The genes a man gets from his mother and father may play an important role in determining whether he is gay or not, according to a new study likely to reignite the “gay gene” debate.

Researchers say it’s the first time the entire human genetic makeup (search) has been scanned in search of possible genetic determinants of male sexual orientation. The results suggest that several genetic regions may influence homosexuality.

“It builds on previous studies that have consistently found evidence of genetic influence on sexual orientation, but our study is the first to look at exactly where those genes are located,” says researcher Brian Mustanski, PhD, a psychologist at the University of Illinois at Chicago.

Those previous studies looked only at the genes located on the X chromosome (search). Genes on this chromosome are only passed to a son from his mother. But this study examined genetic information on all chromosomes, including genes from the father.

The findings show that identical stretches of DNA on three chromosomes were shared by about 60 percent of gay brothers in the study compared to the about 50 percent normally expected by chance.

Gay Gene Debate

A heated debate over the existence of a “gay gene” emerged from a 1993 report published in the journal Science by then-NIH researcher Dean Hamer, PhD. That study linked DNA markers on the X chromosome to male sexual orientation.

Since then, questions arose regarding the validity of those results. Other researchers are attempting to replicate and verify Hamer’s findings. Hamer is also senior author of the current study, which appears in the March issue of Human Genetics.

But researchers say this study takes a different approach. Its goal was not to replicate those findings but to search for new genetic markers associated with male sexual orientation.

“Since sexual orientation is such a complex trait, we’re never going to find any one gene that determines whether someone is gay or not,” says Mustanski. “It’s going to be a combination of various genes acting together as well as possibly interacting with environmental influences.”

Previous studies in male twins have suggested that between 40-60 percent of the variability in sexual orientation is due to genes. The rest is thought to be due to environment and possibly other biologic but nongenetic causes.

Search for Gay Genes

In the study, researchers analyzed the genetic makeup of 456 men from 146 families with two or more gay brothers.

The genetic scans showed a clustering of the same genetic pattern among the gay men on three chromosomes -- chromosomes 7, 8, and 10. These common genetic patterns were shared by 60 percent of the gay men in the study. This is slightly more than the 50 percent expected by chance alone.

The regions on chromosome 7 and 8 were associated with male sexual orientation regardless of whether the man got them from his mother or father. The regions on chromosome 10 were only associated with male sexual orientation if they were inherited from the mother.

Mustanski compares the study’s approach to a search for doctors in a town of 40,000 people, a number that roughly corresponds to the number of human genes.

Rather than guessing that doctors live in a particular type of house and going to only the houses that meet that criteria, researchers in this scenario would knock on every door to ask the residents if a doctor lives on their street. Using a similar approach, researchers were able to locate a few potential genetic neighborhoods that likely contribute to male sexual orientation.

Researchers say the next step is to verify these results in a different group of men to see if the same genetic regions are associated with sexual orientation. If the findings hold up, then Mustanski says they could start to look for the individual genes within these regions linked to sexual orientation.

New Targets for Gay Gene Research

Elliot S. Gershon, MD, professor of psychiatry and human genetics at the University of Chicago, says the study represents an important step forward in understanding how genes affect human sexual orientation.

“It is worth testing genes within a region of linkage to see if one of them has a variant that is more frequent in men who are gay than in men who are not,” says Gershon, who is also currently involved in another study of gay brothers and genetic influences on sexual orientation.

“This report adds to the legitimacy of research on normal variations in human behavior,” Gershon tells WebMD. “There is an argument that has been made in public press that it doesn’t make sense to study conditions or traits that are behavioral. But this suggests that there is a genetic contribution to this particular trait of same sex orientation.”

By Jennifer Warner, reviewed by Michael W. Smith, MD

SOURCES: Mustanski, B. Human Genetics, March 2005 online first edition. Brian Mustanski, PhD, department of psychiatry, University of Illinois at Chicago. Elliot S. Gershon, MD, professor of psychiatry and human genetics, University of Chicago. News release, University of Illinois at Chicago. Council for Responsible Genetics.