Fresh evidence has emerged to back up the idea male sexual orientation is influenced by genes.
Researchers looked at the DNA of 400 gay men and found two chromosomes that appear to be linked to homosexuality.
But it is not known which of the chromosomes’ genes are the important ones and how they affect sexual orientation.
The work, unveiled at the American Association for the Advancement of Science’s annual meeting, backs up a controversial 1993 study. That research, by Dean Hamer, concluded homosexuality tended to be inherited.
Michael Bailey, a psychologist at Northwestern University in Illinois, said: “Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play – we found evidence for two sets that affect whether a man is gay or straight.
“But it is not completely determinative; there are certainly other environmental factors involved. The study shows that there are genes involved in male sexual orientation.
“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.”
Qazi Rahman, a psychologist at King’s College London, told the Guardian: “This is not controversial or surprising and is nothing people should worry about. All human psychological traits are heritable, that is, they have a genetic component,” he said. “Genetic factors explain 30 to 40% of the variation between people’s sexual orientation.
“However, we don’t know where these genetic factors are located in the genome. So we need to do ‘gene finding’ studies, like this one by Sanders, Bailey and others, to have a better idea where potential genes for sexual orientation may lie.”