TUESDAY, Feb. 21 (HealthDay News) -- New research adds a twist to the debate on the origins of sexual orientation, suggesting that the genetics of mothers of multiple gay sons act differently than those of other women.

Scientists found that almost one fourth of the mothers who had more than one gay son processed X chromosomes in their bodies in the same way. Normally, women randomly process the chromosomes in one of two ways -- half go one way, half go the other.

The research "confirms that there is a strong genetic basis for sexual orientation, and that for some gay men, genes on the X chromosome are involved," said study co-author Sven Bocklandt, a postdoctoral researcher at the University of California at Los Angeles.

The link between genetics and sexual orientation has been a hot topic for more than a decade as a few scientists have tried to find genes that might make people gay or straight. In the new study, Bocklandt and colleagues examined a phenomenon called "X-chromosome inactivation."

While females have two X chromosomes, they actually require only one and routinely inactivate the other, Bocklandt said. "That way, both men and women have basically one functional X chromosome," he added. Men have both an X and Y chromosome, but the Y chromosome plays a much smaller role, he said.

Women typically inactivate one of their two X chromosomes at random. "It's like flipping a coin," Bocklandt said. "If you look at a woman in any given (bodily) tissue, you'd expect about half of the cells to inactivate one X, and half would inactivate the other."

In the new study, researchers looked at 97 mothers of gay sons and 103 mothers without gay sons to see if there was any difference in how they handled their X chromosomes. The findings appear in the February issue of the journal Human Genetics.

"When we looked at women who have gay kids, in those with more than one gay son, we saw a quarter of them inactivate the same X in virtually every cell we checked," Bocklandt said. "That's extremely unusual."

Forty-four of the women had more than one gay son.

In contrast, 4 percent of mothers with no gay sons activated the chromosome and 13 percent of those with just one gay son did.

The phenomenon of being more likely to inactivate one X chromosome -- known as "extreme skewing" -- is typically seen only in families that have major genetic irregularities, Bocklandt said.

What does this all mean? The researchers aren't sure, but Bocklandt thinks he and his colleagues are moving closer to understanding the origins of sexual orientation.

"What's really remarkable and very novel about this is that you see something in the bodies of women that is linked to a behavioral trait in their sons," he said. "That's new, that's unheard of."

Still, there are caveats. Dr. Ionel Sandovici, a genetics researcher at The Babraham Institute in Cambridge, England, pointed out that most of the mothers of multiple gay sons didn't share the unusual X-chromosome trait. And the study itself is small, which means more research will need to be done to confirm its findings, Sandovici said.

Ultimately, Sandovici added, the origins of sexual orientation remain "rather a complicated biological puzzle."

And this line of research does have its critics. Some have worried that, in the future, manipulation of a "gay gene" or genes might be used as a method of preventing homosexuality in utero, or perhaps even after. But Bocklandt said these kinds of fears shouldn't stand in the way of legitimate scientific research.

"We're trying to understand one of the most critical human traits: the ability to love and be attracted to others. Without sexual reproduction we would not exist, and sexual selection played an essential role in evolution," he said. "Yet, we have no idea how it works, and that's what we're trying to find out. As with any research, the knowledge you acquire could be used for benefit or harm. But if [scientists] would have avoided research because we were afraid of what we were going to find, then we would still be living in the stone age."

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