For some people, the choice of a same-sex partner may be written in their DNA, this has been the main conclusion of a large study of more than 490,000 men and women in the United States, United Kingdom and Sweden. Taking into account that our sexual behavior is influenced by a multitude of factors, both environmental and genetic, researchers have discovered four genetic variants that occur most frequently in people with same-sex sexual partners. Two of the variants were specific to the choice of sexual partner relative only to men; the other two influence the choice of sexual partner for both men and women.
Together, the DNA differences explained only 8 to 12% of the heritability of having same-sex partners. “There is no gay gene, but non-heterosexuality is influenced by many genetic factors of small effect,” explained Andrea Ganna, a geneticist at the Broad Institute at MIT at the meeting of the American Society of Human Genetics.
The new study is a breakthrough on previous attempts to find “gay genes.” And the main advantage of this study is the size of the sample. It involves hundreds of thousands of volunteers.
Specifically, the researchers examined the DNA data of more than 400,000 participants in the US Biobank and of more than 69,000 people who had a DNA test done by the private company that operates in the field of the genetic diagnosis, 23andMe. All of those participants who provided their DNA data to those research projects also answered a battery of questions, including some about whether they ever had a same-sex partner and how many sexual partners they had so far. The findings were replicated with data from three other studies, including one from Sweden.
Findings from studies as large as this one are more likely to be replicated than small studies done in the past.
Previous genetic studies of sexual orientation may also have been biased because they relied on volunteers. People who volunteer to participate in a study, without being selected at random, may not reflect the general population.
Although this study includes both men and women and the sample is significant enough, it does not address people’s attraction to members of the same sex . Some people who have had sex with a same-sex partner do not consider themselves homosexual and are not exclusively attracted to people of the same sex.
Have you ever had sex with someone of the same gender?
Andrea Ganna and her team found that men who claimed to have had same-sex partners tended to be more exclusively homosexual than women. But people of both sexes ran the gamut of sexual orientations. For example, in the US Biobank dataset, younger people had same-sex partners more often than older people, probably because homosexual activity was illegal in the UK until 1967.
This is the first DNA difference related to female sexual orientation.
The results are consistent with previous studies suggesting that genetics may play a larger role in influencing male sexuality than female sexuality. It is not unusual for a single sex of a species to be more fluid in its sexuality, choosing partners of both sexes, explains psychologist Lisa Diamond, who was not involved in the study. For humans, male sexuality may be more closely linked to genes.
But that does not mean that genes control sexual orientation or behavior. “Same-sex sexuality appears to be genetically influenced, but not genetically determined,” says Diamond. “This is not the only complex human phenomenon for which we see a genetic influence without having a great understanding of how it works.” Other complex human behaviors, such as smoking, drinking alcohol, personality, and even job satisfaction, have some genetic component.
Previous research had suggested that genes influencing sexual orientation were located on the X chromosome, but the current study researchers found no evidence that the X chromosome was involved in mate choice.
Instead, they did find genetic variants known as single nucleotide polymorphisms, or SNPs, located on four other chromosomes. SNPs are naturally occurring spots on DNA, where some people have one DNA base or letter, and other people have another . The variants did not change any genes, but were found near some genes that may be involved.
For example, a variant on chromosome 15 linked to men who have sex with men is also associated with male pattern baldness. Another variant in the study is close to the ORA51A gene on chromosome 11 , which is related to the ability to smell certain chemicals. This is interesting because scent has been linked to attraction in the past. Researchers don’t yet know exactly which genes are involved in mate choice or exactly how they influence behavior, but it’s a big step.
Reference: A. Ganna et al. Large genome-wide analysis of sexual orientation identifies for the first time variants associated with non-heterosexual behavior and reveals overlap with heterosexual reproductive traits. American Society of Human Genetics annual meeting, SCIENCE, 2018, San Diego.
