A group of US scientists have announced that gay men tend to share certain chromosome signatures, indicating that some types of genes can influence orientation.
The study, by researchers at Chicago’s Northwestern University, consisted of analyzing the genetic makeup of four hundred pairs of twins. They discovered that men who identified as openly gay are more likely than others to share similarities in their X chromosome (specifically, in the Xq28 region) and in chromosome 8; they concluded that this means 30 to 40 percent of a man’s sexuality is determined by his chromosomal makeup at conception. They said that the other 60 to 70 percent is purely environmental factors out of his control, such as the hormone levels he is exposed to in the womb.
One researcher on the project, Professor Michael Bailey, told Irish newspaper The Times, “Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play and we found evidence for two sets that affect whether a man is gay or straight.”
He added, “It is not completely determinative, there are certainly other environmental factors involved.”
Regarding a possible prenatal test for a “gay gene,” Hamer said, “Although this could one day lead to a pre-natal test for male sexual orientation, it would not be very accurate, as there are other factors that can influence the outcome.”
The researcher who led the study, Dr. Alan Sanders, who is an associate Professor of Psychiatry at Northwestern University, explained “We don’t think genetics is the whole story. It’s not. We have a gene that contributes to homosexuality but you could say it is linked to heterosexuality. It is the variation.”
The study’s finding correlate and add to Hamer’s 1993 work with the US National Cancer Institute; he discovered part of the X chromosome that he deemed to have an influence on sexual orientation in men, which some promptly dubbed “the gay gene.”
A 2013 study in Canada discovered that one factor that increases the chance a man will be gay is having multiple older brothers; the hormonal levels in each of a person’s pregnancies are influenced by the womb’s previous occupants, among many other factors.
Brain scans have shown structural differences between the brains of gay and straight men; research has also discovered gay men’s brains respond to sex pheromones produced by other men in the same way straight men’s brains respond to the sex pheromones that women produce.
Though studies have also shown similarities between the brain structures of lesbians and straight men, the genes that influence sexual orientation in women have not yet been discovered.