It is nurture and environment more than nature and genes that determine sexual orientation in men, according to the most comprehensive study into the question of the 'gay gene' to date.

While there is a genetic element to homosexuality, this new research concluded that it is not the primary determining factor.

Dr Michael Bailey, of Northwestern University in Chicago, Illinois, examined the blood of 409 gay brothers and heterosexual members of their families.

The results, which were presented yesterday at the American Association for the Advancement of Science's annual conference in Chicago, focused on part of the X chromosome known as 'Xq28'.

The research found this portion of the chromosome is more likely to be shared between two gay brothers than by brothers and their other siblings.

It is also possible that part of 'chromosome 8', one of the 23 pairs of chromosomes housed in the centre of each human cell, could predict homosexuality in men.

However, no one gene was singled out as being anything like a 'gay gene', and there is still no understanding of how these particular parts of genes relate to homosexual behaviour.

Dr Bailey was quoted by The Telegraph as saying that genetics were "not completely determinative" when it comes to homosexual behaviour, and that there are "certainly other environmental factors involved".

Speaking about practical applications of the research, Dr Bailey said: "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."

He added: "The possibility of testing is not the reason why we are conducting this research."

Dr Alan Sanders, the associate Professor of Psychiatry at Northwestern University who led the study, was quoted in The Telegraph describing the idea of a "gay gene" as an "oversimplification".

"We don't think genetics is the whole story. It's not. We have a gene that contributes to homosexuality but you could say it is linked to heterosexuality. It is the variation," he said. 

Qazi Rahman, a psychologist from King's College London speaking in The Times, also said that the use of the phrase 'gay gene' is unhelpful.

"It's a bit like making the proverbial 'haystack' just that bit more smaller so you can find the needles," he said "there is no such thing as one gay gene, there are most likely many genes that influence sexual orientation."

The Xq28 portion of the X chromosome, the chromosome always inherited from the mother, was first linked with homosexuality in 1993 after a study published in the journal Science by American geneticist Dean Hamer.

The media leapt on the story as the discovery of a 'gay gene', but Hamer and his colleagues were less than concrete on the issue.

Angela Pattatucci, a researcher who collaborated with Hamer on the Xq28 studies, said in a piece from The-Scientist.com: "We have made no claims about causality, or even that the Xq28 region contributes to male homosexuality. We have just said that there is a correlation. Causality is what the media suggested - they went further with the results than we did."

While Dr Bailey said that the largest factors in determining sexual orientation were most likely environmental, The Telegraph quoted him clarifying his statement, "Don't confuse 'environmental' with 'socially acquired'. Environment means anything that is not in our DNA at birth, and that includes a lot of stuff that is not social."

Other studies have dismissed the possibility of homosexuality being totally genetic. A 1991 study by J Michael Bailey and Richard Pillard published in the American Journal of Psychiatry found that in pairs of identical twins where one was gay, the other was gay only 52% of the time.

Given that identical twins share 100% of their DNA, this led to the conclusion that while homosexuality might have a genetic element, it is not the determining factor.

The study also found that adoptive brothers, who shared no genetic connections whatsoever, were even more likely to both be gay than genetically linked siblings, suggesting an environmental link.

Daryl Bem, a social psychologist at Cornell University speaking in The Telegraph today, suggests biological influence on sexual orientation may be mediated by childhood experiences. A child's temperament could impact the child to prefer certain activities.

He also observed: "Nobody has found something like this in women."

Other research has recently shed light on the 'Darwinian Paradox' of homosexuality. It was previously thought that if it was genetic, the lack or lessening of procreation involved in homosexuality would cause any 'gay gene' to die out.

However, a new Italian study suggests that genes linked to homosexuality in men could lead to increases in fertility in women. It found that female relatives of gay men tended to have more children on average than those with only straight male relatives.

Richard Lane, of the campaign group Stonewall spoke about studies into the origin of homosexuality in The Times: "The thing that's consistent is that they all point to sexual orientation being something fundamental to a personrather than the lifestyle choice some opponents of equality repeatedly suggest."