The controversy over South African athlete Caster Semenya's gender has given the public a view into the complexities of gender. At first blush, the issue should be fairly straightforward: a person is either a male (with an X and a Y chromosome) or a female (with two X chromosomes). But the reality is that a number of conditions can blur the gender line.

After her 800-meter final on August 19 at the World Athletics Championships in Berlin, the International Association of Athletics Federations announced that they had asked Semenya to undergo tests to verify that she was female, with IAAF spokesman Nick Davies describing the tests as "extremely complex, difficult," according to the journal Nature. (Scientific American is part of the Nature Publishing Group.)

Some people with two X chromosomes can develop masculine characteristics, whereas others with one X and one Y chromosome never develop masculine characteristics, Nature reports. Still, others (most notably, males who are XXY) defy conventional thinking of gender along the lines of XX females and XY males.

Some people with two X chromosomes have medical conditions that elevate androgen levels (which stimulate or control the development and maintenance of masculine characteristics); other people born XY fail to develop as men because of androgen insensitivity syndrome. Whereas XX individuals with plenty of androgens develop male characteristics, XY individuals who are not sensitive to it may grow up with female characteristics. This androgen-insensitivity makes gaining an athletic advantage through these conditions unlikely in most cases, Myron Genel, a pediatrician and expert in sexual development disorders at Yale University, told Nature.

About one in 4,500 babies show ambiguous genitalia at birth, such as a clitoris that looks like a penis, or vice versa, Scientific American reported in a 2007 article. In that story, geneticist Eric Vilain of the University of California, Los Angeles, noted that, lacking the Y chromosome, an embryo will follow the "default" genetic pathway that leads to ovary development, although "antimale" genes are required to make functioning ovaries.

The controversy has also spotlighted the taboos associated with someone who might share both male and female characteristics. (The IAAF has asked Semenya to undergo a number of complex gender tests, according to The Los Angeles Times, so any judgments about her gender are premature at this point.)

Semenya's case is not without precedent. At the 1996 Olympics Games in Atlanta, eight female athletes were determined to have XY chromosomes and were not allowed to compete, The Los Angeles Times reports, adding that further studies showed that they were physiologically female even though their genes said they were male, and they were reinstated. The Times article includes several examples of how genetics and gender don't always match.

Image of Semenya © Erik van Leeuwen