Last week, the International Olympic Committee announced it will determine whether athletes are eligible for women’s events by mandating a once-in-a-lifetime screening for the male-determining gene, SRY.

But this new rule raises many questions – about why “female” is being defined this way, whether there is evidence trans women have an advantage, and whether a “level playing field” in sport is even possible.

In humans and other mammals, the SRY gene determines sex in the early embryo. This gene kickstarts the development of testes and their production of androgens – male hormones (testosterone and its derivatives), which drive male development.

The SRY gene lies on the Y chromosome. Males have an X and a Y chromosome, whereas females have two X chromosomes.

Over the decades, sex tests changed from anatomical inspection to using microscopes to detect the second X in women or the Y chromosome in men. But testing was slow, and misdiagnosed athletes with sex chromosome variation.

So a test was developed to detect the SRY gene directly. This is the test the International Olympic Committee will use.

But it’s not as simple to determine “male” or “female” as you might think. The SRY gene activates a network of dozens of genes that promote testis development or block ovary formation. Variation in any of these genes can produce girls with SRY or boys with no SRY.

Indeed, the scientist who co-discovered the SRY gene has warned that this test misdiagnoses athletes with variant sex genes and chromosomes.

For example, some women have an inactive form of the SRY gene that does not induce testis development. Other women have a typical SRY gene, and testes that produce androgens, but have an inactive form of the molecule that activates androgens, so their........

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