Study Links Homosexuality and Heredity : Science: Research involving sets of brothers reinforces findings of earlier report.

Scientists have found new evidence that a gene inherited from mothers helps make men homosexual, bolstering a study that made headlines in 1993.

“Our result says that genes are involved in male sexual orientation, although they certainly do not determine a person’s sexual orientation,” said Dean Hamer, an author of the study.

“There probably are other biological factors like hormones, for example, and other variables we simply don’t know anything about yet.”

The study follows others that have suggested a biological influence in sexual orientation, but scientists still can’t explain what makes a person homosexual, heterosexual or bisexual.

The latest study does not identify a specific gene. But, like Hamer’s 1993 study, it suggests such a gene resides in a particular region of the X chromosome, one of the microscopic thread-like structures that carry genes. Men inherit the X chromosome from their mothers.

Hamer said there was no way to know how strongly the gene influences the development of homosexuality in men. Researchers looked for such an effect in women but found no evidence of it.

Hamer, a researcher at the National Cancer Institute, reports the work in the November issue of the journal Nature Genetics with colleagues at the institute, the University of Colorado and the Whitehead Institute for Biomedical Research in Cambridge, Mass.

Dr. Elliot Gershon, chief of the clinical neurogenetics branch of the National Institute of Mental Health, said the work does not prove such a gene exists in the chromosome segment, but it is suggestive.

The study focused on a region of the X chromosome that had been identified by the 1993 study.

In a man, this region can get its genetic material from either of his mother’s two X chromosome versions. As a result, two brothers normally have about a 50-50 chance of sharing the same version of the genetic material within this region.

Hamer and colleagues examined 32 pairs of exclusively or mostly homosexual brothers from unrelated families. Twenty-two pairs, or two-thirds, shared the same version of the genetic material, suggesting that it contains a gene predisposing to homosexuality.

By the same logic, Hamer said that if gay brothers shared a version containing a homosexuality gene, then any heterosexual brother of theirs probably would have a different version. The study showed that was the case in nine of 11 families studied.