Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an American geneticist who discovered sex chromosomes. In 1905, soon after the rediscovery of Mendel's paper on genetics in 1900, she observed that male mealworms produced two kinds of sperm, one with a large chromosome and one with a small chromosome. When the sperm with the large chromosome fertilized eggs, they produced female offspring, and when the sperm with the small chromosome fertilized eggs, they produced male offspring. The pair of sex chromosomes that she studied later became known as the X and Y chromosomes. Early life Nettie Maria Stevens was born on July 7, 1861, in Cavendish, Vermont, to Julia (née Adams) and Ephraim Stevens. In 1863, after the death of her mother, her father remarried and the family moved to Westford, Massachusetts. Her father worked as a carpenter and earned enough money to provide Nettie and her sister, Emma, with a strong education through high school. Education During her education, Stevens was near the top of her class. She and her sister Emma were 2 of the 3 women to graduate from Westford Academy between 1872 and 1883. After graduating in 1880, Stevens moved to Lebanon, New Hampshire to teach high school zoology, physiology, mathematics, English, and Latin. After three years, she returned to Vermont to continue her studies. Stevens continued her education at Westfield Normal School (now Westfield State University) She completed the four-year course in two years and graduated with the highest scores in her class. Seeking additional training in sciences, in 1896, Stevens enrolled in newly established Stanford University, where she received her B.A. in 1899 and her M.A. in biology in 1900. She became increasingly focused on histology after completing one year of graduate work in physiology under Oliver Peebles Jenkins and his former student, and assistant professor, Frank Mace MacFarland. After studying physiology and histology at Stanford, Stevens enrolled in Bryn Mawr College to pursue her Ph.D. in cytology. She focused her doctoral studies on topics such as regeneration in primitive multicellular organisms, the structure of single celled organisms, the development of sperm and eggs, germ cells of insects, and cell division in sea urchins and worms. During her graduate studies at Bryn Mawr, Stevens was named a President's European Fellow and spent a year (1901–02) at the Zoological Station in Naples, Italy, where she worked with marine organisms, and at the