Nettie Stevens(1861 — 1912)

Nettie Stevens only became a researcher late in life. After high school, lacking the means to continue, she taught and worked as a librarian for about fifteen years. It wasn't until she was 35, in 1896, that she finally enrolled at Stanford University to study biology: proof that a scientific calling can blossom long after adolescence.

In 1905, while observing the reproductive cells of the mealworm beetle (the flour worm) under the microscope, she noticed that the females had twenty large chromosomes, whereas the males had nineteen large ones and one small one. She then understood that it was this tiny chromosome — the future Y chromosome — that determined the male sex. A landmark discovery drawn from the patient observation of a simple pantry insect.

Her discovery was for a long time credited mainly to men: the famous biologist Edmund B. Wilson had reached similar conclusions almost at the same time, and Thomas Hunt Morgan later received the Nobel Prize for his work in genetics. Nettie Stevens's name, meanwhile, fell into oblivion — an example often cited today of the way women in science are made invisible.

To complete her training, she crossed the Atlantic and worked at the Naples Zoological Station, then at the University of Würzburg alongside Theodor Boveri, one of the fathers of the chromosomal theory of heredity. In just eleven years of her career, she published some forty scientific papers.

Her talent was nonetheless recognized during her lifetime: in 1905, she received the Ellen Richards Prize, awarded for scientific research carried out by a woman. Thomas Hunt Morgan, her former professor, praised in her obituary her keen sense of observation and her exemplary rigor.