text: Two gene variants have been found to be more common in gay men, adding to mounting evidence that sexual orientation is at least partly biologically determined. How does this change what we already knew?
  Didn’t we already know there were “gay genes”?
  We have known for decades that sexual orientation is partly heritable in men, thanks to studies of families in which some people are straight and some people are gay. In 1993, genetic variations in a region on the X chromosome in men were linked to whether they were heterosexual or homosexual, and in 1995, a region on chromosome 8 was identified. Both findings were confirmed in a study of gay and straight brothers in 2014. However, these studies didn’t home in on any specific genes on this chromosome.
  What’s new about the latest study?
  For the first time, individual genes have been identified that may influence how sexual orientation develops in boys and men, both in the womb and during life. Alan Sanders at North Shore University, Illinois, and his team pinpointed these genes by comparing DNA from 1077 gay and 1231 straight men. They scanned the men’s entire genomes, looking for single-letter differences in their DNA sequences. This enabled them to home in on two genes whose variants seem to be linked to sexual orientation.
  What genes did they find and what do they do?
  One of the genes, which sits on chromosome 13, is active in a part of the brain called the diencephalon. Interestingly, this brain region contains the hypothalamus, which was identified in 1991 as differing in size between gay and straight men. This was discovered by neuroscientist Simon LeVay, who says he is excited that the gene discovery seems to fit with what he found.
  Other research has found that this gene, called SLITRK6, is active in the hypothalamus of male mice fetuses a few days before they are born. “This is thought to be a crucial time for sexual differentiation in this part of the brain,” says LeVay. “So this particular finding is a potential link between the neuroanatomy and molecular genetics of sexual orientation.
  What is the other gene?
  This gene is found on chromosome 14 and is mainly active in the thyroid, but also the brain. Called TSHR, it makes a type of receptor protein that recognises and binds to a hormone that stimulates the thyroid. In this way, the gene plays an important role in controlling thyroid function.
  The fact that TSHR seems to be involved in sexual orientation fits with evidence that thyroid function seems to be linked to sexuality. For instance, TSHR function is disrupted in a genetic condition called Grave’s disease, which causes the thyroid gland to become over-active, accelerating metabolism and leading to weight-loss. Grave’s disease is more common in gay than straight men, and some research suggests that gay men tend to be thinner – which might possibly be a result of thyroid overdrive.
  Are all men who have the “gay” variants of these genes gay?
  No, says Sanders, because many other factors play a role, including the environment. “There are probably multiple genes involved, each with a fairly low effect,” he says. “There will be men who have the form of gene that increases the chance of being gay, but they won’t be gay.”
  Because many genes and other factors seem likely to play a role in sexual orientation, this may explain why some people are bisexual or see sexual orientation as a spectrum.
  What about women who are gay? Are there “lesbian genes”?
  Our biological understanding of homosexuality in women lags behind. Some researchers say this is partly because women who have sex with women tend to be more fluid in their sexual orientation.
  There have been studies suggesting that there is a genetic element to homosexuality in women, but more research has been done in men, says Sanders.
  Why should we care about the genetics of being gay?
  The latest findings open the prospect to identifying the whole pathway of genes involved in both homosexual and heterosexual orientation, says Dean Hamer at the US National Institutes of Health, who led the study that pinpointed chromosome X back in 1993. “It adds yet more evidence that sexual orientation is not a ‘lifestyle choice’. But the real significance is that it takes us one step closer to understanding the origins of one of the most fascinating and important features of human beings.”
  Read more: Evolution myths: Natural selection cannot explain homosexuality
  Nature Scientific Reports DOI: 10.1038/s41598-017-15736-4