In normal human development, when a zygote has XY, or male, chromosomes, the SRY—sex-determining region Y—gene on the Y chromosome “instructs” the zygote’s protogonads to develop as testes, rather than as ovaries. The testes then produce testosterone, which issues a second set of developmental instructions: for a scrotal sac to develop and for the testes to descend into it, for a penis to grow, and so on. But the process can get derailed. A person can be born with one ovary and one testicle. The SRY gene can end up on an X chromosome. A person with a penis who thinks he is male can one day find out that he has a uterus and ovaries. “Then, there is chromosomal variability that is invisible,” Anne Fausto-Sterling, the author of “Sexing the Body,” told me. “You could go your whole life and never know.”
All sorts of things can happen, and do. An embryo that is chromosomally male but suffers from an enzyme deficiency that partially prevents it from “reading” testosterone can develop into a baby who appears female. Then, at puberty, the person’s testes will produce a rush of hormones and this time the body won’t need the enzyme (called 5-alpha-reductase) to successfully read the testosterone. The little girl will start to become hairier and more muscular. Her voice may deepen, and her testes may descend into what she thought were her labia. Her clitoris will grow into something like a penis. Is she still a girl? Was she ever?
If a chromosomally male embryo has androgen-insensitivity syndrome, or A.I.S., the cells’ receptors for testosterone, an androgen, are deaf to the testosterone’s instructions, and will thus develop the default external sexual characteristics of a female. An individual with androgen-insensitivity syndrome has XY chromosomes, a vagina, and undescended testes, but her body develops without the ability to respond to the testosterone it produces. In fact, people with complete A.I.S. are less able to process testosterone than average women. Consequently, they tend to have exceptionally “smooth-skinned bodies with rounded hips and breasts and long limbs,” Dreger writes in “Hermaphrodites.”
People with incomplete A.I.S., on the other hand, could end up looking and sounding like Caster Semenya. Their bodies hear some of the instructions that the testosterone inside them is issuing. But that does not necessarily mean that they would have an athletic advantage.