FIONA MACRAE SCIENCE CORRESPONDENT IN CHICAGO
WHAT MAKES A HOMOSEXUAL?
Other studies suggests that conditions in the womb also influence sexual orientation.
For instance, the more older male siblings a man has, the greater chance he will be gay.
Scientists say the phenomenon cannot be explained by the youngest boy being babied and mollycoddled or other differences in the way they are brought up.
It is thought that carrying a male baby in the womb triggers an immune response in the mother, creating antibodies that attack part of the unborn child's brain linked to sexual orientation.
This response gets stronger the more boys a woman carries, raising the odds of homosexuality.
Exposure to hormones in the womb is also likely to be important and some argue that upbringing plays a role in sexuality.
Being gay could be in the DNA.
Scientists have found two stretches of DNA linked to homosexuality in men.
The confirmation of the existence of a ‘gay gene’ or genes will strengthen arguments that homosexuality is a matter of biology, rather than choice.
However, it also raises the prospect of a genetic test that could be used by insurance companies to discriminate against clients or by pregnant women to abort gay babies.
In the study, Chicago University researchers analysed the DNA of more than 400 pairs of gay brothers, recruited at Gay Pride festivals at marches over several years.
This revealed flagged up two pieces of DNA that seem to be linked to homosexuality.
It is not known which of the many genes they contain are key or how they affect the development of sexual orientation.
However, the result, revealed at the American Association for the Advancement of Science’s prestigious annual conference, backs up a contentious 1993 study.
That research, the first to find evidence of a ‘gay gene’, created a storm of controversy, which was further fuelled when other scientists failed to find a genetic link.
Dr Michael Bailey from Northwestern University in Illinois, who contributed to the latest study, said it is the biggest of its kind.
He added: ‘Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play – we found evidence for two sets that affect whether a man is gay or straight.
‘Although this could one day lead to a pre-natal test for male sexual orientation, it would not be very accurate, as there are other factors that can influence the outcome.’
Despite this, he would not would not object to a prenatal test being developed.
He said: ‘Clearly parents should not be allowed to torture or kill babies. But they can currently choose to terminate a pregnancy early on, so they should be allowed to have as much information on the future child as possible.’
Qazi Rhaman, a King’s College London psychologist, said that genes are thought account for up to 40 per cent of a person’s sexual orientation is governed by genes, and that it is likely that many genes are involved.
This would make developing a genetic test incredibly difficult.
He said: ‘There is no real risk of anyone finding a 'genetic test' for sexual orientation based on these or any of the scientific findings about the genetics of sexuality from the past 20 years.
‘The reason is that there is no gay gene. You are not going to be able to develop a test to find them all.’
Dr Rhaman, who has written a book about sexual orientation, added that all psychological traits involve genetics and people should not be afraid about link homosexuality to genes.
Richard Lane, of gay rights organisation, Stonewall, said that while studies into the origins of homosexuality have yet to produce convincing evidence, they do to point to a biological root.
He said: ‘The thing that’s consistent across all of them is that they all point to sexual orientation being something fundamental to a person rather than the lifestyle choice some opponents of equality repeatedly suggest.’
Other studies suggests that conditions in the womb also influence sexual orientation.
For instance, the more older male siblings a man has, the greater chance he will be gay.
Scientists say the phenomenon cannot be explained by the youngest boy being babied and mollycoddled or other differences in the way they are brought up.
It is thought that carrying a male baby in the womb triggers an immune response in the mother, creating antibodies that attack part of the unborn child's brain linked to sexual orientation.
This response gets stronger the more boys a woman carries, raising the odds of homosexuality.
Exposure to hormones in the womb is also likely to be important and some argue that upbringing plays a role in sexuality.
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