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The biological basis of homosexuality

Is there a biological basis for homosexuality? With gay marriage now supported by the state's highest court and homosexuality likely to be a hot-button issue in the presidential campaign, the question of whether sexual orientation is an innate or acquired trait is an increasingly urgent one.

Since at least 1991, some scientific research has suggested a biological basis to homosexuality -- meaning sexual orientation is probably at least partly natural destiny, not a choice. But that point is still open to political and scientific debate, and our understanding of how biology may drive sexual orientation is still fuzzy.

Some data on identical twins suggest that homosexuality -- particularly in men -- is inherited. Other scientists have tried to pin down anatomical differences in brain structure between gay men and straight men.

Understanding homosexuality, or even heterosexuality, involves, among other things, figuring out how the brain, the seat of all complex behavior, becomes male or female in the first place.

Until recently, researchers thought that a surge in the male hormone testosterone set the brain on a male track. Without testosterone, the brain continued developing on a female track.

But, in October, California researchers studying fetal development identified 54 genes that play a role in the expression of sex -- before hormones are ever released.

"This refutes the idea that hormones are the only story in sexual differentiation of the brain. That has been the dogma in the field for 30 years," said Dr. Eric Vilain, an assistant professor of human genetics and urology at the David Geffen School of Medicine at the University of California at Los Angeles, who led the research.

The study in mice "gives us a radical new insight into sexual differentiation of the brain," said Dr. Elliot S. Gershon, a professor of psychiatry and human genetics at the University of Chicago. "It's quite an important paper."

Its implications are many. An estimated one in 4,000 babies is born with "ambiguous genitalia," making it difficult to tell whether the baby is a boy or a girl. By analyzing chromosomes and looking for internal sexual organs such as ovaries or a prostate gland, doctors make their best guess as to the true sex of the child and sometimes perform surgery to make the anatomy conform to that.

DNA analysis of the variations in these 54 genes and other genes that interact with them may help doctors figure out to which gender the child most appropriately belongs, Vilain said.

The 54 genes also may help explain transgenderism, the situation, which affects about one in 50,000 people, in which a person believes he or she was born the "wrong" sex. Some transgendered individuals simply live as the gender they feel they are, regardless of anatomy; others have sex-change surgery.

The UCLA study does not address homosexuality directly. But Vilain said other data suggest that 75 percent of boys who were confused about their gender identity as children grow up to be gay. The new study, he said, may help "pave the way to find out about

gender identity" in such children. Other studies on the genetic roots of homosexuality, which affects at least 3 percent to 4 percent of the population (activists say the figure is higher), are mixed.

Dr. Richard C. Pillard, a professor of psychiatry at the Boston University School of Medicine, has studied male and female homosexuals. In men, he said, sexual orientation is often inherited. In women, "sexuality is not as rigidly set." In identical male twins, his research shows, if one is gay, there's a 50 percent chance that the other one is. Granted, if homosexuality were totally genetically determined, that figure should be 100 percent. On the other hand, in male fraternal twins, there's only a 20 percent chance that if one is homosexual, the other will be.

In 1991, an autopsy study by Simon LeVay at the Salk Institute for Biological Studies in San Diego found that part of the brain called the anterior hypothalamus was twice as large in heterosexual men as in homosexual men, suggesting a biological basis for homosexuality. Because the gay men all had AIDS, it is possible that the disease, rather than their homosexuality, transformed their brains.

And other studies that have tried to draw a biological link to homosexuality have faced problems as well.

In 1993, Dean Hamer, a molecular biologist at the National Cancer Institute, studied 40 pairs of gay brothers and published his results in Science. With a technique called linkage mapping, Hamer identified a region called Xq28 on the X chromosome (inherited from the mother) that was statistically correlated to homosexuality. In 1995, a second study by Hamer and others confirmed that finding.

In 1999, researchers led by George Rice at the University of Western Ontario in Canada studied the same brain region in 52 gay male sibling pairs and reported contradictory findings. Clearly, more research is needed to prove homosexuality is inherited. But Dr. Fred Berlin, a psychiatrist at Johns Hopkins University School of Medicine, said the basic conclusion is already clear: Homosexuality, he said, "is not due to voluntary choice. None of us as kids sat down and said, `Do I want to be attracted to members of the same gender?' "

Judy Foreman is a freelance columnist who can be contacted at foreman@globe.com.

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