A study of gay men in the US has found fresh evidence that male sexual orientation is influenced by genes. Scientists tested the DNA of 400 gay men and found that genes on at least two chromosomes affected whether a man was gay or straight.
A region of the X chromosome called Xq28 had some impact on men's sexual behaviour – though scientists have no idea which of the many genes in the region are involved, nor how many lie elsewhere in the genome.
Another stretch of DNA on chromosome 8 also played a role in male sexual orientation – though again the precise mechanism is unclear.
Researchers have speculated in the past that genes linked to homosexuality in men may have survived evolution because they happened to make women who carried them more fertile. This may be the case for genes in the Xq28 region, as the X chromosome is passed down to men exclusively from their mothers.
Michael Bailey, a psychologist at Northwestern University in Illinois, set out the findings to the annual meeting of the American Association for the Advancement of Science in Chicago on Thursday. "The study shows that there are genes involved in male sexual orientation," he said. The work has yet to be published, but confirms the findings of a smaller study that sparked widespread controversy in 1993, when Dean Hamer, a scientist at the US National Cancer Institute, investigated the family histories of more than 100 gay men and found homosexuality tended to be inherited. More than 10% of brothers of gay men were gay themselves, compared to around 3% of the general population. Uncles and male cousins on the mother's side had a greater than average chance of being gay, too.
The link with the mother's side of the family led Hamer to look more closely at the X chromosome. In follow-up work, he found that 33 out of 40 gay brothers inherited similar genetic markers on the Xq28 region of the X chromosome, suggesting key genes resided there.
Hamer faced a firestorm when his study was published. The fuss centred on the influences of nature and nurture on sexual orientation. But the work also raised the more dubious prospect of a prenatal test for sexual orientation. The Daily Mail headlined the story "Abortion hope after 'gay genes findings' ". Hamer warned that any attempt to develop a test for homosexuality would be "wrong, unethical and a terrible abuse of research".
The gene or genes in the Xq28 region that influence sexual orientation have a limited and variable impact. Not all of the gay men in Bailey's study inherited the same Xq28 region. The genes were neither sufficient, nor necessary, to make any of the men gay.
The flawed thinking behind a genetic test for sexual orientation is clear from studies of twins, which show that the identical twin of a gay man, who carries an exact replica of his brother's DNA, is more likely to be straight than gay. That means even a perfect genetic test that picked up every gene linked to sexual orientation would still be less effective than flipping a coin.
While genes do contribute to sexual orientation, other multiple factors play a greater role, perhaps including the levels of hormones a baby is exposed to in the womb. "Sexual orientation has nothing to do with choice," said Bailey. "We found evidence for two sets [of genes] that affect whether a man is gay or straight. But it is not completely determinative; there are certainly other environmental factors involved."
Last year, before the latest results were made public, one of Bailey's colleagues, Alan Sanders, said the findings could not and should not be used to develop a test for sexual orientation.
"When people say there's a gay gene, it's an oversimplification," Sanders said. "There's more than one gene, and genetics is not the whole story. Whatever gene contributes to sexual orientation, you can think of it as much as contributing to heterosexuality as much as you can think of it contributing to homosexuality. It contributes to a variation in the trait."
Qazi Rahman, a psychologist at King's College London, said the results were valuable for further understanding the biology of sexual orientation. "This is not controversial or surprising and is nothing people should worry about. All human psychological traits are heritable, that is, they have a genetic component," he said. "Genetic factors explain 30 to 40% of the variation between people's sexual orientation. However, we don't know where these genetic factors are located in the genome. So we need to do 'gene finding' studies, like this one by Sanders, Bailey and others, to have a better idea where potential genes for sexual orientation may lie."
Rahman rejected the idea that genetics research could be used to discriminate against people on the basis of their sexual orientation. "I don't see how genetics would contribute more to the persecution, discrimination and stigmatisation of lesbian, gay, bisexual and transgender people any more than social, cultural or learning explanations. Historically, the persecution and awful treatment of LGBT groups has been because politicians, religious leaders and societies have viewed sexual orientation as 'choice' or due to poor upbringing."
Steven Rose, of the Open University, said: "What worries me is not the extent, if at all, to which our genetic, epigenetic or neural constitution and development affect our sexual preferences, but the huge moral panic and religious and political agenda which surrounds the question."
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