I first heard of Nettie Stevens (1861-1912) when I was studying Mendelian genetics in college. She is well known to those of us who studied that branch of genetics (what I call “real” genetics!), but has been largely forgotten despite her immense contribution to the field. What was it? Nothing less than discovering that sex, at least in the species she was studying, was determined by chromosome constitution. And that turned out to be the case for many, many animal species (and even some plants!).

After years of of training, Stevens settled at Bryn Mawr College, and it was there that, while studying the mealworm Tenebrio molitor, she realized that while the cells of females had near pairs of regular-sized chromosomes, males had, among the regular pairs, one regular-sized chromosome paired with a tiny one. Those proved to be the X and Y, respectively, just as in humans. (In some species, like birds and butterflies, the females have the big and little chromosome, called Z and W respectively, and it is females that are heterogametic. In mammals and most insects, it’s the males who are heterogametic, which is what Stevens found.)

Stevens published her result in 1905, the same year that geneticist and cytologist E.B. Wilson also described the chromosomal basis of sex determination.  Today Wilson is largely given the credit for this discovery, but that’s solely because he was more famous than Stevens—and because he was a man, possessor of a Y.

It’s also typical of that era, and of the low status of women in science, that Stevens didn’t gain a regular university research position. It’s ironic that this is almost certainly because she was XX rather than XY!  Eventually she was offered such a position, but died of breast cancer before she could accept it.  She was only 50 years old.

You can read more about Stevens’s work on sex determination, detailed in a journal article, here.

Here’s today’s Doodle of Stevens, clearly based on the photo below it:

There aren’t many pictures of her, but I like this one: