LONDON (AP)-South African runner Caster Semenya has been ordered to undergo a gender verification test, to prove she rightfully won the gold medal in the women's 800 meters at the world championships in Berlin on Wednesday.

Proving one's gender isn't always so easy. Aside from the obvious physical signs, chromosomes usually determine whether a person is male or female. Males are born with XY chromosomes while females have two X chromosomes.

About 1 percent of people are born with some kind of sexual ambiguity, sometimes referred to as intersexuality. These people may have the physical characteristics of both genders, a chromosomal disorder, or simply have ambiguous features. People who have both male and female organs are hermaphrodites.

Until 1999, the International Olympic Committee analyzed chromosomes from saliva samples to confirm the gender of female competitors and prevent men from masquerading as women. Other sports organizations have called the tests unreliable. The tests were scrapped before the 2000 Sydney Games.