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Is Sexuality Hardwired?

Gender Identity Linked With Dozens of Genes
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WebMD Health News

Oct. 20, 2003 -- Through mice, humans are beginning to understand the mystery of sexual identity. A new study provides the first clues that sexuality is hardwired in the brain before birth -- that gender identity and homosexuality are not chosen.

The study turns science on its ear.

"Since the 1970s, the dogma has been that hormones are the driving factor that masculinized or feminized the brain," says lead researcher Eric Vilain, MD, PhD, professor of human genetics, pediatrics, and urology at UCLA's David Geffen School of Medicine.

"Hormones do play a role, but hormones are not the whole story," Vilain tells WebMD. His study appears in the October issue of Molecular Brain Research.

In fact, recent studies at UCLA have also shown that chromosomes affect behavior. "What we've done is try to identify the specific genes involved in gender identity."

Mysteries Within Mouse Brains

In their study, Vilain and his colleagues analyzed mouse brains -- that is, mice that were still fetuses, just 10 days past conception. "This is right before the gonads become testes or ovaries," he explains. "So at the time we looked at their brains, there was no source of hormones."

Researchers also checked each mouse to determine their genetic sex -- whether they had XY (male) or XX (female) chromosomes.

They found more than 50 genes that expressed differently in males and females, reports Vilain. "Some will be turned on at high levels in males but not in females, and in others, it's the opposite situation."

These are the biological clues to gender identity -- one's own perceptions of one's sex, whether they feel they are male or female, Vilain explains.

"It's independent from the way we look, and also independent from sexual attraction," he adds. "Sexual orientation is independent from gender identity. A huge majority of gay men don't feel that they belong to the wrong gender."

But the findings "could explain why some people don't feel right in their bodies or in their gender, why some people are transsexual," he adds. "They have XY chromosomes, penis, scrotum, normal levels of testosterone, but they feel they were not meant to be men. I believe it's mostly biologically determined. Hormones play a role, but this is proof that there are genetic determinants that are independent of hormones."

David Weinshenker, PhD, professor of human genetics at Emory University School of Medicine in Atlanta, says this may be the "first inkling" that gender identity is contained within our genes.

Studies of fruit flies have produced similar results, Weinshenker tells WebMD. "You can change the sexual behavior of the flies by changing one gene."

The complexity of genes that Vilain found in mammals is "surprising" -- and perhaps the "first inkling" that gender identity resides in our genes, he says.

However, "we're a very long way from using gene expression to predict behavior," he says. "There are too many genes involved, and too much influence of environment. Almost everything we are and do is a combination of genes and environment, and trying to pinpoint reason for any particular behavior is tricky."

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