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 鈥済ay gene鈥�.
The researchers discovered that women tend to have more children when they inherit the same 鈥� as yet unidentified 鈥� 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 鈥済ay鈥� 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.
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鈥淲e have finally solved this paradox,鈥� says Andrea Camperio-Ciani of the University of Padua. 鈥淭he same factor that influences sexual orientation in males promotes higher fecundity in females.鈥�
Relative differences
Camperio-Ciani鈥檚 team questioned 98 gay and 100 straight men about their closest relatives 鈥� 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鈥檚 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.
鈥淭his is a novel finding,鈥� says Simon LeVay, a neuroscientist and commentator on sexuality at Stanford University in California. 鈥淲e think of it as genes for 聭male homosexuality鈥�, but it might really be genes for sexual attraction to men. These could predispose men towards homosexuality and women towards 聭hyper-heterosexuality鈥�, causing women to have more sex with men and thus have more offspring.鈥�
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 鈥淴鈥� chromosome, the only sex chromosome passed down by women. 鈥淚t鈥檚 a combination of something on the X chromosome with other genetic factors on the non-sex chromosomes,鈥� 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. 鈥淏ut it鈥檚 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,鈥� she says.
Even if the maternal factors identified by Camperio-Ciani鈥檚 team are linked with male homosexuality, the research team鈥檚 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鈥檚 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 鈥渕aternal鈥� and 鈥渋mmune鈥� 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. 鈥淕enes must develop in an environment, so if the environment changes, genes go in a new direction,鈥� he says. 鈥淥ur findings are only one piece in a much larger puzzle on the nature of human sexuality.鈥�
Journal reference: Proceedings of the Royal Society B: Biological Sciences (DOI: 10.1098/rspb.2004)
