On August 7, 1908, Nettie Stevens published her findings on the role of chromosomes in sex determination in the Journal of Experimental Zoology. She had been studying Tenebrio molitor — the mealworm beetle — for two years. In the beetle's cells, she had noticed something consistent: females had twenty large chromosomes; males had nineteen large chromosomes and one small one. The small chromosome determined male sex.

She called it the "accessory chromosome." We now call it the Y chromosome.

Stevens was born in 1861 in Cavendish, Vermont. She worked as a teacher and librarian for fifteen years before she could afford to attend Stanford, where she earned a B.S. and M.S. in biology. She completed her Ph.D. at Bryn Mawr at forty-two, under the direction of Thomas Hunt Morgan, who would later win the Nobel Prize in Physiology or Medicine — in part for work on chromosomes and heredity that built on Stevens's discoveries.

Her 1905 paper at the Carnegie Institution, Studies in Spermatogenesis, established the chromosomal basis of sex determination in organisms. Edmund Wilson, working independently at Columbia, published similar findings the same year. The discovery was attributed jointly to them for decades — and then, in most textbook treatments, the name Stevens was dropped and Wilson's name remained.

Morgan, who supervised her Ph.D. and published extensively on genetics and chromosomes after her death, wrote an obituary for her that described her as a technician with diligent methods and good instincts. He did not describe her as the person who had found the sex chromosome. His Nobel lecture in 1933 did not mention her name.

Stevens died of breast cancer in 1912, at fifty years old, having been appointed to a research position at Bryn Mawr — an appointment created for her specifically, because there was no official faculty line for a woman — two months before she died. She never received the position's salary. She died before she could occupy the role.