A principal concern is that people who perceive homosexuality as an abnormality will call for attempts to diagnose it, perhaps by a pre-natal test, or attempt to 'cure' it by developing so-called treatments. On the other hand, showing that homosexuality is a normal part of human genetic variation, much like eye or skin colour, could open the way to stronger anti- discrimination laws.
In the US, and to some extent in the UK, it is illegal to discriminate on the grounds of genetically determined characteristics such as sex or race. Finding that homosexuality has a similar genetic basis, albeit less deterministic, could support arguments for stronger legislation.
Dean Hamer, the head of the scientific team that has done the latest research at the US National Cancer Institute near Washington DC, believes this will be the case. 'The legal scholars I have spoken to feel it will have a significant impact,' he said.
Dr Hamer and colleagues are also studying the families of lesbians. They said that preliminary results suggest that female sexual orientation is genetically influenced, but DNA markers - the genetic signposts - have not been detected yet.
As regards developing a test for homosexuality, Dr Hamer emphasises that the research is not designed to test for sexual orientation. He emphasised that one gene, or a group of genes on a region of the X chromosome, cannot explain all the complexities of human sexuality.
Nevertheless, he is concerned that this and other work to unravel the entire genetic makeup of humans - as part of the international Human Genome Project - will raise important ethical issues that have yet to be addressed: 'As efforts to map the human genome progress, there will be increasing concern about how the information will be used.'
Ironically, it was the advent of Aids that enabled scientists to study the physical basis of homosexuality. Doctors' notes began to record the sexuality of patients, enabling researchers to compare physical differences between homosexual and straight men.
In 1991, Simon LeVay, a homosexual scientist working in California, found for instance that homosexual men who had died of Aids had a much smaller anterior hypothalmus - a region of the brain - than heterosexual men. Other research groups found other differences in brain structures, such as the suprachiasmatic nucleus, which is involved in regulating the biological 'clock' and may have a role in sexual activity.
But such physical differencs need not necessarily be the result of genetic differences. For instance, they could have come about as a result of differences in upbringing or perhaps brought about as a result of Aids itself.
Professor LeVay said in his recent book The Sexual Brain that he originally believed in the Freudian view of the origin of homosexuality. 'I remember my mother as having been very close and possessive, and my father as distant, even hostile.'
He later had doubts. 'First, as I got to know large numbers of gay men and lesbian women it became harder and harder to see them, or myself, as the products of defective parenting; we just seemed too normal.'
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