Male sexual orientation may be influenced by genetics, a new study suggests.

The findings, shared at the annual meeting of the American Association for the Advancement of Science in Chicago, revealed that in a study that tested the DNA of 409 gay men, at least two chromosomes may affect a man’s sexual orientation.

"Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play, and we found evidence for two sets that affect whether a man is gay or straight," said Michael Bailey of Northwestern University, who carried out the research.

The study involved drawing blood from 409 gay brothers and heterosexual members of their families. Analysis confirmed that an area on the X chromosome – which men inherit from their mothers -- known as Xq28 has some impact on sexual orientation. Another stretch of DNA on chromosome 8 also affects male sexual behavior, they said.

While the work has yet to be published, the findings confirm the results of a controversial study conducted in 1993 by Dean Hamer, a scientist at the U.S. National Cancer Institute. Hamer studied the family history of more than 100 gay men and found that more than 10 percent of brothers of gay men were gay themselves. This is a vast difference from just 3 percent for the general population. Hamer tied sexual orientation to the mother’s side of the family. In a follow-up work, he found 33 out of 40 gay brothers had similar genetic markers on the Xq28 region of the X chromosome, the Guardian reports.

At the time, Hamer’s study concluded that there is “99.5 percent certainty that there is a gene (or genes) in this area of the X chromosome that predisposes a male to become a heterosexual.”

The latest study confirms Hamer’s findings and suggests that a man’s sexual orientation depends on about 30 to 40 percent of genetic factors, while environmental factors, including the hormones a fetus is exposed to in the womb, may also influence a man’s sexuality.

"It is not completely determinative; there are certainly other environmental factors involved," Bailey said.

The study raises the possibility of prenatal testing to determine whether the child will be gay or straight. “Although this could one day lead to a prenatal test for male sexual orientation, it would not be very accurate, as there are other factors that can influence the outcome,” Bailey said. “Clearly parents should not be allowed to torture or kill babies. But they can currently choose to terminate a pregnancy early on, so they should be allowed to have as much information on the future child as possible.”

Bailey’s colleague, Alan Sanders, says the findings should not be used to test for sexual orientation.

"When people say there's a gay gene, it's an oversimplification," Sanders said. "There's more than one gene, and genetics is not the whole story. Whatever gene contributes to sexual orientation, you can think of it as much as contributing to heterosexuality as much as you can think of it contributing to homosexuality. It contributes to a variation in the trait."