There is no single “gay gene.”
There are, however, signals that nonheterosexual behavior has at least some genetic component, according to Andrea Ganna, PhD, and colleagues.
Five candidate genes found in a half-million subject genetic study each account for less than 1% of the variance in same-sex sexual behavior, the scientists found. Over the entire genome, genetic variants accounted for less than a quarter of such behavior.
None of the genetic signals can reliably predict sexual behavior, Benjamin Neale, PhD, director of genetics in the Stanley Center for Psychiatric Research at the Broad Institute of the Massachusetts Institute of Technology and Harvard Medical School, both in Boston, said in a telebriefing that included Dr. Ganna, a postdoctoral researcher in his lab. Instead, the variability of human sexuality is an entirely natural continuum of human behavior.
“Whether we are attracted exclusively to the opposite sex, the same sex, or both sexes falls along a spectrum that is an integral, and entirely normal, part of the human experience,” said Dr. Neale. “The choice of a sexual partner and the fraction of partners that are of the same sex are all consistent with this diversity being a key feature of our sexual behavior as a species and this diversity is a natural part of being human.”
The study, published in Science, clarifies findings of smaller, previous studies, which determined that sexual behavior is a combination of genetics and environment – although environment is a much more difficult association to assess.
“[Environment] can range from anything in utero, all the way all the way through who you happen to stand next to on the tube in the morning, right? That’s all potentially environmental factors that can have some influence on complex traits and so we don’t really understand,” Dr. Neale said.
However, he added, the concept of individual choice in sexual behaviors was beyond the scope of the study, which strictly centered on genetic associations with sexual behavior.
The study combined genetic information from three extant databases (the U.K. Biobank, National Longitudinal Study of Adolescent to Adult Health, the Molecular Genetic Study of Sexual Orientation, and the Child and Adolescent Twin Study in Sweden) with newly collected data from 23andMe, the technology company that provides at-home genetics tests largely used to determine ethnic origin.
The primary phenotype investigated was a binary measure: self-reported sexual behavior with someone of the same sex (nonheterosexuality) or someone of the opposite sex (heterosexuality).
“The binary variable also collapses rich and multifaceted diversity among nonheterosexuality individuals, wrote Dr. Ganna and his coauthors. Therefore, “we explored finer-scaled measurements and some of the complexities of the phenotype, although intricacies of the social and cultural influences on sexuality made it impossible to fully explore this complexity.”
The team also performed replication analyses on three smaller datasets: the Molecular Genetic Study of Sexual Orientation (2,308 U.S. adult males), Add Health (4,755 U.S. young adults), and the Child and Adolescent Twin Study in Sweden (8,093 Swedish adolescents). Data were available for 188,825 males and 220,170 females overall. When broken down by sexual behavior, data were available for 1,766 homosexual and 180,431 heterosexual males, and 693 homosexual and 214,062 heterosexual females.
The team identified two genes that significantly predicted same-sex sexual behavior (rs11114975-12q21.31 and rs10261857-7q31.2). Two more genes predicted same-sex sexual behavior in males only (rs28371400-15q21.3 and rs34730029-11q12.1), and one additional gene predicted the behavior in females only (rs13135637-4p14).
Three of the single nucleotide polymorphisms (SNPs) nominally replicated those in some of the other datasets, despite the much smaller sample sizes.
“The SNPs that reached genome-wide significance had very small effects (odds ratio, 1.1),” the authors wrote. “For example, in the U.K. Biobank, males with a GT genotype at the rs34730029 locus had 0.4% higher prevalence of same-sex sexual behavior than those with a TT genotype. Nevertheless, the contribution of all measured common SNPs in aggregate was estimated to [account for] 8%-25% of variation in female and male same-sex sexual behavior. … The discrepancy between the variance captured by the significant SNPs and all common SNPs suggests that same-sex sexual behavior, like most complex human traits, is influenced by the small, additive effects of very many genetic variants, most of which cannot be detected at the current sample size.”
The Child and Adolescent Twin Study in Sweden contained the youngest subjects. The polygenic scores in this dataset were significantly associated with sexual attraction at age 15 years, “suggesting that at least some of the genetic influences on same-sex sexual behavior manifest early in sexual development.”
The team also investigated the biological pathways associated with the SNPs. Among the male variants, rs34730029-11q12.1 contains numerous olfactory receptor genes.
“Second, rs28371400-15q21.3 had several indications of being involved in sex hormone regulation. The allele positively associated with same-sex sexual behavior is associated with higher rate of male pattern balding, in which sex-hormone sensitivity is implicated,” they wrote.
This is located near the TCF12 gene, related to a normal gonadal development in mice.
Among women, there were inverse associations with the level of sex hormone–binding globulin, which regulates the balance between testosterone and estrogen.
There were significant associations with some mental health traits, including loneliness, openness to experience, and risky behaviors such as smoking and using cannabis. There were also associations with depression and schizophrenia. The genetic correlations for bipolar disorder, cannabis use, and number of sexual partners were significantly higher in females than in males.
“We emphasize that the causal processes underlying these genetic correlations are unclear and could be generated by environmental factors relating to prejudice against individuals engaging in same-sex sexual behavior,” the authors wrote.
In an interview, Jack Drescher, MD, said he was not surprised by the findings and cited the results a twin study by J. Michael Bailey, PhD, and Richard C. Pillard, MD, as evidence of the complexities surrounding sexual orientation. The study examined the likelihood of one twin having a gay twin (Arch Gen Psychiatry. 1991 Dec;48[12]:1089-96).
“If you were a gay identical twin, you had a 52% chance of having a gay twin, he said. “If you were a gay fraternal twin, you only had a 22% chance of having a gay twin. The chance of an adoptive brother being gay was 11%. If homosexuality were simply a result of simple genetic transmission, then one would expect closer to 100% gay identical twins, since they both have the same genes.”
Dr. Drescher, clinical professor of psychiatry at the Center for Psychoanalytic Training and Research at Columbia University, New York, has written extensively about human sexuality, gender-conversion therapies, and gender.
The study by Dr. Ganna and associates as a whole invalidates several commonly used sexual behavior scales, including the Kinsey Scale, which is solely predicated upon self-reported attraction. The Klein Sexual Orientation Grid, which measures sexual behavior, fantasies, and sexual identification, is similarly problematic, the authors noted.
“Overall, our findings suggest that the most popular measures are based on a misconception of the underlying structure of sexual orientation and may need to be rethought. In particular, using separate measures of attraction to the opposite sex and attraction to the same sex, such as in the Sell Assessment of Sexual Orientation, would remove the assumption that these variables are perfectly inversely related and would enable more nuanced exploration of the full diversity of sexual orientation, including bisexuality and asexuality,” they wrote.
During the telebriefing discussion, Dr. Neale said the study supports the nuances in sexuality espoused by self-identified sexual orientation communities. “I think those things that we’ve learned include the idea that there is more diversity out there in the world. We see that diversity in the genetic analysis. And we reinforce that sort of message that the expanding acronyms in the LGBTQIA+ [lesbian, gay, bisexual, transgender, queer, intersex, asexual, and other] is justified.”
The study “underscores that there is an element of biology and it underscores that there’s an element of the environment,” he said. “And it underscores that this is a natural part of our species and so these are the things that both matter and there’s no way to get away from that idea.”
Several entities funded the study, including the Eunice Kennedy Shriver National Institute of Child Health and Human Development. Dr. Ganna reported no financial conflicts. Two of the researchers and members of the 23andMe research team are 23andMe employees or hold stock options in the company. Another researcher is affiliated with Deep Genomics as a member of its scientific advisory board.
SOURCE: Ganna A et al. Science. 2019 Aug 30. doi: 10.1126/science.aat769.