Apoio | Perguntas e Respostas sobre a Homossexualidade | Influência genética na orientação sexual

Born to be Gay?
(Ask Dr. Weil 19 March 1997)

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    Is being gay a choice or genetic?

    Nobody knows just how a person becomes gay or lesbian, whether it's a choice or triggered by genes. My hunch is there is a significant inborn component; but I don't know whether that's all there is to the story.

    The biological root of sexual orientation is a matter of heated debate among scientists at the moment. In 1991, neuroscientist Simon LeVay, then at the Salk Institute in La Jolla, California, found that a particular region at the center of the brain (called the interstitial nucleus of the anterior hypothalamus) was much larger in straight men than in gay men or in women. He claimed this showed that sexual orientation was biological.

    Then, in July 1993, Dean Hamer of the National Cancer Institute in Washington, DC, reported that he had found a genetic factor called Xq28 that seemed to help determine male sexual orientation. In his study of 40 pairs of gay brothers, 83 percent shared a very similar region on the tip of their X chromosomes, which are inherited from the mother. Later, he looked at 33 more sets of brothers and found that 67 percent shared the region.

    But no one since has been able to duplicate Hamer's findings. Instead, researchers have found all sorts of problems with his work. His original study even became the subject of an investigation by a watchdog office on ethics, the national Office of Research Integrity, based on an allegation by a member of his lab that he skewed his selection of subjects and omitted data from his final report.

    The latest research makes matters even more muddy. One study looked at markers on the X chromosome in multiple sclerosis patients and got very similar results to Hamer's pivotal study, demonstrating that he could have been looking at something very different from gayness. In another, psychologist Scott Hershberger at the University of Kansas could find no evidence of heritability among self-identified gay men. He did find a 35 percent rate of heritability for attraction to the same sex, whether male or female. (In general, researchers have not found the same sort of genetic linkage to sexual orientation among women as among men.)

    Two other studies of twins concluded that male sexual orientation is about 50 percent heritable, but that number declined significantly in a second round of one of them.

    All this research, of course, is politically charged. Some people think discovery of a "gay gene" would make sexual orientation a biological necessity and thus help eliminate discrimination. Others think such a discovery would lead to genetic testing before birth, abortion of gay and lesbian fetuses, and efforts at biological intervention - or ways to "cure" sexual orientation.

    Geneticists I know say it's very difficult to study whether homosexuality has a genetic base. For one thing, it's hard to get people to participate in such studies in the first place. Then, it's not clear whether you're getting accurate information. Finally, scientists disagree about how to identify a person as gay or lesbian.

    Researchers say the picture is getting more and more complicated. If there is a genetic root to homosexuality, they say, it definitely won't be the action of just one gene, but the interaction of many. Even Dean Hamer, a strong proponent of a genetic cause, points to the studies of twin brothers to show that half or more of the variability in sexual orientation is not inherited. And some critics of his work and other researchers looking for a biological root to sexual orientation question the essential assumption that men have to be either gay or straight. Sexuality is a continuum of behavior, they say, not a species. I agree.

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