It鈥檚 time to throw out the idea of a 鈥済ay gene鈥�. According to the largest ever genetic study of sexual behaviour, there are probably a large number of genes that collectively influence sexuality, each with tiny effects.
Previous studies have identified individual genes that may influence how sexual orientation develops in boys and men. But studies like this have always been relatively small. Most human traits seem to be influenced by hundreds or thousands of genes, each with a small effect. So genetic studies need to include genomes from at least hundreds of thousands of people to capture them.
鈥淩esearch in the past has not had anywhere close to the amount of data necessary,鈥� says Robbee Wedow at the Broad Institute of MIT and Harvard.
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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鈥� owners to complete questionnaires, including on their sexual behaviour.
This gave the team genetic data and information on the corresponding sexual behaviour for around 477,000 people. 鈥淭his is an enormous leap in terms of sample size,鈥� 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 鈥� 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鈥檚 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鈥檛 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鈥檚 sexuality. Given that the team had already found that around a third of the variation in this behaviour can be explained by a person鈥檚 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鈥檛 ensure genes are passed to the next generation?
鈥淎s a biologist and a gay man, I鈥檓 very curious about this,鈥� says Jeremy Yoder at California State University Northridge. 鈥淚f it鈥檚 many genes, with many, many small effects, the capacity of natural selection to purge those variants from the population goes down proportionally,鈥� he says. 鈥淵ou 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.鈥�
Discrimination concerns
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 鈥� 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 鈥� 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鈥檛 statistically significant. But this completely failed to predict a person鈥檚 sexuality.
That doesn鈥檛 mean that others won鈥檛 try. Direct-to-consumer tests use similar polygenic scores, some with a similar predictive power, says Melinda Mills at the University of Oxford. 鈥淧eople still buy the services and believe in them,鈥� she says.
The team say it was only a matter of time before someone ran a study like this 鈥� the data was out there. They also consulted with LGBTQ+ groups throughout the project. 鈥淭his is the best way you could possibly do this study,鈥� says Yoder. 鈥淸But] once this is published, it鈥檚 out of their hands.鈥�
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. 鈥淥ur results really underscore that this behaviour is a normal part of human variation,鈥� he says.
鈥淏ecause we have so many different [genetic regions] involved, it implies that there are multiple genetic ways to end up queer,鈥� says Yoder. 鈥淎nd that鈥檚 kind of a poetic idea.鈥�
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