Researchers looking for a genetic signature of homosexuality have been barking up the wrong tree, according to a trio of researchers in the United States and Sweden. Instead, the scientists posit, epigenetic influences acting on androgen signaling in the brain may underlie sexual orientation. In a paper published last week (December 11) in The Quarterly Review of Biology, they propose a model describing how epigenetic markers that steer sexual development in males could promote homosexual orientation in females, and vice versa. The scientists offer their model to explain both the tendency of homosexuality to run in families, and the fact that so far no “homosexual gene” has been identified.

“It’s a very provocative, very interesting new twist that is plausible,” said Margaret McCarthy, a neuroscientist at the University of Maryland who studies how hormones influence brain development and was not involved in producing the model. But, she cautioned, so far the theory “is not supported by any data.”

Indeed, Andrea Ciani, an evolutionary psychologist at the University of Padova, thinks that a variety of factors, including genes and epigenetics, influence sexual orientation. “It’s a little bit vain to think we’ll find the answer to homosexuality as a whole.”

The model was developed by William Rice, an evolutionary geneticist at the University of California, Santa Barbara; Sergey Gavrilets, a mathematician at the University of Tennessee; and Urban Friberg, an evolutionary biologist at the University of Uppsala. The notion that epigenetics, rather than genetics, is the primary force promoting homosexuality sprang from several observations, explained Rice.

First, evidence shows that homosexuality can run in families. Still, only 20 percent of identical twins are both gay, said Rice. Furthermore, linkage studies looking for a genetic underpinning to sexual orientation have not turned up any “major” homosexual genes, Rice noted. “This made us suspicious that something besides genes produces heritability that isn’t genetic.” Epigenetics fits the bill.

The model focuses on the role of epigenetics in shaping how cells respond to androgen signaling, an important determinant of gonad development. The researchers suggest that androgens are also important factors in molding sexual orientation, and that various genes involved in mediating androgen signaling are regulated by epigenetic modifications.  These epigenetic marks, they argue, can be passed on between generations.