By LINDSEY TANNER AP Medical Writer
CHICAGO (AP) - A large study of gay brothers adds to evidence that genes influence men's chances of being homosexual, but the results aren't strong enough to prove it.
Some scientists believe several genes might affect sexual orientation. Researchers who led the new study of nearly 800 gay brothers say their results bolster previous evidence pointing to genes on the X chromosome.
They also found evidence of influence from a gene or genes on a different chromosome. But the study doesn't identify which of hundreds of genes located in either place might be involved.
Smaller studies seeking genetic links to homosexuality have had mixed results.
The new evidence "is not proof but it's a pretty good indication" that genes on the two chromosomes have some influence over sexual orientation, said Dr. Alan Sanders, the lead author. He studies behavioral genetics at NorthShore University HealthSystem Research Institute in Evanston, Illinois.
Experts not involved in the study were more skeptical.
Neil Risch, a genetics expert at the University of California, San Francisco, said the data are statistically too weak to demonstrate any genetic link. Risch was involved in a smaller study that found no link between male homosexuality and chromosome X.
Dr. Robert Green, a medical geneticist at Harvard Medical School, called the new study "intriguing but not in any way conclusive."
The work was published Monday by the journal Psychological Medicine. The National Institutes of Health paid for the research.
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