Nettie Stevens discovered XY sex chromosomes. She didn’t get credit because she had two X’s.
Wilson still believed environmental factors played a role in determining sex. Stevens said it was purely the chromosomes. Neither view could be confirmed absolutely at the time of the discovery.
.. It’s a classic case of the “Matilda effect,” a term named after the abolitionist Matilda Gage. The effect is the phenomenon that women’s accomplishments tend to be co-opted, outright stolen, or overshadowed by those of male peers. Stevens is far from the only woman scientist to have this happen to her: Rosalind Franklin, whose work was crucial to the discovery of DNA, got similarly sidelined later in the 20th century.