Happy birthday, Nettie Stevens, who was born today in 1861. She was an American biologist and geneticist who discovered that an organism's sex is determined by its chromosomes. . . . . . . . #womeninscience #wome. #science #womenscientist #male #female #genes #genetic #chromosome #biology #biologist #nature #life #animals #humans #discovery #discover #humanlife #living #xy #x #y