Daughters inherit many things from their fathers, but a select few get something unusual—a Y chromosome. Women typically have two X chromosomes while men have an X and a Y, but some XY people are born with female genitals and a uterus. They’re almost always raised as girls from birth, and their hidden Y chromosome only becomes obvious during puberty. That’s because they don’t develop working ovaries, and without these organs providing a flood of hormones, they don’t menstruate, grow body hair, or develop larger breasts on their own. They’re also sterile.

This condition, known as Swyer syndrome, is often caused by changes to SRY, a gene on the Y chromosome that acts as a master switch for maleness. Human