The largest study to date into the genetic basis of sexuality has found that there is no single gene associated with same-sex sexual behaviour.
The findings, based on the genomes of nearly 500,000 participants, reflect the results of previous, smaller studies - although sexual preference has a genetic component, no single gene has a determining effect on sexual behaviours.
Dr Ganna and his colleagues used a genome-wide association study (GWAS) to look at the genomes of hundreds of thousands of people for single-letter DNA changes called SNPs. If lots of people with a certain trait – here, for same-sex sexual preferences – share a common SNP, it is likely that the SNP is related to the given characteristic.
In order to obtain such a large sample size, the team used genomic data that had previously been collected as part of broader projects. These included DNA data and responses from participants of the UK BioBank study, and individuals who had used the US commercial 23andMe service.
The team split the study participants into two groups – those who reported having had sex with someone of the same sex, and those who didn't. They then analysed more than one million SNPs to identify those that were more frequent among individuals who reported similar sexual behaviours.
Using this approach, the scientists found that common sexual variants could explain between 8-25 percent of the differences in sexual behaviour. Of these, five specific SNPs could confidently be associated with same-sex sexual behaviours.
In addition, they could identify between hundreds to thousands of further genomic locations that also play a role in sexual preference, reinforcing the complexity of the genetic component of sexual identity. These findings also indicate that on the genetic level, there is no single continuum from straight to gay, said senior study author Dr Brendan Zietsch at the University of Queensland in Brisbane, Australia, writing in The Conversation.
'This is a solid study,' said Professor Melinda Mills, a sociologist at the University of Oxford in the UK, who studies the genetic basis of reproductive behaviours, and was not involved in the study.
Nonetheless, she cautioned in comments to Nature News that the findings might not be representative of the general population – a caveat the study authors also acknowledge. The majority of study participants were largely of European ancestry, and tended towards a higher age bracket. In addition, people whose biological sex and self-identified gender did not match were not included in the study.