A pride march in Belgrade last month.
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. A 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.
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.
But Rice and others caution that the research is still preliminary and based on a small sample. 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 What causes homosexuality scientific size 10 times larger than his last year.
Twin studies suggested, moreover, that gene sequences can't be the full explanation. That's why some have suggested that epigenetics—instead of or in addition to traditional genetics—might be involved.
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 What causes homosexuality scientific methylation, in which a methyl group is attached to What causes homosexuality scientific DNA regions. Recent studies, however, have shown that some marks are passed on to the next generation.
In a 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. Such ideas inspired Tuck Ngun, a postdoc in Vilain's labto study the methylation patterns atregions 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. 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.
One gene is important for nerve conduction, whereas another has been implicated in immune functions.
To test how important the five regions are, the team divided the discordant twin pairs into two groups.
They looked at the associations between specific epi-marks and sexual orientation in one group, then tested how well those results could predict sexual orientation in the second group.
Just why identical twins sometimes end up with different methylation patterns isn't clear.
If Rice's hypothesis is right, their mothers' epi-marks might have been erased in one son, but not the other; or perhaps neither inherited any marks but one of them picked them up in the womb. In an earlier review, Ngun and Vilain cited evidence that methylation may be determined by subtle differences in the environment each fetus experiences during gestation, such as their exact locations within the womb and how much of the maternal blood supply each receives.
By Meredith Wadman Nov. By Jeffrey Mervis Nov. By Frankie Schembri Nov. By Gretchen Vogel Nov. How to contact the news team.