US scientists have identified several different stretches of human DNA which could contain genes that influence male sexuality. The team, based at the University of Illinois, say that multiple genes interacting with environmental influences are the most likely explanation for differences in sexual orientation. The results, based on a scan of all the 22 pairs of 'non-sex' human chromosomes (all those apart from the X and Y chromosomes) were published online in the journal Human Genetics.
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. For the latest study, also lead by Hamer, the researchers focused on the non-sex chromosomes - or autosomes - numbered 1-22.
The researchers looked at 456 individuals from 146 families with two or more gay brothers. They found that around 60 per cent of these siblings shared identical sections of DNA on chromosomes seven, eight and ten - slightly more than the 50 per cent expected by chance. The shared region of DNA on chromosome ten was only linked to sexual orientation when inherited from the mother, the scientists said. However, the regions on chromosomes seven or eight were linked to male sexuality regardless of whether the man inherited them from his father or mother.
First author Brian Mustanski said the next step would be to confirm the findings, and to look at the genes contained within the pinpointed regions of DNA. But Mustanski also stressed that 'since sexual orientation is such a complex trait, we're never going to find any one gene that determines whether someone is gay or not'. Elliot Gershon, professor of psychiatry and human genetics at the University of Chicago, says that the study 'adds to the legitimacy of research on normal variations in human behaviour'.
In October 2004, Italian scientists reported a survey which suggested that the mothers and maternal aunts of gay men have more children than those of heterosexual men. The researchers, based at the University of Padua, said their study provided 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.