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One gene is important for nerve conduction, whereas another has been implicated in immune functions.
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After several rounds of analysis-with the help of a specially developed machine-learning algorithm-the team identified five regions in the genome where the methylation pattern appears very closely linked to sexual orientation. Such ideas inspired Tuck Ngun, a postdoc in Vilain's lab, to study the methylation patterns at 140,000 regions in the DNA of 37 pairs of male identical twins who were discordant-meaning that one was gay and the other straight-and 10 pairs who were both gay. Specifically, they argued that inherited marks that influence a fetus's sensitivity to testosterone in the womb might "masculinize" the brains of girls and "feminize" those of boys, leading to same-sex attraction. In a 2012 paper, Rice and his colleagues suggested that such unerased epi-marks might lead to homosexuality when they are passed on from father to daughter or from mother to son. Recent studies, however, have shown that some marks are passed on to the next generation.
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Such "epi-marks" can remain in place for a lifetime, but most are erased when eggs and sperm are produced, so that a fetus starts with a blank slate. During development, chromosomes are subject to chemical changes that don't affect the nucleotide sequence but can turn genes on or off the best known example is methylation, in which a methyl group is attached to specific DNA regions. That's why some have suggested that epigenetics-instead of or in addition to traditional genetics-might be involved. For example, the identical twin of a gay man, despite having the same genome, only has a 20% to 50% chance of being gay himself.
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Twin studies suggested, moreover, that gene sequences can't be the full explanation. The discovery generated worldwide headlines, but some teams were unable to replicate the findings and the actual genes have not been found-not even by a team that vindicated Hamer's identification of Xq28 in a sample size 10 times larger than his last year. Researchers thought they were hot on the trail of "gay genes" in 1993, when a team led by geneticist Dean Hamer of the National Cancer Institute reported in Science that one or more genes for homosexuality had to reside on Xq28, a large region on the X chromosome. But Rice and others caution that the research is still preliminary and based on a small sample. The new work, from Eric Vilain's lab at the University of California (UC), Los Angeles, is "exciting" and "long overdue," says William Rice, an evolutionary geneticist at UC Santa Barbara, who proposed in 2012 that epigenetics plays a role in sexual orientation. It finds that epigenetic effects, chemical modifications of the human genome that alter gene activity without changing the DNA sequence, may have a major influence on sexual orientation. Yet actual "gay genes" have been elusive.Ī new study of male twins, scheduled for presentation at the annual meeting of the American Society of Human Genetics (ASHG) in Baltimore, Maryland, today, could help explain that paradox. Indeed, over the past 2 decades, researchers have turned up considerable evidence that homosexuality isn't a lifestyle choice, but is rooted in a person's biology and at least in part determined by genetics. "Baby, I was born this way," Lady Gaga sang in a 2011 hit that quickly became a gay anthem.