It is, per google, Women in Science Day (or it was yesterday when I wrote this, don’t @ me). Which gives me an extraordinarily long list of people to choose from. So who better than the woman who identified sex chromosomes in beetle sperm?
Today’s daily Moment of Science… the letters X and Y, brought to you by Dr. Nettie Stevens.
We take a lot of what we can do with genetics for granted now. We can sequence the genome of a virus in mere days, match a perpetrator to a stain on a dress, and even go on national television to instantly find out that you owe back child support. Science is lit.
But go back to the turn of the twentieth century and genetics are kinda like Bitcoin in the aughts; five people sorta had a clue what the vocab was, but nobody knew what it was going to turn into or how it would be used.
Enter Nettie Stevens, whose career took a meandering old time to launch on account of being a woman who was born in 1861. If you’ve ever felt like your career took some pains to start up, take solace in this. Stevens graduated from high school in 1880, it took until 1903 to complete her PhD and start work as a researcher at Bryn Mawr College, and she shaped the foundation of our understanding of genetics. So I’m pretty sure your life isn’t over if you don’t have a million followers on TikTok by the time you’re 23.
Think about where we were in history at the turn of the century with regards to genetics. Mendel’s research in the 1860s established that traits could be inherited, but his work had all but disappeared before enjoying a popular resurgence in scientific research around this time. Also, several scientists in the 1880s had confirmed that chromosomes… were a thing?
But we hadn’t put it all together yet. What was an inherited trait, what did you get from your environment, and what did chromosomes have to do with all of it?
Studying cytology and experimental morphology, Stevens got to work researching a variety of insect chromosomes. She found the chromosomes of the males and females of the species were different. The females had two chromosomes with an X shape, and males had one X shaped and a smaller one that seemed like it was missing a leg, which she dubbed the Y chromosome.
Under Nettie Stevens’ microscope, it looked like chromosomes were a hard link to sex determination, but did external factors have any influence over which combination of these chromosomes you got?
Stevens then examined beetle sperm under a microscope. As you do. She found that individual sperm come pre-packaged with either an X or a Y chromosome. As eggs carry just an X chromosome, it seemed this was the one variable at play.
Thus the XY sex determination system was discovered. It’s random chance determined by what sperm races down the birth canal the fastest. We now know of many other systems of sex determination through the animal kingdom, but let’s save haplodiploids and ZW sex determination for another day.
This was a long time ago in a scientific galaxy far far away. They weren’t even close to figuring out various intersex conditions and all the factors beyond chromosomes that go into determining gender. This was a giant leap forward in our understanding of what chromosomes could factor into. It would have blown some minds if you’d told them the SRY gene that’s typically found on the Y chromosome, the sole gene on the chromosome responsible for sex determination, can just pop onto an X chromosome once in a while, causing an XX male. Or that there are XXY men. And XXX women. And XY women. And women with just one X chromosome.
Shit’s complicated, and understanding chromosomal sex determination was a huge first step.
Nettie Stevens passed away at only 50 years of age to breast cancer, having published 38 papers, contributing incalculable amounts of knowledge to the scientific world in under a decade of work.
This has been your daily Moment of Science, suggesting that if you’re feeling blue about not changing the world by age thirty, you’ll be vaccinated and out of lockdown soon. Start a podcast.
“haplodiploids and ZW sex determination for another day” can’t wait.