A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Is there a "gay gene"?

July 28, 1995

Dear Cecil:

More and more often I have been coming across claims that homosexuality is genetic and not learned. Who proved this? Did they really isolate a gene that predisposes one to develop homosexual interests or did they just find a pair of identical twins separated at birth who were later discovered to have developed into homosexual adults? Are there similar genes for pedophiles, necrophiles, or foot fetish — [question truncated]

Cecil replies:

Sometimes you just have to know when to shut up, chum. Despite what you may have seen, nobody except headline writers seriously thinks there's a "gay gene," i.e., if you've got it you're gay and if you don't you're not. However, there may be some genetic basis for homosexuality, as shown by the following research:

Brother studies. Dean Hamer et al collected data on 76 gay men and more than 1,000 of their male relatives. Homosexuality among maternal uncles and sons of maternal aunts was 4-6 times more common than in the general population. Paternal relatives showed much less difference. This suggests a genetic mechanism involving the X chromosome, the sex chromosome men inherit from their mothers. Hamer then studied 40 pairs of gay brothers. In 33 cases they found the brothers had the same five DNA markers in a region of the X chromosome called Xq28. The odds of this occurring by chance are less than 1 in 100.

Brain studies. Having examined the brains of 19 gay men who had died of AIDS plus those of 16 heterosexual men, Simon LeVay found that a brain region known as INAH 3 was much smaller in the gays than in the straights. INAH 3 is located in the hypothalamus, a part of the brain thought to be associated with sexual behavior. The difference wasn't just due to AIDS; six of the heterosexual men had died of AIDS, too. This research has yet to be replicated and even if it is we don't know whether a small INAH 3 is the cause or effect of gayness.

Bug studies. Two scientists, S.-D. Zhang and W. F. Odenwald, found that by tweaking the genes of fruit flies they could induce gay group gropes — five or more males flies would link together in chains or circles and lick each other's genitalia. Interesting, but it's a long reach from the sexual behavior of fruit flies to that of humans.

So OK, maybe your genes have some influence on your sexual preference. But how you get from DNA markers to Castro Street — that is, how genes influence that rat's nest of thoughts and behavior we call sexual identity — we're still a long way from finding out.

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