One step closer to tying homosexuality to the human genome

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The largest study yet of the genetic roots of homosexuality links sexual preference in men to two regions of the genome.

At least in men, homosexuality may be a function of genetics, according to a study of more than 400 pairs of gay brothers. The research, published yesterday (November 18) in Psychological Medicine, confirms the role of a stretch of the X chromosome in determining sexual preference in men, a finding first suggested more than 20 years ago. Geneticist Dean Hamer, scientist emeritus at the National Institutes of Health, published a study in 1993 that proposed that Xq28, a region of the X chromosome, might play a role in determining whether a man was gay. “When you first find something out of the entire genome, you’re always wondering if it was just by chance,” Hamer told Science of the new study, adding that the research “clarifies the matter absolutely.”

Hamer, who recently wrote an opinion piece in The Scientist about the responsibilities of researchers who study sexual orientation, only studied 38 pairs of brothers in his 1993 study, but he told New Scientist that he sees the new paper as confirmation of his work. “Twenty years is a long time to wait for validation, but now it’s clear the original results were right,” he said. “It’s very nice to see it confirmed.”

But as was the case in 1993, not all researchers are convinced that science is homing in on the biological roots of sexual preference. Even the senior author on the Psychological Medicine paper, Northwestern University psychologist Michael Bailey, had his doubts. “I thought that [Hamer] did a fine but small study,” he told Science. “If I had to bet, I would have bet against our being able to replicate it.”

But when Bailey, who also wrote an opinion piece for The Scientist on the search for the biological roots of homosexuality, and his colleagues analyzed single nucleotide polymorphisms (SNPs) in the DNA of the brother pairs, they found five SNPs that were commonly shared by all the gay men. And those SNPs clustered in the Xq28 region on the X chromosome and in the 8q12 region of chromosome 8.

Bailey and his colleagues are now working on a genome-wide association study to confirm the results of their genetic linkage research. This analysis, which will include DNA samples from more than 1,000 additional gay men, may narrow the search for genetic signals for homosexuality down to individual genes. “It looks promising for there being genes in both of these regions,” Bailey told Science. “But until somebody finds a gene, we don’t know.”

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