Despite what one might think, it is not always easy to tell who is male and who is female.
In sporting events, officials who watch athletes produce a urine specimen usually can see whether genitals match their proclaimed sex. But that can still leave room for doubt, as with South African runner Caster Semenya.
The problem, said Dr. Joe Leigh Simpson, a pediatric geneticist at Florida International University, is that genetic or hormonal abnormalities can affect any organ system, including the gonads and external genitalia. When such anomalies occur, "it can produce confusion" because hormone levels and other aspects of physiology may not match appearance.
Moreover, there is "no single process" for determining sex because every case is different, he said. For years, sports authorities considered only the sex chromosomes: If they are XX, the athlete is female; XY and he is male. Technicians would swab the athlete's mouth to remove some cells, look at the sex chromosomes and make a determination.
But at the 1996 Atlanta Olympics, eight female athletes were determined to have XY chromosomes and were booted from the Games. Further studies, however, showed that they were physiologically female even though their genes said they were male, and they were reinstated.
Genes are only a blueprint, and sometimes nature doesn't follow the blueprint precisely. Take the examples of XY athletes who appear to be women.
At least five enzymes are required to synthesize testosterone, the hormone that produces most male characteristics. Occasionally one of those enzymes is defective. When that happens, the genitals are typically male, but tiny, the person doesn't have much body hair and is generally feminized. Such people are normally eligible to compete as women.
In other genetic males, the receptor that the testosterone binds to is defective and it doesn't matter how much testosterone is present. That male is classified as androgen insensitive, but the results are the same: feminization. Genitalia are typically female. Often, such people grow up as females and don't find out they have male genes until they don't get menstrual periods. They are often tall, slender and attractive.
Neither anomaly gives the person strength or endurance beyond that of a normal woman and subjects are allowed to compete as women.
The condition of being genetically female, or XX, but appearing male can often be traced to congenital adrenal hyperplasia, in which the adrenal glands produce excess testosterone. The woman may look like a boy with tiny male genitals, but once a month will pass blood. If the condition is caught early in life, doctors usually recommend surgery to create female genitalia. But many persons with the condition live normal lives as men.
Another possible cause for a disconnect between genetics and appearance is mosaicism, in which the individual has more than one set of chromosomes, in some cases certain cells could be XX and others XY. That occurs because of a faulty division at an early stage in the embryo, or if two embryos fuse, and can produce a variety of mixed sexual signals.
The ultimate determinant in sporting events: Does the abnormality give an unusual benefit to the female?