Nettie Stevens

Nettie Stevens was an American geneticist who provided key evidence that an organism's sex is determined by a specific pair of chromosomes. Studying the mealworm beetle in 1905, Stevens observed that males produced two visibly different types of sperm cells, one carrying a small chromosome she labeled Y and the other carrying a larger chromosome labeled X, while females carried only the X type, and she correctly concluded that which chromosome a sperm cell contributed at fertilization determined the offspring's sex. The geneticist Edmund Beecher Wilson published closely related findings the same year, and the discovery is now generally credited jointly to both researchers, though Stevens's account is considered to have stated the chromosomal determination of sex more directly. Stevens carried out her research at Bryn Mawr College after earning her doctorate there in 1903, and her work helped establish that inherited traits, including something as fundamental as sex itself, could be tied to specific, physically identifiable chromosomes.