CHICAGO – In Gregg Mierow’s family, there were six boys, brothers who grew into two groups as they reached maturity: Three are gay, and three are straight.

“It seems innate to me,” Mierow, who works in advertising and as a yoga teacher in Chicago, said of his homosexuality. “It doesn’t seem like there’s any choice involved, and it seemed very clear even when we were very young.”

Mierow stumbled upon a chance to help prove that hunch at the Northalsted Market Days festival four years ago. Spotting a banner reading, “Wanted! Gay Men with a Gay Brother,” he stopped by the booth and volunteered for what he thought would be little more than a survey.

Instead, Mierow found himself part of the Molecular Genetic Study of Sexual Orientation – the most extensive study yet to search for a genetic basis for homosexuality – embarked upon by a team of Chicago researchers from local universities.

The scientists hope that by gathering DNA samples from 1,000 sets of gay brothers like the Mierows they will be able to find genetic linkages smaller studies failed to detect. They’ll be recruiting brothers again at the Halsted Street festival this weekend.

The results may ignite controversy, the researchers acknowledge, both by providing ammunition in the raging cultural war over homosexuality and by raising fears about ethically questionable applications like genetic profiling and prenatal testing.

But, they argue, the research is essential to our biological understanding of sexual behavior.

“If there are genetic contributions to sexual orientation, they will not remain hidden forever” the march of genetic science can’t be stopped,” said Timothy F. Murphy, bioethicist adviser to the study. “It’s not a question of whether we should or should not do this research, it’s that we make sure we’re prepared to protect people from insidious uses of this science.”

While the question of whether homosexuality is a choice remains a hot topic for pundits, scientists are largely in agreement that sexual orientation is at least partially determined by biology.

Inspired by the accumulating circumstantial evidence of genetic factors, researchers in the early ’90s began trying to narrow down the wide expanse of DNA to a few promising regions. By comparing the genetic codes of gay brothers, who also share 50 percent of their genes, a “linkage study” tries to detect areas that show up in both men at a frequency higher than chance, suggesting one or more genes in that region might be linked to sexual orientation.

In 1993, geneticist Dean Hamer announced his group had found such a region on the X chromosome, which males inherit from their mothers. But the number of brother pairs used in the study was small and subsequent studies failed to replicate Hamer’s findings, questioning the result.

“In complex gene scenarios, people figured out that you need a larger sample size in order to get reasonable statistical power,” said Dr. Alan Sanders, a psychiatrist at Evanston Northwestern Healthcare and the leader of the current study.

To increase the chances of finding genetic areas associated with homosexuality, Sanders proposed assembling almost 10 times the sibling pairs of previous studies. The project received funding in 2001 and began recruiting subjects at gay pride festivals, through gay-oriented publications and on the Internet.

So far the Chicago researchers have obtained blood or saliva DNA samples and survey data from more than 600 brother sets, with several hundred other volunteers in the process of submitting one or the other. Sanders hopes to publish his findings from the first wave of DNA samples in a scientific journal sometime next year.

Sanders cautioned a linkage study can single out only regions of the genetic code, not individual genes.

“One of the advantages of linkage studies is that we don’t have to know those things ahead of time,” Sanders said. “It’s a big advantage here because we don’t know about the biology of sexual orientation yet, so we can find the genes first and then study the biology.”

At this point, the researchers do not know what types of genes they may find; they could be related to hormones, sexual development or a completely unexpected system.