There’s no such thing as a ‘gay gene’ finds largest study of sexuality
small effect. So genetic studies need to include genomes from at least hundreds of thousands of people to capture them.
“Research in the past has not had anywhere close to the amount of data necessary,” says Robbee Wedow at the Broad Institute of MIT and Harvard.
Wedow and his colleagues collected data from two huge sources: the UK Biobank, which holds the genomes of 500,000 people aged between 40 and 70, and 23&Me, a company that has sold consumer genetic tests to over 5 million people. Both organisations ask the genomes’ owners to complete questionnaires, including on their sexual behaviour.
Read more: What do the new ‘gay genes’ tell us about sexual orientation?
This gave the team genetic data and information on the corresponding sexual behaviour for around 477,000 people. “This is an enormous leap in terms of sample size,” says John Perry at the University of Cambridge, who co-authored the study.
First, the team looked at the similarity of the genomes of people who said they had sex with a person of the same sex. This helped them estimate how much of sexuality is linked to genetics – if a trait is largely determined by genetics, you would expect people with that trait to have similar genomes. The team found that around a third of the variation in this behaviour can be explained by a person’s genes.
The team then took all the genetic data they had and looked for particular genes that correlated with whether a person was gay. They identified five genetic variants that seem to be associated with same-sex sexual behaviour to a statistically significant degree. Two of these were found in both men and women, but two were specific to men and one was specific to women. It isn’t clear what these genes might do to affect sexuality. One is known to be associated with the sense of smell, and another has been linked to male pattern balding.
But crucially, all five of these genes only had a small effect and were far from being predictive of a person’s sexuality. Given that the team had already found that around a third of the variation in this behaviour can be explained by a person’s genes, this suggests there are many other genes that contribute at a very low level.
This sheds light on an evolutionary question surrounding homosexuality: how could same-sex sexual behaviour have endured if it doesn’t ensure genes are passed to the next generation?
“As a biologist and a gay man, I’m very curious about this,” says Jeremy Yoder at California State University Northridge. “If it’s many genes, with many, many small effects, the capacity of natural selection to purge those variants from the population goes down proportionally,” he says. “You can have people who are completely straight who are nevertheless carrying lots of variants that, if they came together in another genetic background, would maybe predispose someone to same-sex sexual activity.”
Some worry that, in the wrong hands, findings from such research could be used to create a genetic test for homosexuality. This could be problematic, because the LGBTQ+ community is subject to prejudice and discrimination – in some countries, homosexuality is punishable by death.
But such a test would be impossible, says Wedow. Based on their data, Wedow and his colleagues created what is known as a polygenic score – a method of assigning a weight to each of the genetic factors identified that appear to be linked to same-sex sexual behaviour, even if it isn’t statistically significant. But this completely failed to predict a person’s sexuality.
That doesn’t mean that others won’t try. Direct-to-consumer tests use similar polygenic scores, some with a similar predictive power, says Melinda Mills at the University of Oxford. “People still buy the services and believe in them,” she says.
The team say it was only a matter of time before someone ran a study like this – the data was out there. They also consulted with LGBTQ+ groups throughout the project. “This is the best way you could possibly do this study,” says Yoder. “[But] once this is published, it’s out of their hands.”
At the same time, research like this can bring comfort to some members of the LGBTQ+ community, says Wedow, who is a cis gay man who grew up in a conservative part of the US. “Our results really underscore that this behaviour is a normal part of human variation,” he says.
“Because we have so many different [genetic regions] involved, it implies that there are multiple genetic ways to end up queer,” says Yoder. “And that’s kind of a poetic idea.”