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”.
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.”
Journal reference: Proceedings of the Royal Society B:
Biological Sciences (DOI: 10.1098/rspb.2004)
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