While earlier studies had focused solely on the X chromosome, one of the two sex chromosomes, the present study examined all 22 pairs of non-sex chromosomes in addition to the X chromosome. The other sex chromosome, called Y, was not explored because it is not believed to contain many genes.

Identical stretches of DNA on three chromosomes — chromosomes 7, 8 and 10 — were found to be shared in about 60 percent of the gay brothers in the study, compared to about 50 percent expected by chance. The region on chromosome 10 correlated with sexual orientation only if it was inherited from the mother.

“Our study helps to establish that genes play an important role in determining whether a man is gay or heterosexual,” said Mustanski. “The next steps will be to see if these findings can be confirmed and to identify the particular genes within these newly discovered chromosomal sequences that are linked to sexual orientation.”

Other researchers involved in the study were Dean Hamer, at the National Institutes of Health; Nicholas Schork and Caroline Nievergelt, at the University of California at San Diego; Michael DuPree, at Pennsylvania State University; and Sven Bocklandt, at the University of California at Los Angeles.

The study was supported in part by grants from the National Science Foundation and the National Institutes of Health.  Text for this article comes from a UI press release.