A large new study that analyzed the genetic data of thousands of people suggests that the drive to have sex with people of the same sex may stem from a very complex interaction of many different genes.

Sexuality may emerge from a very complex 'genetic architecture.'
In 1993, geneticist Dean Hamer and colleagues first made the case for a heavy genetic component underlying human sexual orientation.

This and other studies indicating that genes likely play a key role in sexuality gave rise, in time, to the emergence of a controversial concept: the "gay gene," a single genetic player that could determine sexual orientation.

However, sexuality is not influenced by a single gene. Recent studies have suggested that different genetic loci — positions on a chromosome where different genes are located — are linked to sexuality and same-sex sexual behavior.

Now, a very large study that assessed the genetic information stored by two different databases — the UK Biobank and 23andMe — confirms that the genetic background of sexual orientation is not down to just one gene.

The first author of the study is Andrea Ganna, Ph.D., from the Broad Institute of MIT and Harvard in Cambridge, MA.

"The biological factors that contribute to sexual preference are largely unknown, but genetic influences are suggested by the observation that same-sex sexual behavior appears to run in families and is concordant more often in genetically identical (monozygotic) twin pairs than in fraternal twin pairs or siblings," the researchers write in their study paper, which now appears in the journal Science.

The researchers received funding from many state and academic institutions, including the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which sponsored one of the researchers "specifically to investigate the genetics of sexual orientation."

Many of the scientists involved in this study are also members of the 23andMe Research Team, employees of 23andMe, and hold stock or stock options in 23andMe. This company has, in the past, expressed a particular interest in revealing more about the genetics of sexual orientation.

'No single genetic determinant'
One main question that the researchers aimed to answer was, "What genes are involved, and what biological processes do they affect?"

To find out, they conducted a genome wide association analysis on the data of over 408,000 people in the UK Biobank and over 68,000 people who provided genetic information through 23andMe.

The researchers only included in their analysis the data of people who had reported having had sex with someone of the same sex, with someone of the opposite sex, or both.

The researchers found five genetic variants "significantly associated" with same-sex sexual behavior — that is, the likelihood of having sex with someone of the same sex.

However, the researchers also noted that many more genetic loci likely underpin same-sex sexual behavior, and that the relationship between genetic factors is so complex that it would be impossible to pinpoint a predictive pattern.

"We identified genome wide significant loci associated with same-sex sexual behavior and found evidence of a broader contribution of common genetic variation," the researchers write.

They add, "We established that the underlying genetic architecture is highly complex; there is certainly no single genetic determinant."

"Rather, many loci with individually small effects, spread across the whole genome and partly overlapping in females and males, additively contribute to individual differences in predisposition to same-sex sexual behavior."

As for the genetic variants that seemed to be common in different cases of same-sex sexual behavior, some of them are also linked to biological pathways involved in sex hormone regulation, as well as the sense of smell.

However, how this information contributes to a better understanding of the genetic and biological mechanisms underlying sexual behavior remains to be seen.

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