Scientists claim to have identified two genetic variations that appear to be more common in gay men, adding a firm piece of evidence to the decades-old theory that homosexuality is at least partly reliant on a person’s genes.
The research, published in Nature, marks the first time individual genes have been identified that may help determine sexual orientation in boys and men. The team led by Alan Sanders at North Shore University, Illinois, found these genetic pointers by scanning the entire genomes of 1,077 gay and 1,231 straight men.
The scientists then looked for differences in the subjects’ DNA sequences, eventually settling on genetic areas where single nucleotide polymorphisms (SNP) variations indicate single-letter differences in DNA. This brought them to two genes: one on chromosome 13 and one on chromosome 14.
The first gene – called SLITRK6 – plays a role in part of the brain called the diencephalon, which has previously been shown in 1991 by neuroscientist Simon LeVay to differ in size between homosexual and heterosexual men. The second gene – called the thyroid stimulating hormone receptor (TSHR) gene – is, as the name suggests, linked to activity in the thyroid.
“Because sexuality is an essential part of human life – for individuals and society – it is important to understand the development and expression of human sexual orientation,” said Sanders.
“The goal of this study was to search for genetic underpinnings of male sexual orientation, and thus ultimately increase our knowledge of biological mechanisms underlying sexual orientation.”
The genome-wide association study (GWAS) chimes with research in the 1990s, such as that by Dean Hamer, who studied the link between chromosome band Xq28 and sexual orientation with men. Sanders emphasised, however, that a number of other factors could play a part in determining if someone is gay or straight, including a person’s environment, as well as a combination of other genes.
“There are probably multiple genes involved, each with a fairly low effect,” he said. “There will be men who have the form of gene that increases the chance of being gay, but they won’t be gay.”
The study is limited in its relatively small sample size, as well as the fact that most of the self-identifying gay or straight men come from European ancestry. Nevertheless, the GWAS does suggest the prospect of charting a host of genes involved in sexual orientation.
“What we have accomplished is a first step for GWAS on the trait, and we hope that subsequent larger studies will further illuminate its genetic contributions,” said Sanders.
“Understanding the origins of sexual orientation enables us to learn a great deal about sexual motivation, sexual identity, gender identity, and sex differences, and this and subsequent work may take us further down that path of discovery.”
The prospect of being able to discern a person’s sexuality based on their genes remains a controversial subject. Earlier this year a study claimed to be able to tell whether a man was gay or straight using an artificial intelligence system, based on a single photograph. There was uproar, with one prominent LGBT group slamming the research as “dangerous and flawed“, potentially putting lives in danger for gay men in countries where homosexuality carries the death penalty.
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