The Molecular Genetic Study of Sexual Orientation is the most extensive study of its kind.
It was embarked upon by a team of researchers from Chicago universities four years ago.
Researchers hope that by gathering DNA samples from 1,000 sets of gay brothers they will be able to find genetic linkages smaller studies failed to detect.
The results of the study could lead to massive controversy, 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.
However, the scientists involved argue that the research is essential to our biological understanding of sexual behaviour.
"If there are genetic contributions to sexual orientation, they will not remain hidden forever. The march of genetic science can't be stopped," Timothy F. Murphy, the bio ethicist adviser to the study, told the Chicago Tribune.
"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."
Scientists are largely in agreement that sexual orientation is at least partially determined by biology.
Gregg Mierow, a participant in the study, has two gay brothers and three straight brothers.
"It seems innate to me," Mierow, who works in advertising and as a yoga teacher in Chicago, told the Chicago Tribune.
"It doesn't seem like there's any choice involved, and it seemed very clear even when we were very young."
Twins studies have consistently suggested that there are both genetic and environmental components to homosexuality.
Identical twins, who share 100% of their genes, show a higher chance of both being gay, compared to non-identical twins who only share half their genetic code but the same environment.
Researchers in the early nineties began comparing the genetic codes of gay brothers, who also share 50% of their genes in linkage studies.
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.
"In complex gene scenarios, people figured out that you need a larger sample size in order to get reasonable statistical power," Dr. Alan Sanders, a psychiatrist at Evanston Northwestern Healthcare and the leader of the current study told the Chicago Tribune.
To increase the chances of finding genetic areas associated with homosexuality, Sanders proposed collecting 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.
But he warns that 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."
"The genes would probably be doing their work by affecting sexual differentiation of the brain during prenatal life," J. Michael Bailey, a Northwestern University psychology professor involved with the project told the Chicago Tribune.
"But what scientists are increasingly appreciating is that genes can affect a trait in ways you could never have guessed."
The project will be searching for gay men with gay brothers at this weekend's Halstead Street festival.