Genetic factors, along with cultural and early experiences, influence male homosexuality, Italian scientists said on Wednesday.
Researchers at the University of Padua said the genetic components are linked to the X chromosome, which boys always inherit from their mother, but they are probably also on other chromosomes.
“The key factor is that these genes both influence homosexuality in men, higher fecundity in females and are in the maternal and not the paternal line,” Andrea Camperio-Ciani, who headed the research team, said in an interview.
The fact that the genetic factors also raise female fertility counters an argument against a genetic basis of homosexuality - that because gay men are less likely to reproduce, the gene would disappear from the population. However, the new findings suggest that this is offset by higher female reproduction.
More than a decade ago scientists in the United States reported that they had found evidence of a “gay gene” in men. But other researchers questioned the finding when they could not duplicate the results.
Camperio-Ciani and his team suggest several genes could be involved, including those on the X chromosome.
In their research, which is reported in the Proceedings of the Royal Society, they found an increase in homosexuality in the maternal line of gay men they studied, which suggests the X chromosome.
“We know that at least one of these genetic factors is on the X chromosome but that it not enough, there must be other genetic factors that are important but are elsewhere,” Camperio-Ciani explained.
The results are based on a study of 98 homosexual and 100 heterosexual men and about 4,600 of their relatives. The scientists compared the frequency of gay men on the maternal and paternal lines of the families.
Among homosexuals there were a greater number of gay men in the maternal line of the family, as well as greater fertility in the female relatives.
“We can no longer say that it is impossible to have a gene that influences homosexuality because we found out that genes might have different effects depending on gender,” Camperio-Ciani concluded.
But he added that cultural and individual experience also play a part.
Revision date: June 20, 2011
Last revised: by Sebastian Scheller, MD, ScD