Semenya may well prove to be a straightforward female. Another possibility is that she has a rare birth defect such as androgen insensitivity syndrome, in which women possess a male Y chromosome, but their bodies don't respond to male sex hormones, causing them to develop female genitalia.
Compulsory gender testing of female athletes competing at the 1996 Olympic games in Atlanta, Georgia, identified eight out of 3387 tested as possessing a Y chromosome – and all were subsequently allowed to compete as women. After this the International Olympic Committee abandoned gender-verification testing.
Conditions like androgen insensitivity may in fact be disadvantageous. "Their bodies are insensitive to testosterone, whereas in the case of normal women they're perfectly responsive to testosterone," says Genel. "There's certainly no reason why you would preclude a female athlete simply on the basis of her possessing a Y chromosome."