I used to ask my students how you tell if someone is male or female. I always got some giggles and a few eyerolls. It's obvious, right? Mind you, we mostly talked about this in human terms---determining sex throughout the animal kingdom isn't just a matter of innies and outies.
The International Olympic Committee has been concerned with this very question for a long time. The first issue was simply a concern of men dressing as women to compete for their countries---something solved by having athletes walk naked in front of a panel of doctors. This wasn't a particularly popular solution with athletes, so the IOC moved to a Barr Body test. In humans, women typcially have 2 "X" chromosomes and men have one. In women, one of the X chromosomes turns off in each cell, migrates to the side, and hibernates. (It's not the same one in each cell---hence patterns in calico cats, for example.) The IOC decided that anyone who had a Barr Body must be female.
But then the IOC found out that they were disqualifing men who were for all visible purposes male. They just happened to have an extra chromosome. Why were they men? Because a gene from the Y chromosome (the SRY gene) had been transferred to the X chromosome during sperm formation. There's lots of gene exchange (a/k/a "crossing over") happening during sperm and egg formation. Things sometimes get stuck in the wrong place or in the wrong way. No matter---the gene functioned as it should have, causing the embryo to be flooded with testosterone at the right time to make the child a male. Now the IOC determined that anyone with an SRY gene would be viewed as male. Here again, there were issues. If that gene doesn't kick on at the right time or is counteracted in other ways, the embryo will keep developing as a female.
It's not so easy, is it?
The recent news about the runner from India failing a gender test and being stripped of a medal made me think about all of this again. Why did she fail? Too many Y chromosomes. Physically, she is female. At the cellular level she has some DNA that should have produced a male when she was in the womb, but it didn't happen. To me, there is something deeply unfair about disqualifying someone because their dad had a mutant sperm. This isn't like doping or cross-dressing. She didn't choose her chromosomes---it isn't anything she has any more control over than any other trait. It's discrimination at the molecular level.
I tend to think of gender as something of a continuum, not an either/or. At one end, you have the XX fertile female...and at the other, an XY fertile male. In the middle? People missing a part or having something extra. Gender is really left to the individual to identify---not society. I understand that the IOC is looking to keep countries honest and ensure a level playing field in a variety of ways, but I worry about the precedent they may be setting. How many of us are unknowingly carrying all sorts of genetic oddities that might mean an employer, insurer, or group would prevent us from participating?