Okay, so it's not exactly that simple; but researchers have found a gene that keeps females being...well, female. FOXL2, which was known to be involved with ovary growth, apparently keeps a woman's body from turning into a man. Switch off FOXL2, and ovaries will start turning into testes and testosterone-producing cells will develop. Just think what this says about the human gender binary, or future less invasive methods for people undergoing gender reassignment.
*obligatory SCIENCE IS COOL flailing*
Sorry, that's all the commentary you get, since my tonsils currently feel like they're turning into...uh...inflamed...spikey...polka-dotted sadness*. Which I guess is better than testicles? Check out the article for a more detailed explanation, or for the genetically-inclined, read the original paper here.
*My wit fails me when I don't feel well. I apologize.