Nettie Stevens

Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an early American geneticist. She discovered sex chromosomes. 
Stevens saw two kinds of sperms in male mealworms. One had a large chromosome and one had a small chromosome. They were sex chromosomes, which later became known as the X and Y chromosomes.
Stevens eventually became fully qualified. She received a PhD in cytology, with Thomas Hunt Morgan as her advisor. She died from cancer. She had worked at Bryn Mawr College, Naples Zoological Station and the University of Würtzburg during her short career.
One paper, written in 1905, won Stevens an award of $1,000 for the best scientific paper written by a woman. Her major sex determination work was published by the Carnegie Institution of Washington in the two part monograph, "Studies in Spermatogenesis".