History of Genetics
The Chromosome Theory Of Heredity
Meanwhile, in America a bridge between zoology and Mendelism was being constructed at Columbia University, New York City. There, Edward Beecher Wilson was promoting the search for parallels between Mendelian heredity and the theory of the chromosome. Subsequently the embryologist Thomas Hunt Morgan, studying the determination of sex and the hereditary transmission of mutations in the fruit fly (Drosophila), discovered what he called "sex-limited" inheritance, and in 1911 he associated it with a specific chromosome. This led in 1915 to his groundbreaking text The Mechanism of Mendelian Heredity, which he coauthored with the three young men working under him: Alfred Henry Sturtevant, Hermann Joseph Muller, and Calvin Bridges. Here the authors distanced themselves from any suggestion that the factorial hypothesis ("gene" hypothesis) can account for the embryological development of the hereditary characters. A whole mysterious world, they explained, lies between the factor and the character. So they challenged the claim that "until we know something of the reactions that transform the egg into the adult" our hereditary theories "must remain superficial." The Mendelian theory, they argued, does not explain development, nor does it pretend to. Yet "it stands as a scientific explanation of heredity, because it fulfills all the requirements of any causal explanation" (Morgan, p. 280).
Here the authors were seeking to separate genetics from embryology. At the same time, they were developing its relation with the cytologists' study of the chromosomes. This association had at first seemed improbable since many organisms were known to have very few chromosomes—Pisum has seven pairs, the fruit fly four, and the horse thread worm (Parascaris) only two. Assuming the Mendelian factors for the numerous hereditary factors are able to recombine in all possible combinations, as Mendel claimed, surely they could not be tied together on just a few chromosomes. Fortunately in 1909 F. A. Janssens in Belgium had observed "cross-over" patterns between chromosomes that he called "chiasmata," a discovery that gave Morgan the idea that chromosomes might exchange parts, thus permitting recombination. This insight was followed by Sturtevant's suggestion that the frequency of such recombinations between factors could be used as a measure of the distance separating them along the chromosome, and the mapping of genes became a reality. Already the simple picture of Mendelian heredity had been complicated by the discovery of interaction between factors, association of factors (linkage), and the determination of a character by many factors (polygenic inheritance). The latter brought blending heredity under the explanatory arm of Mendelian heredity. The degree of blending was determined by the number of genes involved. Ancestral inheritance was absorbed into Mendelian heredity. What had once been likened to fluids mixing was now attributed to a finite and fixed number of Mendelian factors.
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