BBC World Service's John Newell: Is this research needed?

Previous research suggested that male homosexuality is passed on from mother to son.

But the new work, covering more people, aims to show that the particular genetic features implicated are no more common in gay men than would be expected.

"Because our study was larger than the original one, we certainly had adequate power to detect a genetic effect as large as was reported in that study," said the team from the University of Western Ontario in the journal, Science.

The latest research says there is no gay gene at position Xq28

"Nonetheless, our data does not support the presence of a gene of large effect influencing sexual orientation," they concluded.

However, both the studies targeted only one part of the X chromosome. The authors of the new study say that: "These results do not preclude the possibility of detectable gene effects elsewhere in the genome."

Equality needed

Griffith Vaughan Williams from the UK Campaign for Homosexual Equality says conflicting research results in this field are beside the point: "The most important thing is, that however I and other homosexuals are created, we are treated as equals."

When the first study was published in 1993, there was concern it might lead to pre-natal screening and abortion of foetuses carrying the gene.

The new study used DNA from 52 pairs of gay brothers. These were recruited via advertisements in two Canadian gay news magazines. Mr Williams expressed surprise that so many homosexuals were willing to help in such experiments.

The researchers looked to see if the gay brothers shared more of the candidate genetic markers than would be expected. Any pair of brothers will share about half their DNA on any particular chromosome.

They found that 46% of the 52 pairs of brothers shared three key markers. In the previous study, which considered five markers, those scientists reported that 83% of 40 pairs shared the markers.

Conflicting results

"It is unclear why our results are so discrepant from the original study," say the scientists in Science. This is strong language for a scientific journal, implying the scientists believe that mistakes were made in the first study.

However, none of the team would speak to the BBC to confirm this. The new study has been criticised by the lead author of the old study, Professor Dean Hamer, over how the subjects were selected.

The 1993 study was the most powerful piece of evidence for a strong genetic factor in male homosexuality.

But other studies, such as one showing that identical twins are more likely to both be gay than non-identical twins, means that the nature-nurture debate will continue.