A handy explainer for the oft-cited (but rarely elucidated) factoid that every single person on Earth was once female — or, at a minimum, gender-neutral.
Between 1 in 500 and 1 in 1000 men have an extra X chromosome, a condition known as Klinefelter's Syndrome. In mice, these extra female genes have an unexpected effect, as the second X chromosome actually makes mice more masculine.
A few years back, we learned that the Y chromosome was essentially rotting, shedding hundreds of its genes over the last 300 million years. That isn't wrong, but it turns out reports of the Y chromosome's eventual extinction were premature.
There's a longstanding belief that, on average, women are healthier than men, and with good reason. Women live longer, and studies reveal women fight off disease better than their male counterparts. But where does this advantage come from? Turns out it's all thanks to some microRNA on the X-chromosome.