A new study has found no evidence that male homosexuality is influenced by a gene passed from mother to son.
The study, reported in today's edition of the journal Science, does little to settle the issue of whether there is a gene on the X chromosome that predisposes men to homosexuality. Men inherit their single X chromosome from their mothers.
The gene was first predicted in 1993 based on studies of families with two gay brothers.
"The initial finding hasn't been disproven, but it probably hasn't been replicated to most people's satisfaction," said Alan Sanders, a psychiatrist and geneticist at the University of Chicago. "Maybe it will be, and maybe it won't be."
Dean Hamer, the scientist who first reported the connection, criticized the new study for not using families where homosexuality predominated on the mother's side. The researchers, argued Hamer, of the National Cancer Institute in Bethesda, Md., were making it harder on themselves to find the gene.
Several studies have supported the idea that genetics plays a role in homosexuality. But scientists say genetics cannot be the whole story.
For example, some gay men have an identical twin who is gay, and other gay men have an identical twin who is not gay. The term "gay gene" is misleading, scientists say, because there are likely to be many subtle genetic influences on sexual orientation.
What Hamer found
In the 1993 research, Hamer and his colleagues studied the genetic makeup of 40 families. As the researchers reported then in Science, genetic features on a region of the X chromosome tended to correlate with homosexual orientation. A 1995 study by the same researchers also suggested the link. An actual predisposing gene in the region has not been reported.
The result was intriguing to many people. Some thought the study supported the notion that homosexuality is not a choice.
But many scientists were cautious about saying Hamer's findings meant the "gay gene" really existed. While geneticists are adept at finding genes that cause clear-cut conditions, such as sickle cell anemia or cystic fibrosis, it has been much harder to pin down genes that underlie more complex traits.
For instance, scientists know that certain disorders, such as schizophrenia and manic depression, have a genetic component. But they have not been able to pin down the culprit genes. That's probably because many genes, as well as other influences, contribute to the disorders.
Results need to be seen again
Scientists don't put much stock in any one study unless other studies find the same result.
"The replication of a genetic finding in these behavioral (traits) is very difficult," said Melvin McInnis, a psychiatrist at Johns Hopkins University School of Medicine in Baltimore.
Sexual orientation is likely to be just as complicated, he said. The result of the new study - by George Rice, George Ebers and Carol Anderson at the University of Western Ontario and Neil Risch at Stanford Medical School in California - doesn't mean the first study was incorrect. Although the new study examined the same spot on the X chromosome in 104 gay men, it was done on different people. It could be, McInnis said, that an X chromosome gene is at work in the first group of people but not the second.
Sanders has also tried to replicate the findings of Hamer, although his results have not appeared in a scientific journal. He said his study was more in the middle - it didn't replicate Hamer's findings, but it didn't exclude the possibility as strongly as the new study did.
More should be studied
The only way to solve the problem is to study more people, the scientists said. Even then, if there is an effect of a gene on the X chromosome, scientists may never be able to pinpoint it if the effect is very small, Sanders said.
