Scientists in the US looked at the whole genomes of around 1000 homosexual men and 1200 heterosexual men, finding several stretches of DNA where there were more differences between the groups than would be expected by chance. But the researchers caution their results are “best described as speculative”.
The authors of the study, published in Scientific Reports, were looking for differences in single letters of DNA between the groups of homosexual and heterosexual men. The two regions of the genome with the most differences were near genes whose roles might be linked to sexual orientation.
One gene is linked to the development of a brain region which can differ in men depending on their sexual orientation while the other gene is linked to thyroid function, which has previously been linked to male homosexuality, according to the authors.
“It is well established from twin and family studies that sexual orientation is partly heritable – that is, that whether someone is straight or bisexual or gay depends partly on their genetic makeup,” Dr Brendan Zietsch of the University of Queensland told the Australian Science Media Centre.
“Because there are so many bits of DNA (millions) that vary among people, finding the relevant bits is like finding a needle in a haystack,” continued Zietsch.
Genome-wide association studies, such as this one, try to find variations in DNA that are linked to a particular trait. But according to Zietsch, this study was very small compared to similar studies, which often include more than 100,000 people.
Dr Nina McCarthy of the University of Western Australia cautioned that “findings from small studies are less likely to be robust and less likely to be generalizable compared to large studies”.
“As this study was carried out in European men, we do not know whether the findings will apply to homosexuality in women, or even to homosexuality in non-European men.”
According to McCarthy, with any genome-wide association study “it’s really important to appreciate that association does not imply causation”.
“All that is required to see a genetic association in this study is for slightly more homosexual men to carry the genetic variant than heterosexual men, and many times this will simply be due to chance.”
Although genome-wide association studies may be able to give clues about which genes contribute to the biology of human sexuality, just as with most complex human traits, it’s unlikely to be down to a single gene, explained McCarthy.
According to Dr Ilan Dar-Nimrod of the University of Sydney, sexual orientation has been a “constant area of investigation” since 1993, when a study was published linking male homosexuality to a region on the X chromosome.
“At the time, [the region] was dubbed as the ‘gay gene’ in popular media, despite being a region that contains multitude of genes, and despite other studies failing to replicate the finding,” he explained.
Dar-Nimrod said that advocates for gay community “have made the argument that if there is a genetic underpinning for sexual orientation, then a person’s same-sex attraction will be viewed as unchangeable, inborn, and therefore beyond any reproach from viewing such attraction as a choice”.
However, this line of argument is a “double-edged sword” he said, because it can draw “clear divides between groups of people”.
“In this instance, it results in a heterosexual person feeling a clear ‘us’ and ‘them’ that leads to distancing from ‘them’ (i.e. gay people). Such a feeling may fuel prejudice.”
Dar-Nimrod expressed hope that “sensationalising headlines such as ‘scientists discover gay genes’ are a thing of the past,” noting that the discourse leading up to the plebiscite on same-sex marriage in Australia reflected a “movement towards seeing sexual choices among consensual adults as their own business, rather than a moral issue.”
Prepared by the Australian Science Media Centre and used here with permission.