Nettie Stevens (July 7, 1861 – May 4, 1912) was an American geneticist who discovered sex chromosomes, which later became known as the X and Y chromosomes. Using observations of insect chromosomes, Stevens discovered that, in some species, chromosomes are different between the sexes and when chromosome segregation occurs in sperm formation; this difference leads to outcomes of female versus male progeny. Her discovery was the first time that observable differences of chromosomes could be linked to an observable difference in phenotype or physical attributes (i.e., whether an individual is male or female). Stevens was one of the first American women to be recognized for her contribution to science. Most of her research was completed at Bryn Mawr College, where she expanded the fields of genetics, cytology, and embryology. This photograph of Stevens was taken in the 1900s.

Photograph credit: Carnegie Institution of Washington; restored by Adam Cuerden