Nettie Stevens

Pioneering geneticist who discovered sex chromosomes

Nettie Stevens (1861–1912) revolutionized genetics through her discovery of X and Y chromosomes in 1905, overturning centuries of misconception about gender determination. Working with mealworms at Bryn Mawr College, she demonstrated that sex is determined by specific chromosomes rather than environmental factors - a foundational concept in modern biology.

Her meticulous Microscopic Studies on Chromosome Behavior proved that males produce two types of gametes (X and Y), while females produce only X-bearing eggs. This contradicted her contemporary Thomas Morgan's initial theories, though he later acknowledged her breakthrough. Despite institutional sexism that limited her academic positions, Stevens published 38 papers that shaped early cytogenetics.

Stevens' work enabled later discoveries like sex-linked genetic disorders and IVF gender selection techniques. The American Philosophical Society now awards a medal in her name, recognizing her as one of science's unsung heroines.