Nettie Stevens — who’s honored in today’s Google Doodle — lived at the end of the 1800s, at a time when women mostly married and stayed home, or were teachers or nurses if they wanted to work. Instead, Stevens became a research scientist and her discoveries changed genetics forever.

Stevens was interested in understanding how a person, or animal, is born male or female. She began studying chromosomes — thread-like structures that keep DNA molecules inside the nucleus of cells. And she became one of the first scientists to find that sex is determined by a particular combination of the sex chromosomes X and Y.

She published her research, conducted on the yellow mealworm, in 1905. It eventually evolved into the XY sex-determination system we know today: The father’s sperm, which can carry either X or Y chromosomes, determines the sex of the offspring. Before Stevens’ work, scientists thought that the mother or the environment determined if a child was born male or female.

Her theory wasn’t accepted immediately by the scientific community, according to Nature. But that same year, another researcher, Edmund Wilson of Columbia University, also reached her same conclusion. And eventually, it became widely accepted.

Stevens was born in Vermont 155 years ago today. She went on to study at Stanford University and got her PhD in biology at Bryn Mawr College in Pennsylvania in 1903. She kept teaching and doing research on the chromosome makeup of various insects until her death of breast cancer at the age of 50.

Correction July 7th, 1:30PM ET: A previous version of this article said that Nettie Stevens died of breast cancer at the age of 45. She was 50. The article has been changed.