The research, published in the journal Science, raises ethical issues on the genetic influences on human sexual preferences. Finding a gene that predisposes a person to homosexuality could soon lead to pre-natal diagnostic tests which might in theory be offered to pregnant women to determine whether to abort a foetus carrying the gene.
Unusually, the researchers make a plea at the end of their research paper for such information about genetic predispositions not to be used to discriminate against sexual preferences: 'We believe that it would be fundamentally unethical to use such information to try to assess or alter a person's current or future sexual orientation, either heterosexual or homosexual, or other normal attributes of human behaviour.' The scientists, led by Dean Hamer, an Aids researcher at the US National Cancer Institute near Washington DC, said: 'Scientists, educators, policy makers and the public should work together to ensure that such research is used to benefit all members of society.'
Although they feel confident of being able to identify one or more genes involved in a tendency towards homosexuality, the scientists emphasised that their findings cannot explain all male homosexuality. Dr Hamer said it is likely homosexuality arises from a variety of causes, both genetic and environmental.
Other research groups had found that homosexual preferences sometimes tend to run in families, indicating a genetic basis, but the latest research has located a region of the X chromosome - which men inherit from their mothers - that is strongly implicated.
Dr Hamer and colleagues studied the family histories of 114 homosexuals and found that 13.5 per cent of the gay men's brothers were also homosexual, compared with 2 per cent in the general population. They also found maternal uncles and maternal male cousins were more likely to be homosexual. In some families, gay relatives could be traced back three generations. Because homosexual uncles and male cousins of the gay subjects were raised in different households, the scientists hypothesised a genetic factor on the X chromosome was responsible.
They analysed the X chromosomes of 40 pairs of gay brothers using genetic markers, which are like signposts. They found that 33 of the paired brothers had coinherited genetic markers on the same region of the X chromosome, known as Xq28.
This region represents only 0.02 per cent of the human genome - the entire genetic makeup - but may carry several hundred genes, so there is considerable work to be done to identify the precise gene, or genes, involved, Dr Hamer said.
'Our research implies that being gay or straight relies to some extent on a genetic predisposition. We can only speculate on what the gene does. Once we have the gene, we'll be able to understand it,' he said.
The scientists do not know why seven of the 40 pairs of gay brothers do not appear to have the same genetic markers.
Dr Hamer said these gay men may have inherited other genes that are associated with homosexuality, or they might have been influenced by 'environmental factors or life experiences'.
In 1991, Simon LeVay, a British-born scientist at the Salk Institute for Biological Studies at San Diego, California, found slight structural differences in the hypothalamus of the brains of gay men. Other reseachers have also identified structural differences in other areas of the brain, but were not able to say whether this resulted from environmental influences or genetic inheritance.
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