N. M. (Nettie Maria) Stevens

Born in Cavendish, Vermont, in 1861, Nettie Maria Stevens came to research later than many scientists of her era. She taught school for years before studying at Stanford University and then earning a doctorate at Bryn Mawr College, where she built a reputation for precise, patient work in cytology and embryology.

Stevens is best known for her 1905 studies of mealworms, in which she showed that males produce two kinds of sperm and that these differences are tied to what became known as the X and Y chromosomes. That work helped establish the chromosomal basis of sex determination and became an important foundation for early genetics.

Although her life was short—she died in 1912—her influence was lasting. She is now remembered as one of the scientists who helped connect chromosomes with heredity at a moment when genetics was just beginning to take shape.