Study probes inheritance of sexuality

Genetic factors that influence homosexuality in men might also affect the number of children borne by their female relatives, Italian scientists claim. A new survey suggests that although gay men may have fewer children than heterosexual men, their mothers and maternal aunts have more children than those of men who are not homosexual. The researchers say their study provides an explanation for the fact that homosexuality could be passed on from one generation to the next, even if gay men don't have children themselves.

The scientists, based at the University of Padua, interviewed 98 gay and 100 straight men about their closest relatives, a total of 4600 people. They found that female relatives of homosexual men had an average of 2.7 babies, compared to 2.3 babies born to female relatives of heterosexual men. This could mean that mothers and maternal aunts of gay men have inherited some genetic factor that confers a reproductive advantage, which counterbalances the fact that gay men have fewer children. 'For a long time it has been a paradox', said study leader Andrea Camperio-Ciani, adding 'but we have found that there might be a set of genes that, in males, influences homosexuality but in females increases fecundity'.

Earlier studies aimed at identifying genes that influence male sexuality have produced conflicting results. In 1993, a group lead by US scientist Dean Hamer reported an association between variations in a particular region of the X chromosome and male homosexuality. But these findings were not confirmed in a larger study, carried out by a Canadian group in 1999. Referring to the new study, Camperio-Ciani stressed that it does not show that a single gene is responsible for sexual orientation. However the fact that the trait only appears to be passed through female relatives suggests that an X-chromosome gene could be involved. 'It's a combination of something on the X-chromosome with other genetic factors on the non-sex chromosomes', he said.

Most geneticists think that sexuality, like other aspects of human behaviour, is influenced by many different genetic and non-genetic factors. The Italian team think that a proposed X-chromosome gene would only account for around 14 per cent of the likelihood that a man will be gay. Other factors could include the environment in the womb, additional genes and life experiences. 'Our findings are only one piece in a much larger puzzle on the nature of human sexuality', concluded Camperio-Ciani.