Julio and Mauricio Cabrera are gay brothers who are convinced their sexual orientation is as deeply rooted as their Mexican ancestry. They are among 1,000 pairs of gay brothers taking part in the largest study to date seeking genes that may influence whether people are gay.

The Cabreras hope the findings will help silence critics who say homosexuality is an immoral choice.

If fresh evidence is found suggesting genes are involved, perhaps homosexuality will be viewed as no different than other genetic traits like height and hair color, said Julio, a student at DePaul University in Chicago.

Adds his brother, "I think it would help a lot of folks understand us better."

The federally funded study, led by Chicago-area researchers, will rely on blood or saliva samples to help scientists search for genetic clues to the origins of homosexuality. Parents and straight brothers also are being recruited.

While initial results aren't expected until next year -- and won't provide a final answer -- skeptics are already attacking the methods and disputing the presumed results.

Previous studies have shown that sexual orientation tends to cluster in families, though that doesn't prove genetics is involved. Extended families may share similar child-rearing practices, religion and other beliefs that could also influence sexual orientation.

Research involving identical twins, often used to study genetics since they share the same DNA, has had mixed results.

One widely cited study in the 1990s found that if one member of a pair of identical twins was gay, the other had a 52 percent chance of being gay. In contrast, the result for pairs of non-twin brothers, was 9 percent. A 2000 study of Australian identical twins found a much lower chance.

Dr. Alan Sanders of Evanston Northwestern Healthcare Research Institute, the lead researcher of the new study, said he suspects there isn't one so-called "gay gene."

It is more likely there are several genes that interact with nongenetic factors, including psychological and social influences, to determine sexual orientation, said Sanders, a psychiatrist.

Still, he said, "If there's one gene that makes a sizable contribution, we have a pretty good chance" of finding it.

Many gays fear that if gay genes are identified, it could result in discrimination, prenatal testing and even abortions to eliminate homosexuals, said Joel Ginsberg of the Gay and Lesbian Medical Association.

However, he added, "If we confirm that sexual orientation is an immutable characteristic, we are much more likely to get the courts to rule against discrimination."

There is less research on lesbians, Sanders said, although some studies suggest that male and female sexual orientation may have different genetic influences.

His new research is an attempt to duplicate and expand on a study published in 1993 involving 40 pairs of gay brothers. That hotly debated study, wrongly touted as locating "the gay gene," found that gay brothers shared genetic markers in a region on the X chromosome, which men inherit from their mothers.

That implies that any genes influencing sexual orientation lie somewhere in that region.

Previous attempts to duplicate those results failed. But Sanders said that with so many participants, his study has a better chance of finding the same markers and perhaps others on different chromosomes.

If these markers appear in gay brothers but not their straight brothers or parents, that would suggest a link to sexual orientation. The study is designed to find genetic markers, not to explain any genetic role in behavior.

And Sanders said even if he finds no evidence, that won't mean genetics play no role; it may simply mean that individual genes have a smaller effect.

Skeptics include Stanton Jones, a psychology professor and provost at Wheaton College in Wheaton, Ill. An evangelical Christian, Jones last month announced results of a study he co-authored that says it's possible for gays to "convert" — changing their sexual orientation without harm.

Jones said his results suggest biology plays only a minor role in sexual orientation, and that researchers seeking genetic clues generally have a pro-gay agenda that will produce biased results.

Sanders disputed that criticism.

"We do not have a predetermined point we are trying to prove," he said. "We are trying to pry some of nature's secrets loose with respect to a fundamental human trait."

Jones acknowledged that he's not a neutral observer. His study involved 98 gays "seeking help" from Exodus International, a Christian group that believes homosexuals can become straight through prayer and counseling. Exodus International funded Jones' study.

The group's president, Alan Chambers, said he is a former homosexual who went straight and believes homosexuality is morally wrong.

Even if research ultimately shows that genetics play a bigger role, it "will never be something that forces people to behave in a certain way," Chambers said. "We all have the freedom to choose."

The Cabrera brothers grew up in Mexico in a culture where "being gay was an embarrassment," especially for their father, said Mauricio, 41, a car dealership employee from Olathe, Kan.

They had cousins who were gay, but Mauricio said he still felt he had to hide his sexual orientation and he struggled with his "double life." Julio said having an older brother who was gay made it easier for him to accept his sexuality.

Jim Larkin, 54, a gay journalist in Flint, Mich., said the genetics study is a move in the right direction.

Given the difficulties of being gay in a predominantly straight society, homosexuality "is not a choice someone would make in life," said Larkin, who is not a study participant.

He had two brothers who were gay. One died from AIDS; the other committed suicide. Larkin said he didn't come out until he was 26.

"I fought and I prayed and I went to Mass and I said the rosary," Larkin said. "I moved away from everybody I knew ... thinking maybe this will cause the feelings to subside. It doesn't."

(Agencies via China Daily October 16, 2007)