Campaigners object to gay being labelled a lifestyle...
Scientists say they have shown how male homosexuality could be passed from generation to generation...
Nature encourages mothers to pass on a "gay trait" to their male offspring by boosting their fertility, the Italian University of Padova team believes.
This would keep the pattern of gay inheritance alive, they told the Royal Society's Biological Sciences journal.
Critics of the theory argue a gay gene would eventually be wiped out because gay couples do not procreate.
There is controversy about whether sexual orientation is a matter of choice, the authors of the study admitted to the journal.
Campaigners say equality for homosexual people is the more important issue.
Back in 1993, US researchers suggested male homosexuality was passed from mother to son after they found strong patterns of inheritance in family trees.
There might be...reproductive advantages associated with male homosexuality.
The study authors
It has also been noted that homosexual males are more often the younger siblings of a number of older brothers.
Scientists have said it might be that the mother develops some kind of resistance to the male Y chromosome in her offspring that makes subsequent baby boys more likely to be born gay.
Scientists doing DNA studies on homosexual brothers pinpointed 'culprit' genetic material to a region of the X chromosome that mothers pass on to their offspring.
But other researchers in the US have not been able to replicate these findings.
Andrea Camperio-Ciani and colleagues argue genetic factors favouring homosexual male offspring could make women more fertile.
"Our data resolve this paradox by showing that there might be, hitherto unsuspected, reproductive advantages associated with male homosexuality," they said.
They looked at 98 homosexual and 100 heterosexual men and their relatives, which included more than 4,600 people overall.
Is a "gay gene" passed from mother to son?
The female relatives on the mother's side of the homosexual men tended to have more offspring than the female relatives on the father's side.
This suggests that these women who, in theory, pass on the gay trait to their male offspring are also more fertile.
In comparison, the female relatives on both the mother's and the father's side of the heterosexual men did not appear to be as fertile, having fewer offspring.
The researchers believe the homosexuality-increased fertility trait must be passed down on the female X chromosome.
They pointed out that this would not explain the majority (80%) of cases, and that cultural factors might be important.
"It is clear that our findings, if confirmed by further research, are only one piece in a much larger puzzle on the nature of human sexuality," they said.
In 2002, the Nuffield Council on Bioethics produced a report into the possible link between genes and behaviour, which included sexual orientation.
It concluded: "There are numerous problems with genetic and other biological research into sexual orientation which mean that any reported findings must be viewed with caution."
It said many of the genetic studies were too small to draw definite conclusions from.
Alan Wardle from the gay rights charity Stonewall said: "This is an interesting debate and there may well be a genetic element, but it's not conclusive.
"It does not really matter whether it is nature or nurture.
"The important thing is getting equality for homosexual people."