18 December 2012

Nothing in Biology Makes Sense: Making sense of same-sex orientation in humans

This week over at Nothing in Biology Makes Sense, I'm taking a look at a much-heralded new journal article that purports to solve an evolutionary puzzle that has particularly personal interest to me: how same-sex sexual orientation could evolve in the face of its selective costs. Of course, I've previously discussed a long list of possible answers to this question — but the new paper suggests that the best solution may lie in the epigenetics of sexual development.

Epigenetics is an appealing explanation for same-sex attraction because we have, at best, a fuzzy picture of the genetic basis of sexual orientation. Homosexuality definitely "runs in families". That is, people with gay or lesbian parents, siblings, aunts, or uncles are more likely to be gay or lesbian themselves; and pairs of identical twins, who share pretty much all their genetic code, are more likely to have the same sexual orientation than pairs of fraternal twins, who share only half their genes.

Yet more sophisticated methods to identify specific genes associated with sexual orientation have failed to find any consistent candidates. (Though, as a caveat, the only genetic association study [PDF] I've seen suffers from small sample size and considers a very small number of markers by modern standards.) Moreover, while identical twins share sexual orientation more than fraternal twins, they don't share it with complete fidelity — only about 20% of gay men who are identical twins have twin brothers with the same orientation.

For an explanation of what exactly epigenetics is, a full description of the new study, my evaluation of it all, and even some gratuitous — if, I hope, educational — beefcake, you'll have to go read the whole thing.◼

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