Elusive answers to Darwin's riddle: Is a homosexual born or made? Theories abound over possibility of genetic link

IF THERE is a 'gay gene' that predisposes a man to being homosexual, and presumably less likely to father children, why has it not been wiped out by natural selection?

The question is essentially unanswerable for many reasons. First, no one has yet admitted to having identified and isolated a gene that causes homosexuality, only that a small region of the X chromosome is implicated in a predisposition to being gay in some men. Even so, a predisposition resulting in a lower fertility rate would lose out in Darwinian terms to an alternative version of the gene that confers in men a strong heterosexual attraction.

There are other examples of genes that do not seem to confer any advantage but which are common enough in the population not to be maintained through random mutations.

Cystic fibrosis, for instance, is extremely common in northern Europeans. One in every 2,500 babies have both copies of the defective gene, and as a result suffer from the disorder.

Steve Jones, professor of genetics at University College, London, said scientists do not know why such genes become so common despite their apparent disadvantages in terms of Darwinian evolution. 'In general terms we do not know why most genetic variation is there.'

One suggestion is that a genetic predisposition to homosexuality would somehow confer an advantage to sisters of the man, because in primitive societies he would be less likely to set up his own home and more likely to help to raise his sisters' children. But Professor Jones said there is 'absolutely no evidence' on this.

Richard Dawkins, a zoologist at Oxford University, has two possible explanations for why natural selection has not removed homosexuality. 'One arises from the fact that homosexual males do sometimes copulate with women. In primitive societies where dominant males defended harems, homosexual males might have obtained privileged access to women because the dominant males - wrongly - saw them as no threat. Hence the genes 'for' homosexuality were passed on.'

The other possible explanation is that the gene is not 'for' homosexuality as such but increases the likelihood of homosexual behaviour.

The gene may have one effect under certain environmental conditions - say bottle feeding - but a totally different effect given a different condition - say breast feeding.

'When our wild ancestors were subject to natural selection, the triggering condition for the gene to cause homosexuality would very likely have been different. If I am right, another way to put it is that the gene wasn't always a gene 'for' homosexuality. When it was subject to natural selection, it might have been a gene 'for' something quite different.'

(Photograph omitted)