The biology of sexual orientation has been one of the most vexing — and politically charged — questions in human genetics. For the first time, researchers have found associations between homosexuality and markers attached to DNA that can be influenced by environmental factors.
Twin studies and family trees provide strong evidence that sexual orientation is at least partly genetic. When one identical twin is gay, there is about a 20% chance that the other will be as well. But because this rate is not 100%, it is thought that environmental factors play a role as well. One of the best characterized is the ‘older brother effect’: the chance of a man being gay increases by 33% for each older brother he has. The reason is not clear, although one hypothesis holds that the mother’s immune system begins to react against male antigens and alter the fetus’s development.
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Source Credits: Sara Reardon in Nature