Stevens, Nettie Maria (1861–1912)

US biologist whose experiments with a species of beetle showed that sex was determined by a specific chromosome. This was the first direct evidence that the units of heredity postulated by Austrian biologist Gregor Mendel were associated with chromosomes.

Stevens was born in Cavendish, Vermont, and worked as a librarian; not until the age of 35 did she begin to study at Stanford University, moving to Bryn Mawr College for her PhD. She spent research periods at marine and zoological laboratories in Europe, and was an associate professor at Bryn Mawr from 1905.

Stevens studied regenerative processes in lower invertebrates.

From there she moved on to working on the development of the roundworm, examining its regenerative properties after exposure to ultraviolet radiation, and showed that even in very early embryonic life, cells were restricted in their regenerative capabilities.