In this Aug. 20, 2007, file photo, Julio Cabrera poses in a local park in Chicago. Cabrera and his brother Mauricio, of Fort Worth, Texas, are among almost 800 gay brothers nationwide who donated blood or saliva to help scientists search for genetic clues about the origins of homosexuality. AP
CHICAGO 鈥� A large study of gay brothers adds to evidence that genes influence men鈥檚 chances of being homosexual, but the results aren鈥檛 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鈥檛 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 鈥渋s not proof but it鈥檚 a pretty good indication鈥� 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 鈥渋ntriguing but not in any way conclusive.鈥�
The work was published Monday by the journal Psychological Medicine. The National Institutes of Health paid for the research.
The researchers say they found potential links to male homosexuality in a portion of chromosome X and on chromosome 8, based on an analysis of genetic material in blood or saliva samples from participants.
Chromosome X is one of two human sex chromosomes; the other is chromosome Y, present only in men.
The study authors note that animal research suggests a gene located in one region of chromosome X may contribute to some sexual behavior; it鈥檚 one of the same regions cited in the new study.
Specific causes of homosexuality are unknown. Some scientists think social, cultural, family and biological factors are involved, while some religious groups consider it an immoral choice.
Study participant Dr. Chad Zawitz, a Chicago physician, called the research 鈥渁 giant step forward鈥� toward answering scientific questions about homosexuality and helping reduce the stigma gays often face.
Being gay 鈥渋s sort of like having certain eye color or skin color 鈥� it鈥檚 just who you are,鈥� Zawitz said. 鈥淢ost heterosexuals I know didn鈥檛 choose to be heterosexual. It鈥檚 puzzling to me why people don鈥檛 understand.鈥�