The idea that sexuality is a choice is an archaic idea. The hypothesis that there is a genetic element to homosexuality has floated around for a while; gay twins are surely enough to prompt suspicion on the matter. But it took a long time for convincing evidence in support of this to surface. Just last year, for example, the biggest study of gay brothers to date backed up what had been proposed 20 years earlier: stretches of DNA on the X chromosome and chromosome 8 are strongly associated with sexual orientation.
It’s clearly not all down to genes, though. Studies that have looked at identical twin pairs have found that if one sibling is gay, there is roughly a 20% chance that the twin will also have the same sexual orientation. And each time a woman has a male pregnancy, the chance that her next son will be homosexual increases by 33%. These percentages clearly indicate another factor is involved.
When we talk about traits like this, the argument is often over “genetics or environment,” but what about something that is a bridge between the two? This is where epigenetics swoops in. Epigenetics refers to tags that can be added to or removed from DNA that alter the expression of a particular gene. Your DNA sequence has got to be read before proteins can be made; the tags get in the way of the scanning machinery and thus prevent expression. Imagine something getting stuck in a production line. Importantly, these can be influenced by environmental factors, but as Nature points out, they are not necessarily shared between identical twins.
To find out whether epigenetics may also play a part in homosexuality, researchers from the University of California, Los Angeles, looked at patterns of epigenetic tags – or more specifically, chemicals called methyl groups – in 37 identical twin pairs in which only one was gay, so called “discordant” for sexual orientation. They also included 10 pairs of heterosexual twins as a control group.
After combing their genomes, they discovered five marks that were present in higher frequencies in the gay men compared with their heterosexual twin brothers. They then applied an algorithm to the regions identified and found that these tags were able to predict whether or not the individual sampled was gay with an accuracy of 67%.
To the researchers’ knowledge, they write, this is the first biomarker-based predictive model for sexual orientation.
So what are the genes affected by these marks, and what environmental factor is influencing them? This study unfortunately can’t tell us that, but it’s an interesting starting point that will hopefully spur further research.