SCIENTISTS have found the strongest evidence yet for the existence of genes that increase the chance of a man being gay. The study, which involved more than 400 gay brothers, identified two small areas on the male genome that appear to be linked to sexuality.
Michael Bailey, from Northwestern University in Chicago, who carried out the research, said: “Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play and we found evidence for two sets that affect whether a man is gay or straight.”
In the study, presented yesterday at the American Association for the Advancement of Science’s annual conference in Chicago, scientists took blood from 409 gay brothers and heterosexual members of their family.
On average, siblings share 50 per cent of their DNA, but ought to have a higher chance of sharing any genes involved in determining sexuality.
In the 1990s, the American geneticist Dean Hamer identified an area that appeared to influence male sexuality on the X chromosome, which men inherit from their mothers.
However, the results have remained controversial.
The latest research confirmed that this region on the X chromosome, known as Xq28, is more likely to be shared by the gay pairs of brothers than by the brothers and their other siblings.
The study also identified a second genetic region, on Chromosome 8, which also appeared to predict whether a man would be homosexual.
However, while the pairs of brothers shared gene variants in these regions, there were not individual genes that stood out across all the participants in the study.
The findings suggested that overall a man’s sexuality depends about 30 to 40 per cent on genetic factors, while the rest depends on environmental factors, such as the hormones a baby is exposed to in the womb.
“It is not completely determinative, there are certainly other environmental factors involved,” Professor Bailey said, adding that this did not imply that social factors or personal choice played a role.
Qazi Rahman, a psychologist from King’s College London, said that the study was the most compelling evidence so far of a genetic influence for male sexuality, but added that the term “gay gene” should be avoided: “It’s a bit like making the proverbial ‘haystack’ just that bit more smaller so you can find the needles – there is no such thing as one gay gene, there are most likely many genes that influence sexual orientation.”
That means it would not be possible to develop an accurate test to determine sexuality before birth, for instance.
“The possibility of testing is not the reason why we are conducting this research,” Professor Bailey said. No genes had been discovered that influenced female sexuality, he added.
Previously, homosexuality has been viewed as a “Darwinian paradox” because if male homosexuality were genetic, and gay men reproduced less than heterosexuals, the trait should eventually disappear from the population over time.
This has led some critics to dismiss the genetic argument.
However, there is now some evidence that genes linked to male homosexuality could increase fertility in women. One study by Italian scientists found that female relatives of gay men tended to have more children than those of straight men.
Richard Lane, of the campaign group Stonewall, said that while studies into the origins of homosexuality had yet to produce firm evidence, they did point to a biological root. He said: “The thing that’s consistent is that they all point to sexual orientation being something fundamental to a person rather than the lifestyle choice some opponents of equality repeatedly suggest.”
Comment: Apart from the fact that the “Darwinian paradox” indicating that Darwin’s theory of evolution is now outmoded, this study probably adds little to the argument that is not already known – intuitively at least – already.
But it does serve as confirmation and indicate this may be one of the main value of genetic studies, rather than seeing them as some sort of panacea for illness and disease.