Beginning in 1907, Thomas Hunt Morgan extended Sutton's insights by conducting laboratory studies of the fruit fly, Drosophila melanogaster. With his students Alfred Henry Sturtevant, Calvin Blackman Bridges, and Hermann Joseph Muller, he established what is now called classical genetics. Morgan and his students found new phenomena that added to Sutton's chromosome theory of heredity. The first finding, achieved in 1920, was X-linked inheritance, in which white-eyed flies showed a sex-linked inheritance of the trait in a modified 3:1 ratio. In other words, cross-breeding hybrid red-eyed flies resulted in all the female offspring having red eyes, whereas half the male offspring had white eyes.
Morgan's team explained this modified ratio by proposing that the eye-color genes are carried on the X chromosome, of which females have two but males have only one. The female's two X chromosomes can be homozygous (the genes carried are AA or aa) or heterozygous (the genes carried are Aa) for an X-linked gene. Males are always homozygous, because the small Y chromosome lacks almost all the genes found on the X chromosome. Thus, they can carry only one gene for the trait: AY or aY. Since males can carry no second copy of the gene for the trait, they will express the white-eyed trait if they inherit the gene for it from a hybrid red-eyed parent. This discovery further strengthened the case in favor of the chromosomal theory of inheritance.
Several additional X-linked mutations arose by 1913. Morgan reported that these mutations produced unusual ratios when subjected to breeding analysis. He explained these findings by proposing that genes could trade places between two homologous chromosomes. Morgan's finding, called crossing over, was used to map the genes along the length of the X chromosome, and Sturtevant used Morgan's data to construct the first linkage map. Working independently, maize geneticists, especially Barbara McClin-tock, later demonstrated the same phenomenon.
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