| [ sExLife ] in KIDS 글 쓴 이(By): claudia (가 아님...맧) 날 짜 (Date): 1998년01월21일(수) 03시14분03초 ROK 제 목(Title): Cap: 동성애 유전자? [ sExLife ] in KIDS 글 쓴 이(By): guest (guest) 날 짜 (Date): 1998년01월21일(수) 01시22분01초 ROK 제 목(Title): 동성애 유전자? 도움이 되지 않을까 해서 1996년 11월에 New Scientist지에 실린 글을 옮깁니다. 글 끝에쯤에 아까 John/Joan이야기의 주역인 John Money박사도 잠깐 나오는데 아마 이 사건이 다시 터지기 전의 일인 것 같군요. 이미 1년 전의 기사라 그새 어떤 진전이 있는지는 모르겠습니다만, '애를 못 갖는 게이 유전자가 만일 존재한다면 어떻게 자연 선택을 살아남을 수 있는가'에 대한 재미있는 가설들이 있군요. -------- Copyright 1996 New Scientist IPC Magazines Ltd New Scientist September 28, 1996 SECTION: Features, Pg. 32 LENGTH: 4227 words HEADLINE: Born to be gay? BYLINE: Vittoria D'Alessio (Vittoria D'Alessio is a freelance journalist workingin London and Hong Kong.) BODY: SOME parents cannot stomach the news that their son has embraced a gay lifestyle. But when Roger Gorski tells them that their sons have inherited "an immutable, behaviourally expressed, bimodal trait" rather like left-handedness, homosexuality assumes a whole new mantle of respectability. "I recently spoke at a seminar for Mormon parents of gay children in Salt Lake City and it was amazing," says Gorski. "I felt like I was talking in a vacuum, with every word sucked up. It was the parents who were coming out." Such meetings are all in a day's work for the neurobiologist from the University of California in Los Angeles. He welcomes the chance to tell angst-ridden parents a few biological facts: that homosexuality is a normal genetic variant, that it is not a lifestyle choice, a disease, or a mental illness, and that in all probability it was passed down from the person's mother. Vindicated Everyone leaves these cosy assemblies smiling - Gorski has dispensed some soothing news, gay men feel vindicated, parents can go back to loving their homosexual sons unconditionally. And Dean Hamer, the molecular geneticist from the National Institutes of Health near Washington DC, who, in 1993, discovered where in the genome the gay gene nestles, can pat himself on the back for fostering goodwill on earth. Gorski has also contributed to the idea that gayness is biological in origin. In 1992, he showed that the anterior commissure, a bundle of nerves that connects a small region of the right and left sides of the brain's cortex, is larger in gay men than in straight. All the same, Gorski is the first to acknowledge that in the name of community spirit, the scientific fellowship is quietly blending fact with biological theory. For in reality, today's scientists are about as baffled by homosexuality as they have ever been. In the intervening three years, Hamer's spectacular finding has started to look shaky. Some geneticists have poked holes in his original methodology, and others have failed to confirm the existence of the gay gene with their own studies. But those disappointments have done nothing to dull the intense scientific and public interest in what triggers gayness. And if human genetics has drawn a blank in the past few years, new studies of fruit fly courtship are reinforcing the idea that, in the end, homosexuality will come down to the difference of a gene or two. Meanwhile, one Cornell psychologist has introduced a fresh twist to the ancient argument that homosexuality is an acquired rather than innate behaviour - with his controversial "exotic to erotic" theory of sexual orientation. That gayness is at least partly genetic is just about indisputable. A man with a gay identical twin brother - that is a brother who to all intents and purposes shares all his genes - has a 52 per cent chance of being gay, and a man with a gay non-identical twin only a 22 per cent chance, according to a 1991 study from psychologist J. Michael Bailey at Northwestern University in Chicago. X-rated In their study, Hamer's team showed that specific genetic markers on a region of the X chromosome called Xq28 were shared by 33 of 40 pairs of gay brothers. Chance dictates that the brothers inherit either of their mother's X chromosomes at the same rate, that is that only 20 pairs of brothers should haveshared the markers. Hamer concluded that within a region of 4 million DNA base pairs on the tip of the long arm of the X chromosome, lies a locus related to sexual orientation. He calls this locus GAY-1. Harvard biologist Evan Balaban loathes the over-simplification of the gay gene topic by both scientists and journalists that accompanied Hamer's original identification of GAY-1. "I wish that some of the work was done a bit more carefully and presented more conservatively," said Balaban. "It can only lead to misunderstanding and disappointment." Certainly, with the air still ringing from the trumpets and cymbals that accompanied the release of Hamer's original research paper, it comes as a great anticlimax that other scientists have been unable to reproduce his findings. George Ebers, a neurologist at the University of Western Ontario in London, failed when he searched for the same genetic landmarks on Xq28 of 54 pairs of gay Canadian brothers. Even Hamer seems to be having problems repeating his own work with the same degree of conviction. In a recent repeat study of 33 pairs of gay brothers, he found that only 67 per cent shared genetic markers on Xq28, compared with the 82 per cent identified in his first study. One of the most persistent criticisms of Hamer's research has been his ready acceptance, after interviews with the family members of gay men, that there were considerably more gay relatives on the mother's side of the families than the father's. It was this observation that led him to confine his search to the X-chromosome, instead of the other 22. But, says Balaban, "if you ask about medical history by interviewing family members, who knows more about it, men or women? Women of course." But Hamer is quick to defend his methodology. "We did several controls. We asked the same people who the lesbians were in the family, and from their answers we found they were evenly distributed between the two sides of the family." And even if Hamer is right, and GAY-1 does contain the key to male homosexuality, the work is far from over. GAY-1 contains several hundred genes, and only by teasing out the gene or genes responsible for sexual orientation will there be a chance of working out the biochemical basis of why some men are turned on by other men. Hamer refuses to say how far his search has progressed. But if the genetics of sexual orientation in humans has reached deadlock, work on sexual orientation in fruit flies is winging ahead. Way back in 1963, the gene responsible for same-sex courtship in the fruit fly (Drosophila melanogaster) was tracked down to a region on the third chromosome (the species only has four chromosomes) by Kulbir Gill, a visiting Indian scientist to Yale University in New Haven, Connecticut. Jeffrey Hall, a geneticist at Brandeis University in Waltham, Massachusetts, later named the genetic variant fruitless. Flies do it Fruitless males court females as well as other males, making them bisexual rather than gay. Now, Hall, molecular biologist Bruce Baker of Stanford University in Palo Alto in California, and neurobiologist Barbara Taylor from Oregon State University in Corvallis, have completed a three-year project that created an array of reproductively dysfunctional cousins of the original fruitless variant. The results are expected to show that one of the new fruitless variants courts males, but not females, making it a true "gay" fruit fly. The biochemical function of the fruitless gene remains a mystery. Still, Hall has his suspicions. He believes that the unmutated version of the fruitless gene that regular flies carry is central in setting up a region in the nervous system that teaches male fruit flies to recognise the cues from females and ignore other cues that are useless for reproduction. And although he is quick to point out the risks of extrapolating from flies to humans ("You don't have to treat human chromosomes with X-rays to get homosexuality - it is found everywhere in nature in extremely high percentages"), Hall says it is conceivable that a DNA homologue to the fruitless gene may exist in the human brain. Over the past decade, scientists from Amsterdam to California have been picking through brain matter in their search for structures that differentiate gay men from their straight brothers. Three findings stand out. Gorski's work on the anterior commissure, and the discoveries that the superchiasmatic nucleus (which contains the "body clock") is larger in gay men than straight men, and that another region of the hypothalamus, usually referred to as INAH-3 and whose function is a mystery, is smaller. Most researchers agree that it's impossible to say what any of those findings actually mean - except that they support the idea that biology, in the shape of genes, hormones and brain structures, correlates with gayness in some men. Daryl Bem, a psychologist from Cornell University in Ithaca, New York, is not impressed by that insight. "Correlation is not cause," he says. "Sexual orientation is not transmitted by genes." Instead, Bem argues that when it comes to sexual orientation, a child is born with a clean slate. Genetic inheritance affects the child's temperament and behaviour, and that, in turn, moulds their final sexual orientation. Bem takes his cue from a 1981 study of about 1000 gay men and women, and 500 straight men and women living in the San Francisco Bay area. That study, which was run by researchers at the Kinsey Institute for Research in Sex, Gender and Reproduction in Bloomington, Indiana, found that 63 per cent of gay men reported that as children they disliked "boys' games", such as football, compared to 10 per cent of straight men. On the flip side, 48 per cent of gay men had enjoyed "girls' games", such as playing house, compared to 11 per cent of straight men. Those findings have been confirmed by studies that look at boyhood play, and then wait and see who becomes gay and who straight, and also by a massive "meta-analysis" that pooled the data from 48 separate studies. Bem speculates that a boys-will-be-boys boy who enjoys a bit of rough and tumble will detach himself from the gentler, more introspective world of little girls, and so will come to regard females as dissimilar and exotic. This sentiment is translated into "physiological tingles and jolts" in the presence of the opposite sex which at first seem offensive (hence the widely held view among boys that "girls are yuck", and vice versa), but which during adolescence fire up sexual desire. Genes may well help hard-wire a boy's brain so that he acts in a fashion that doesn't conform to his gender, becomes distanced from other boys and later finds them sexually attractive. "It's psychologically determined," says Bem. "You can take a 'sissy' boy, make him play football, and it will only serve to make him feel even more as if he doesn't fit in." Bem's theory is all-encompassing. It explains both male and female homosexuality (the Kinsey study also showed that lesbians tended to prefer boy's games as children) and heterosexuality, as well as some long-standing conundrums, such as why children who are not related, but are raised together - for example adopted children and children raised on kibbutzim - never fancy one another as adults: the familiar is the antithesis of the exotic. Curious but cautious Biologists are treating Bem's theory with a mixture of caution and curiosity. Hall says that there are at least two reasons why the theory should be given due consideration. First, it leaves a role for genes. No one has ruled out the idea that genes code for sexual orientation through a simple activity like childhood play, says Hall. Second, science has far from ruled out an experience-based element to homosexuality. After all, roughly half of men with gay identical twin brothers are straight. But Bem admits that there is a least one more vital piece of evidence he needs to support his theory. Whereas most animals, be they rats or adult humans, that are confronted with unfamiliar animals or objects become (non-sexually) aroused - heart rate increases and breathing becomes more shallow, for example -Bem has yet to show that the same thing happens when a boy who does not conform to his gender norms is in the presence of other boys. "That's the first experiment we plan to do," he says. Meanwhile, whether Bem, and the eclectic bunch of geneticists, neuro-anatomists and psychologists who are studying gayness, will ever completely understand the forces that dictate sexual orientation remains to be seen. * * * Do Gay Men Make Better Brothers? When Zimbabwe president Robert Mugabe last year described homosexuals as "lower than dogs" he was reflecting a popular sentiment that gay sex is a perversion because it breaks the laws of nature. In his lurid way, Mugabe was disseminating the lay version of the notion that evolution could not possibly select for a gay gene because homosexuality hampers reproduction. On the contrary, argue geneticists like Dean Hamer of the National Institutes of Health near Washington DC, who discovered the location of the gay gene, nature may select for a gay gene because it carries a distinct gain for the species as a whole. For instance, gay men may make better brothers, giving their siblings' children - who share a fair chunk of their uncle's genetic material - the edge in the survival stakes. The kinship theory reasons that natural selection works on averages, not on individuals, picking the genes that help more people than they hurt. Thus a gay gene could be perpetuated by the heterosexuals who carry it silently, and selected for by the cumulative reproductive success of the tribe. But there are alternative explanations for the persistence of the gay trait. Perhaps the putative gay gene sits on a bit of the chromosome that is unstable, so that fresh variants pop up to replenish the supply. Gayness may also be the by-product of a gene whose primary impact is to increase the number of children women have. That's not such a far-fetched idea when you consider that women can carry the identical gene to gay men (see Diagram), and that the gene is supposed to effect human sexual behaviour. There is even some evidence, albeit tentative,for that idea. After studying several thousand homosexuals, Ray Blanchard, a psychologist from the University of Toronto, concluded that a man is more likely to be gay if he is the youngest in a longer than average line of brothers - with each additional older brother, the odds of homosexuality increased by 33 per cent. Other reading: "Homosexuality in men and number of older brothers" by R. Blanchard and A. F. Bogeart, American Journal of Psychiatry, 1996, vol 153, p 27-31. * * * The feminine mystique While men's sexual orientation is more or less bimodal - gay or straight - with very little gradation in between, women express their sexuality across a wide range. Far more women than men categorise themselves as bisexual, for instance. That complexity has needled science well-nigh into pretending lesbianism does not exist. So too has the small matter that most researchers in the field are men. "Male scientists are less interested in female sexuality," says psychiatrist Richard Pillard of Boston University in Massachusetts. "They get impatient with the nuances." Finally, the research bias cannot be divorced from the political reality that sexual liaisons between women, with their coy image of playful romps and gentle caresses, incite less public intolerance than the thought of sexual encounters between men. Lesbianism may have escaped scrutiny simply because scientists have less incentive to find a biological justification for it. With the dearth of reliable information, most scientists can only speculate on the origins of female sexual attraction. "What is it that makes a woman more responsive to the social context than the gender of a partner? Maybe women have more genes than men relating to homosexuality, or maybe they can ignore their genes more easily," says Pillard. The few attempts to understand the biological basis of lesbianism do, however, reveal that it has a strong genetic component. A woman has a 48 per cent chance of being a lesbian if her identical twin is also a lesbian. The Xq28locus, however, does not play a role in female homosexuality as it may in male homosexuality, and unlike in men (see Box), birth order has no effect on a woman's sexual preferences. Only nature's slip-ups provide a window on the biological processes that mayinfluence female sexuality. In congenital adrenal hyperplasia (CAH), a defective gene causes male hormones to be pumped into the bloodstream of the female fetus. The result is a baby girl who has the outside genitalia of a boy - an aberration usually put straight by a series of operations between birth and the end of puberty. Sexologist John Money of Johns Hopkins University in Baltimore has found that an incredible 37 per cent of CAH females identify themselves as lesbian or bisexual, suggesting that male hormones may play a role in female homosexuality. Further reading: "Exotic becomes erotic: a developmental theory of sexual orientation" by D. Bem, Psychological Review, 1996, vol 103, p 320-335. |