Many, many genes influence a person’s likelihood of having same-sex partners, according to a new large-scale study published on 30 August in Science (1).
A 1993 study reported a link between a region of the genome located on the X chromosome and homosexuality and the x28 gene was quickly dubbed the ‘gay gene’ (2). However, other studies have since been unable to replicate the same finding (3). And this latest study seems to disprove any notion that sexuality can be attributed to a single gene. In fact, decades of genetic research suggest most human traits are too complex to be attributed to a single gene and are a complex interplay of genetics and the environment.
The international team of researchers, in collaboration with 23andMe — a privately-held personal genomics and biotechnology company — performed a large-scale genome-wide association study (GWAS) based on samples from nearly half a million people from the United States, the United Kingdom, and Sweden. Samples included almost 400,000 from the UK Biobank project and more than 68,000 from 23andMe.
The authors found that homosexuality cannot be attributed to one single gene. Instead, is more likely influenced by a multitude of genetic variants as well as environmental factors. But they were able to connect five genetic variants to same-sex sexual behaviour, two in both men and women, two found only in men and one found only in women. However, since these genetic differences explain less than one per cent of the variation in same-sex behaviour, numerous others must be involved. In other words, whereas each gene may have only a teeny tiny effect, the combined effects could be significant.
In addition, they found overlaps between a genetic predisposition to same-sex sexual behaviour and traits such as openness to experience, male pattern baldness, and interestingly, a predisposition to mental health problems. However, the latter could also be caused by the stigma in some cultures surrounding same-sex sexual behaviour, which could cause or worsen mental health issues.
One thing certainly seems clear, you cannot meaningfully predict a person’s sexual preference from their DNA alone. In fact, the authors estimate that one-third of the variation in same-sex behaviour can be explained by genetics. However, the study also has some notable limitations. Samples were mainly obtained from people with European ancestry and all age ranges were not accounted for. Additionally, questions were not specifically designed to examine genes associated with sexual preference and behaviours were self-reported.
Researchers are making significant steps toward understanding the role of genes in sexuality. Nonetheless, the search for the genetic code of human sexuality is definitely contentious. To address this controversy and help avoid any confusion or misinterpretation surrounding the latest results, the authors have created a website to answer questions about the study. And to help ensure the scientific data is not misinterpreted by the media – as can so often happen.
(1) Ganna, A. et al. Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. Science (2019). DOI: 10.1126/science.aat7693
(2) Hamer, D.H.. et al. A linkage between DNA markers on the X chromosome and male sexual orientation. Science (1993). DOI: 10.1126/science.8332896
(3) Rice, G. et al. Male homosexuality: absence of linkage to microsatellite markers at Xq28. Science (1999). DOI: 10.1126/science.284.5414.665