Researchers have found two gene variants that are more common in gay men, adding to the belief that sexual orientation is at least partly determined by genetics.

Scientists at Illinois' North Shore University compared DNA sequences from 1,077 gay and 1,231 heterosexual men, scanning their genomes for single-letter differences. One gene was found on chromosome 14 and is mainly active in producing and maintaining thyroid. Interestingly, Grave's Disease, a condition which causes the thyroid to be overactive, is more common in gay men. (It also causes an accelerated metabolism and lower body weight.)

The other gene, located on chromosome 13, is active in a part of the brain called the diencephalon.

Writes Cosmos:

Interestingly, this brain region contains the hypothalamus, which was identified in 1991 as differing in size between gay and straight men. This was discovered by neuroscientist Simon LeVay, who says he is excited that the gene discovery seems to fit with what he found.

Other research has found that this gene, called SLITRK6, is active in the hypothalamus of male mice fetuses a few days before they are born. “This is thought to be a crucial time for sexual differentiation in this part of the brain,” says LeVay. “So this particular finding is a potential link between the neuroanatomy and molecular genetics of sexual orientation.

A link between genetics and homosexuality was first theorized decades ago—in fact a 1995 report from the National Institute of Health (NIH) identified a region on chromosome 8. But none of the previous scholarship honed in on a specific gene. These new findings, published in Scientific Reports, may also enable us to understand how sexual orientation develops from the womb and throughout life. (It is worth noting, though, that the study did not address homosexuality in women.)

Many in the LGBT community look forward to uncovering the biological "cause" of homosexuality: “It adds yet more evidence that sexual orientation is not a ‘lifestyle choice,'" says NIH geneticist Dean Hamer. But it could create a whole new set of problems—homophobes labeling being gay a "disability," or even fearful parents aborting gay babies.

Still, scientists insist human sexuality will never be revealed as some kind of "on/off" switch.

“There are probably multiple genes involved, each with a fairly low effect,” says North Shore University's Alan Sanders, who led the recent study. “There will be men who have the form of gene that increases the chance of being gay, but they won’t be gay.”