Scientists have long wondered, if being gay is indeed caused--at least partially--by genetic factors, how is it that these genes don't die out since gay men are significantly less likely to reproduce and pass them on from generation to generation. A new Italian study suggests the "gay gene" is passed on by female relatives of gay men--including their mothers, maternal aunts, and siblings--who tend to have larger families than women without gay relatives and are therefore more likely to pass along the genetic factor, The [London] Independent reports. The study, published in The Proceedings of the Royal Society of London Series B, Biological Sciences, suggests that the gene or genes predisposing men to homosexuality are also somehow linked to the desire among females to have more than the average number of children. Because of this link, these genetic factors will never die out since the mothers, maternal aunts, and sisters of gay men will continue to spread the genes by having large families.
Andrea Camperio-Ciani of the University of Padova in Italy and his colleagues studied the extended families of 98 gay men and 100 heterosexual men to answer the question "If male homosexuality has a genetic component and homosexuals reproduce less than heterosexuals, then why is this trait maintained in the population?" Camperio-Ciani told The Independent, calling the question the "Darwinian paradox" of homosexuality. "Our data resolves this paradox by showing that there might be hitherto unsuspected reproductive advantages associated with male homosexuality."
The study suggests that the genetic factors linked with homosexuality tended to be passed on by women because the gay men studied had more maternal than paternal gay relatives, linking sexual orientation to the mother's side of the family. Camperio-Ciani said other research has suggested that some of the genetic predisposition to homosexuality is carried on the X chromosome, which men inherit from their mothers. But he adds that there are almost certainly many other genes linked with sexual orientation. He also says his study suggests that about 20% of the predisposition toward being gay is caused by genetic factors, but didn't say what specific data led him to that conclusion and what constituted the other 80%.