and 1863, in the garden of the Augustinian Abbey of St. Thomas in BrŁnn (now Brno in the Czech Republic), Mendel raised some 29,000 plants of the species Pisum sativum (peas) in multiple generations so that he could track how seven different traits were inherited. In 1865 Mendel reported on his experiments and their results in his paper "Experiments in Plant Hybridization" (Versuche Łber Pflanzen-Hybriden).
Mendel chose his project in order to gain a clearer understanding of what happens in the artificial fertilization of plants, a process that, with it unpredictable outcomes, seemed to defy understanding. Mendel designed his experiment in order "...to determine the number of different forms under which the offspring of the hybrids appear, or to arrange those forms with certainty according to their separate generations, or definitely to ascertain their statistical relations." To that end he chose to experiment with peas, choosing them for their ease of cultivation, for their lack of marked disturbances in their fertility in subsequent generations of self-pollenation, and for the ease of protecting them from foreign pollen. As grist for the mill of his mathematical analysis he chose seven characteristics of the plant that seem to come in binary form: that is, for example, the flowers are either violet or white and nothing in between, the peas are wrinkled or smooth and nothing in between, and so on.
Mendelís experimental method was simple, though tedious. When each of the plants he grew achieved maturity, he subjected it to self-pollenation: in other words, he fertilized the seeds of any given plant with that plantís own pollen and prevented any other pollen from entering the plant. In that way he ensured that, through successive generations, the offspring of any given plant would carry only the hereditary factors of that plant and of no other.
Consider, for example, the trait (Mendel called it a differentiating character) of the peas being either smooth or wrinkled. When a plant produced wrinkled peas, Mendel found that subsequent generations of that plant, subjected to self-fertilization, yielded only wrinkled peas. That plant, then, was purebred. When a plant produced smooth peas, it would produce offspring that either yielded only smooth peas in subsequent generations (purebred plants) or yielded a mixture of plants yielding smooth peas and wrinkled peas (hybrid plants). The hybrid plants, Mendel discovered, always yielded the same pattern: for every offspring plant that yielded wrinkled peas (and was, therefore, purebred) three offspring plants yielded smooth peas and of those three, one was purebred in smooth peas and the other two were hybrids that would repeat in their own offspring the pattern that we can express, in Mendelís notation, as
in which S represents smooth peas and W represents wrinkled peas. As Mendel put it, "...it is now clear that the hybrids form seeds having one or other of the two differentiating characters, and of these one-half develop again the hybrid form, while the other half yield plants which remain constant and receive the dominant or the recessive characters in equal numbers."
Mendelís notation above gives a somewhat unclear picture of heredity. But Mendel understood clearly enough. As he noted in his paper, "Experimentally, therefore, the theory is confirmed that the pea hybrids form egg and pollen cells which, in their constitution, represent in equal numbers all constant forms which result from the combination of the characters united in fertilization." In other words, for every trait (differentiating character, as Mendel called it) that the plant manifests that plant obtains from its parents an equal amount of the determining factor (what we call genes), so, for example, the hybrids described above (SW) obtain a smooth gene (S) from one parent and a wrinkled gene (W) from the other parent. When those offspring plants produce their own gametes (pollen and eggs) the genes must separate (into S and W in this case) so that they can combine with the genes from the other parent plant. In the case of self-fertilization of a hybrid (as in Mendelís experiment) we have S and W from the pollen combining with equal probabilities with S and W in the egg to yield four equally likely possibilities, which we can express in a modified form of Mendelís notation as
Thus Mendel produced a properly scientific theory of heredity.
Unfortunately, scientists mostly ignored Mendelís work. In some measure that may have reflected the fact that Mendelís theory seemed to disallow evolution. Mendel had treated the traits of his peas as immutable and for the purpose of his experiment that was the correct assumption to make. But if traits canít change, then there exists no possibility of one species evolving into another. Of course, over long spans of time traits do mutate, but that fact had yet to be discovered.
By 1900 biologists were beginning to understand that theories of heredity based on phenotype (the actual physical body of the organism) were inadequate to account for the facts of heredity. They were ready for a theory, like Mendelís, based on genotype and in that last year of the Nineteenth Century three men rediscovered Mendelís work and brought it to the attention of other biologists.
From 1892 to 1900 Carl Correns (1864 Sep 10 Ė 1933 Feb 14) repeated Mendelís experiments on plants and also rediscovered Mendelís work. Hugo de Vries (1848 Feb 16 Ė 1935 May 21) did the same at the same time. And the agronomist Erich von Tschermak-Seysenegg (1871 Nov 15 Ė 1962 Oct 11) republished Mendelís ideas in June of 1900. Working more or less independently, they reconfirmed Mendelís law of segregation and law of independent assortment. Other biologists replicated Mendelís experiments and extended them, bringing in new techniques of analysis. By the 1930's biologists had developed Mendelís work into such a robust theory of heredity that they combined it with the theory of evolution to create the modern synthesis, an ongoing project that, when complete, will give a full accounting for all the observed facts of life on Earth.
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