American geneticist Nettie Stevens was born on July 7, 1861, in Cavendish, Vermont. Stevens changed biology forever when she discovered sex chromosomes—the long, intricate DNA molecules today universally known as the XY chromosomes—which dictate whether an organism develops as male or female.
Despite her world-altering discovery, Stevens was largely denied credit during her lifetime because a prominent male scientist, Edmund Beecher Wilson, published a similar finding at roughly the same time. While Wilson only looked at the male contribution to genetics, Stevens was actually the first to look at both sexes and correctly deduce the entire chromosome pairing system, though textbooks credited Wilson for decades.
Image via Wikimedia Commons, public domain
