February 25, 2021 13:43 IST Updated: February 25, 2021 13:43 IST Updated: February 25, 2021 13:43 IST By studying the cell division in the male common mealworm, Nettie identified a large chromosome and a small chromosome – we now call these X and Y Share Article AAA By studying the cell division in the male common mealworm, Nettie identified a large chromosome and a small chromosome – we now call these X and Y Nettie Stevens (1861 – 1912) was an American geneticist who discovered that sex is determined by chromosome. Nettie Maria Stevens was born on July 7, 1861, in Cavendish, Vermont. The family moved to Westford, Massachusetts, after her mother’s death. In 1896, she joined the then newly-established Stanford University, earning her undergraduation and postgraduation degrees there. She received a Ph.D. in cytology (the study of structure and function of cells) from Bryn Mawr College in 1903. Her Ph.D advisor was the geneticist Thomas Hunt Morgan.