Cytologist Nettie Stevens is credited with finding evidence to conclude that chromosomes have a role in sex determination, making the connection between heredity and the sex of offspring.
Raised and educated in Vermont, Nettie excelled in school, studying science at Westfield Normal School, teaching, and eventually moving on to study and research at Stanford University in 1896. During her time at Stanford, she spent summers performing research at Hopkins Seaside Laboratory, and worked at many other labs over the course of her academic career. Stevens earned her BA & MA at Stanford, and a PhD from Bryn Mawr College, where she met biology professors Thomas Hunt Morgan and Edmund Beecher Wilson, who were influences on her work, and who recommended her for funding for research on heredity by the Carnegie Institution on Washington.
Stevens was not the first to suggest that heredity factors influence sex. Her research observing the chromosomal makeup of mealworms, noting that male worms always received the smaller chromosome of the variants, and the females the larger, leading her to publish her X.Y chromosomal model.
Stevens work beyond this discovery include an assistant professorship at Bryn Mawr and continued affiliation with the Carnegie Institute.
Sources: Smith, Kaitlin, "Nettie Maria Stevens (1861-1912)". Embryo Project Encyclopedia (2010-06-20). ISSN: 1940-5030 http://embryo.asu.edu/handle/10776/2028. Accessed 7 Feb. 2019.
Image credit: The Incubator (courtesy of Carnegie Institution of Washington) [Public domain], via Wikimedia Commons