Nettie Stevens

Nettie Stevens (1861-1912), American biologist and geneticist whose research helped prove that chromosomes determine the sex of an organism. Researchers had previously believed that gender was influenced by food and temperature conditions during the early stages of an organism's development.

Stevens studied Tenebrio molitor beetles. She found that unfertilized eggs in female beetles always contain an X chromosome. Sperm from male beetles, however, either contain an X chromosome or a Y chromosome. Eggs fertilized by sperm carrying the X chromosome produced female beetles. The combination of egg and Y-chromosome sperm produced male beetles. Stevens published her discovery that chromosomes determine sex in 1905. Edmund Beecher Wilson, a biologist at Columbia University in New York City, made this same discovery at about the same time. Continued research with chromosomes by Stevens yielded other significant results. For example, she established that chromosomes exist as paired structures in body cells, and she determined that certain insects have unusually large numbers of chromosomes.

Nettie Maria Stevens was born in Cavendish, Vermont. She worked as a librarian for several years before enrolling at the Normal School (now Westfield State College) in Westfield, Connecticut. In 1896, when she was 35, she transferred to Stanford University in California, where she earned a B.A. degree. In 1900 Stanford awarded her an M.A. degree in physiology. Stevens then moved to Pennsylvania to attend Bryn Mawr College, where she received her Ph.D. degree in 1903. As part of her doctoral program, she spent a year in Europe studying at the Naples Zoological Station in Italy and the University of Wurzburg in Germany. After graduation, Stevens remained at Bryn Mawr, teaching and conducting research financed by grants and awards. These honors included an assistantship from the Carnegie Institution in Washington, D.C., and the Ellen Richards Prize for study at the Naples Zoological Station. At Bryn Mawr, Stevens held positions of research fellow in biology (1903-1904), reader in experimental morphology (1904-1905), and associate in experimental morphology (1905-1912).

Stevens's research career spanned only nine years (1903-1912), but she published 40 papers in that short time. In addition to her work with chromosomes, she also studied the morphology (form and structure) and taxonomy (scientific classification) of ciliate protozoa and the regeneration of cells in developing roundworms after exposure to ultraviolet radiation. This research showed that very young embryonic cells could not fully regenerate.