What we know—and don't know—about the biology of homosexuality

The media was abuzz this week after an international group of researchers proposed that scientists may have been looking for the biological underpinnings of homosexuality in the wrong place. Although scientists have spent the last few decades scouring our genome for a “gay gene,” William Rice, Urban Friberg, and Sergey Gavrilets suggest in The Quarterly Review of Biology that homosexuality may have its roots in epigenetics, rather than in genetics.

According to the authors, much of we know about homosexuality suggests that it is not simply a result of direct genetic inheritance. First, despite thorough genome-wide research, no study has been able to find a gene or genetic marker that is consistently associated with homosexuality. Second, although twenty to fifty percent of the variation in sexual orientation appears to be inherited in some way, identical twins don’t necessarily share a sexual orientation; if one twin is gay, there’s only a twenty percent probability that the other twin is, too. This low probability (or “concordance”) suggests that simple genetic inheritance might not drive sexual orientation. Finally, the authors argue that any purely genetic “fitness-reducing phenotypes” like homosexuality would be selected against and weeded out of the gene pool.

Instead, the researchers suggest, epigenetic inheritance via “epi-marks” might be responsible for sexual orientation. Epi-marks are physical changes in our genetic material (such as chemical modification or changes in DNA packaging proteins) that regulate gene activity without actually changing the sequence of bases.