Thomas Hunt Morgan and his Work on Drosophila

American zoologist and geneticist Thomas Hunt Morgan was known for his experimental research with the fruit fly (Drosophila) by which he established the chromosome theory of heredity. He showed that genes are linked in a series on chromosomes and are responsible for identifiable, hereditary traits. Morgan’s work played a key role in establishing the field of genetics. He received the Nobel Prize for Physiology or Medicine in 1933.

Morgan apparently began breeding Drosophila in 1908. In 1909 he observed a small but discrete variation known as white-eye in a single male fly in one of his culture bottles. Aroused by curiosity, he bred the

fly with normal (red-eyed) females. All of the offspring were redeyed. Brother-sister matings among the F1 generation produced a second generation with some white-eyed flies, all of which were males. To explain this curious phenomenon, Morgan developed the hypothesis of sex-limited—today called sex-linked—characters, which he postulated were part of the X chromosome of females.

Other genetic variations arose in Morgan’s stock, many of which were also found to be sex-linked. Because all the sex-linked characters were usually inherited together, Morgan became convinced that the X chromosome carried a number of discrete hereditary units, or factors. He adopted the term gene, which was introduced by the Danish botanist Wilhelm Johannsen in 1909, and concluded that genes were possibly arranged in a linear fashion on chromosomes. Much to his credit, Morgan rejected his skepticism about both the Mendelian and chromosome theories when he saw from two independent lines of evidence—breeding experiments and cytology—that one could be treated in terms of the other.

In collaboration with American geneticists Alfred Henry Sturtevant, Calvin Blackman Bridges, and Hermann Joseph Muller, who were graduates at Columbia, Morgan quickly developed the Drosophila work into a large-scale theory of heredity. Particularly important in this work was the demonstration that each Mendelian gene could be assigned a specific position along a linear chromosome “map.” Further cytological work showed that these map positions could be identified with precise chromosome regions, thus providing definitive proof that Mendel’s factors had a physical basis in chromosome structure. A summary and presentation of the early phases of this work was published by Morgan, Sturtevant, Bridges, and Muller in 1915 as the influential book The Mechanism of Mendelian Heredity. To varying degrees Morgan also accepted the Darwinian theory by 1916.

In 1928 Morgan was invited to organize the division of biology of the California Institute of Technology. He was also instrumental in establishing the Marine Laboratory on Corona del Mar as an integral part of Caltech’s biology training program. In subsequent years, Morgan and his coworkers, including a number of postdoctoral and graduate students, continued to elaborate on the many features of the chromosome theory of heredity.

In 1924 Morgan received the Darwin Medal, and in 1939 he was awarded the Copley Medal by the Royal Society of London. Among Morgan’s most important books are Evolution and Adaptation (1903), A Critique of the Theory of Evolution (1916), Heredity and Sex (1913), and The Theory of the Gene.

Fig. 1. Sex-linked inheritance of white eyes in Drosophila flies. Encyclopedia Britannica, Inc.