Survival of genetic homosexual traits explained

Italian geneticists may have explained how genes apparently linked to male homosexuality survive, despite gay men seldom having children. Their findings also undermine the theory of a single “gay gene”.

From the newscientist:

The researchers discovered that women tend to have more children when they inherit the same - as yet unidentified - genetic factors linked to homosexuality in men. This fertility boost more than compensates for the lack of offspring fathered by gay men, and keeps the “gay” genetic factors in circulation.

The findings represent the best explanation yet for the Darwinian paradox presented by homosexuality: it is a genetic dead-end, yet the trait persists generation after generation.

“We have finally solved this paradox,” says Andrea Camperio-Ciani of the University of Padua. “The same factor that influences sexual orientation in males promotes higher fecundity in females.”

Relative differences
Camperio-Ciani's team questioned 98 gay and 100 straight men about their closest relatives - 4600 people in total. They found that female relatives of gay men had more children on average than the female relatives of straight men. But the effect was only seen on their mother’s side of the family.

Mothers of gay men produced an average of 2.7 babies compared with 2.3 born to mothers of straight men. And maternal aunts of gay men had 2.0 babies compared with 1.5 born to the maternal aunts of straight men.

“This is a novel finding," says Simon LeVay, a neuroscientist and commentator on sexuality at Stanford University in California. “We think of it as genes for ‘male homosexuality’, but it might really be genes for sexual attraction to men. These could predispose men towards homosexuality and women towards ‘hyper-heterosexuality’, causing women to have more sex with men and thus have more offspring.”

Camperio-Ciani stresses that whatever the genetic factors are, there is no single gene accounting for his observations. And the tendency of the trait to be passed through the female line backs previous research suggesting that some of the factors involved are on the male “X” chromosome, the only sex chromosome passed down by women. “It’s a combination of something on the X chromosome with other genetic factors on the non-sex chromosomes,” he says.

Immune system
Helen Wallace, of the UK lobby group GeneWatch, welcomes the new research that moves away from the controversial single-gene theory for homosexuality. “But it’s worth noting that the data on the sexuality of family members may be unreliable, so more studies are likely to be needed to confirm these findings,” she says.

Even if the maternal factors identified by Camperio-Ciani’s team are linked with male homosexuality, the research team’s calculations suggest they account for only about 14% of the incidence.

Their findings also support earlier findings that when mothers have several sons, the younger ones are progressively more likely to be gay. This might be due to effects changes to the mother’s immune system with each son they carry.

But Camperio-Ciani calculates the contribution of this effect to male homosexuality at 7% at most. So together, he says, the “maternal” and “immune” effects only account for 21% of male homosexuality, leaving 79% of the causation still a mystery.

This leaves a major role for environmental factors, or perhaps more genetic factors. “Genes must develop in an environment, so if the environment changes, genes go in a new direction,” he says. “Our findings are only one piece in a much larger puzzle on the nature of human sexuality.”