Biologist Nettie Maria Stevens (1861-1912) discovered that chromosomes determine sex.

Nettie Maria Stevens was a biologist and cytogeneticist and one of the first American women to be recognized for her contributions to scientific research. "She...produced new data and new theories," wrote Marilyn Bailey Ogilvie in Women in Science, "yet beyond these accomplishments passed along her expertise to a new generation.... illustrat[ing] the importance of the women's colleges in the education of women scientists." Although Stevens started her research career when she was in her thirties, she successfully expanded the fields of embryology and cytogenetics (the branch of biology which focuses on the study of heredity), particularly in the study of histology (a branch of anatomy dealing with plant and animal tissues) and of regenerative processes in invertebrates such as hydras and flatworms. She is best known for her role in genetics--her research contributed greatly to the...