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Researchers have conducted a genome-wide association of over 470,000 people to explore whether there are genetic variants that predict whether an individual will engage in same sex behavior.



"Taylor Swift tells us that "shade never made anyone less gay" and neither will this research," - Dr Traude Beilharz, a Senior Research Fellow in the Department of Biochemistry & Molecular Biology at Monash University.



Science: A perpetual quest for explanation



Scientific research endeavors to explain all aspects of life – what constitutes a healthy person, why we suffer from disease, why we engage in certain behaviors, and even our sexual orientation.



You may be familiar with the Kinsey Scale. First published in 1948 by Alfred Kinsey, Wardell Pomeroy, and Clyde Martin, the scale accounted for research findings that demonstrated individuals did not fit exclusively into heterosexual or homosexual categories. As a result, the scale measures sexuality on a continuum, from exclusively heterosexual to exclusively homosexual.



Whilst Kinsey received much criticism for his research methods and broad claims, he arguably kick-started research into human sexual orientation. Now, over 70 years later, scientists have very different tools at their disposal: methods to sequence the human genome.



The role of genetics in shaping human sexual behavior



In a new study published in Science, an international team of scientists that specialize in human genetics set out to study the role of genetics in shaping sexual behavior by using information provided by 470,000 people.



The researchers wish to make the following statement known: "Our findings should not in any way be interpreted so as to imply that the experiences of LGBTQ individuals are “wrong” or “disordered.” In fact, this study provides further evidence that diverse sexual behavior is a natural part of overall human variation. Our research is intended to improve our understanding of the genetic basis of same-sex sexual behavior. It should not be misconstrued to disparage LGBTQ people."



The study rationale is based on the fact that previous studies have found strong evidence that same-sex sexual behavior is partly heritable, and therefore is influenced partly by genetics: studies of same-sex sexual behavior in twins show that theheritability is between 30 and 40 percent. This percentage is similar to that of other behavior and personality traits. On the other hand, research also shows that genetics is not the only factor that influences sexual behavior – environment and life experiences contribute also.



The research goals were to:



1. Find genetic markers associated with same-sex sexual behavior



2. Examine biological pathways that might be associated with same-sex sexual behavior



3. Better understand the complexity of sexual orientation by exploring genetic differences between females and males; between behavior, attraction, and identity; and between different sexual behaviors



4. Estimate the extent to which genetic variants associated with same-sex sexual behavior have also been associated with personality, reproductive health, and mental health



A GWAS approach



Andrea Ganna et al. used information from 470,000 research participants that had contributed DNA and provided self-report questions regarding their sexual behavior to large-scale research projects or direct-to-consumer genetic companies, such as the UK Biobank and 23andMe, Inc. Customers of 23andMe Inc. have the option to consent to partake in research, and so the sample used in the study have given their permission for their DNA to be analyzed.



Each person’s DNA was genotyped and the researchers used a genome-wide association study (GWAS) approach to:





Identify genetic markers associated with same-sex sexual behavior





Explore the extent to which the genetics of same-sex sexual behavior is the same for females and males





Look for evidence of overlap between the sets of genetic variants that influence same-sex sexual behavior and those that influence the proportion of same-sex partners among people that, at least once, had sex with someone of the same sex





Look for evidence of overlap between the genetic markers that influence same-sex sexual behavior and those that influence other personality, reproductive, and psychiatric traits



"This is one of the largest studies to-date examining genetic contributions to same-sex sexual behavior," says James Morandini, a Postdoctoral Fellow at the Social Cognition Individual Differences Laboratory, School of Psychology, the University of Sydney.



No single "gay gene"



From the results, the researchers could not identify any patterns among genetic variants that could be used to predict or identify a person's sexual behavior.



"What we found was that there is no one “gay gene” – instead, there are many, many genes that influence a person’s likelihood of having had same-sex partners," says Brendan Zietsch, an ARC Future Fellow in the School of Psychology at the University of Queensland.



Zietsch continues: "Individually, each of these genes has only a very small effect, but their combined effect is substantial. We could be statistically confident about five specific locations; we could also tell with high confidence that there are hundreds or thousands of other locations that also play a role, though we couldn’t pinpoint where they all are."



The authors note that some of these gene variants are linked to the pathways implicated in sex hormones and olfaction. “Our findings provide insights into the biological underpinnings of same-sex sexual behavior,” say Ganna. "But [they] also underscore the importance of resisting simplistic conclusions because the behavioral phenotypes are complex, because our genetic insights are rudimentary, and because there is a long history of misusing genetic results for social purposes."



Morandini adds, "These findings suggest that the genetic architecture of same-sex behavior is complex and that there is no single genetic etiology for same-sex behavior."



Study limitations



The authors acknowledge that there are limitations to the study, including the self-report questionnaires which are subject to misinterpretation and for which a participant may perhaps feel uncomfortable providing an answer.



"We expect that some proportion of the responses may be misreported," they note.



Studying the potential genetic underpinnings of certain traits is no easy task: "For a study of complex trait genetics, where we are looking at millions of geneticmarkers each with small effects, we need a large sample size; for such large groups, detailed information about individuals’ sexual behaviors often isn’t available. Further, fragmenting data into smaller subgroups could reduce the ability to detect these small effects," the authors add.



Beilharz declares: "For now, to understand what it means to be gay, it might just be easier to have a conversation."



Reference: Ganna et al. 2019. Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. Science. DOI: 10.1126/science.aat7693.