Genes on Chromosomes

Gregor Mendel’s hereditary studies on garden peas were “rediscovered” in 1900 and, with them, the basis for genetics. Thomas Hunt Morgan was one of many biologists at the turn of the twentieth century who accepted Darwin’s theory of evolution but rejected his notion of natural selection, and also Mendel’s findings. One of the three “rediscoverers” of genetics was the Dutch botanist Hugo de Vries, who in 1886, while working with the evening primrose, found evidence for mutations (sudden changes in body form).

Morgan, a zoologist at Columbia University, in 1907 began research on Drosophila melanogaster —the common fruit fly—in an attempt to show that mutations, rather than the gradual variation proposed by Darwin, were the basis for natural selection. He selected fruit flies because 1,000 could be housed in a one-quart milk bottle, they produced one generation every twelve days, males and females were readily distinguishable, and mutations were readily detected. After three years of breeding, the first mutant was detected: a white-eyed fly. Subsequent breeding revealed that females were exclusively red-eyed and that only some males had white eyes.

In 1910, he proposed the chromosomal theory of heredity. Each chromosome contained a collection of small units called genes that were arranged on the chromosome as “beads on a string.” Moreover, some traits (such as yellow body color or rudimentary wings) are linked to the sex-determining chromosome. In 1913, Morgan’s student Alfred H. Sturtevant found that each gene could be assigned a specific position on a chromosome map, which became the basis for mapping the human genome.

In this illustration of the human karyotype, all twenty-two chromosome pairs are displayed, along with the XX and XY sex chromosome pairs.

The gene was the missing link in Mendel’s theory of heredity and Darwin’s theory of evolution, and Morgan filled in that gap. (By 1916, Morgan had accepted Darwin’s theory of natural selection.) Thus, Morgan was awarded the 1933 Nobel Prize for determining the role of chromosomes in heredity. As an additional legacy, of the students who worked with Morgan or one of his students, five were awarded their own Nobel Prizes.