In the court battles over Proposition 8, the underlying question was whether homosexuality was a choice or biological. Critics often say “you can’t find a single gay gene”, but humans are squishy — they are resultant of about thirty thousand genes which build and detract and modify one another like ripples in a pond. Saying there’s no single gay gene is as accurate as that there is no single hair color gene, but both have every appearance of being genetic, and geneticists are quite convinced this is the case. And honestly, so am I. Nobody would intentionally choose to grossly limit the number of people they can choose partners from, while simultaneously opening themselves to hate crimes from complete assholes.
Most geneticists consider sexual orientation a phenotype — namely, an observable set of properties that varies among individuals. Although physical phenotypes like height and weight are easier to quantify, behavioral phenotypes are intensely studied in animals and humans. Research from many directions leads to a strong conclusion: Human sexual orientation has deep biological roots.
Gay genes appear paradoxical at first blush. From the perspective of natural selection, how could they persist in the population if they lead to fewer offspring? Recent research has uncovered several plausible explanations. For example, one set of studies found that the same inherited factors that favor male homosexuality actually increase the fecundity of female maternal relatives. By balancing the number of offspring, they would contribute to maintaining these genes over the course of evolution. This explanation may not be exclusive but serves to illustrate that the Darwinian problem is not necessarily overwhelming.
The article also talks about epigenetics, which I really don’t understand outside of the dictionary definition. I should probably go toddle off to Scicurious’ place and see if she’s talked about it in her archives.
Update 29 Aug 2011: So this is one of those blog posts I keep going and linking back to, but the article I originally linked has disappeared. Sigh. Here’s an alternate report discussing a study of gay brothers’ genetics. While genes might not be the only factor, while epigenetics might still play a role, it’s fairly obvious at this point that just like there’s no single gene that controls how tall you’ll be, sexuality might be an emergent property of a number of unrelated (and advantageous for other reasons) genes coupled with some epigenetic factors.