Wat wordt beweerd over het ontstaan van homoseksualiteit?

Mothers' genetic skew linked to gay sons

10:26 09 November 04   Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.  

There has been much debate over “gay genes”. Now an intriguing study raises another possibility: in some cases, variations in the genetic program we inherit from our parents, rather than in the genes themselves, might determine sexual preference.

Our genome is “programmed” by the addition of chemical markers called methyl groups to the DNA, which shut down genes. One of the most dramatic examples of methylation is the shutdown of one of the two X chromosomes (one from each parent) in every woman’s cells, a process called X-inactivation (New Scientist print edition, 10 May 2003).

Normally, this process is random; either of the X chromosomes can be inactivated.

But when Sven Bocklandt of the University of California, Los Angeles, compared blood and saliva samples from 97 mothers of gay men with samples from 103 mothers without gay children he found this process was extremely skewed in the mothers with gay sons, with one X chromosome being far more likely to be inactivated than the other.

"I like males"

Only 4% of the mothers without gay sons showed this skewing, compared with 14% of mothers with at least one gay son. Among mothers with two or more gay sons, the figure was 23%.

Such skewing is generally associated with genetic disorders, but the mothers all appear to be healthy. Their daughters also seemed unaffected, with only 1 out of 24 showing skewing.

Bocklandt suspects that whatever is causing the skewed methylation of the X chromosome also affects the methylation of certain genes on the chromosomes the women pass on to their sons. Mothers might not be resetting their own “I like males” program, he told a meeting of the American Society of Human Genetics in Toronto last week.

“I’m not absolutely persuaded, but it’s an interesting hypothesis,” says Ian Craig of the Institute of Psychiatry in London. “Until you’ve got some molecular way to test it, it’s just a nice idea.”

Alison Motluk, Toronto

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Survival of genetic homosexual traits explained

00:01 13 October 04   NewScientist.com news service  

Italian geneticists may have explained how genes apparently linked to male homosexuality survive, despite gay men seldom having children. Their findings also undermine the theory of a single “gay gene”.

The researchers discovered that women tend to have more children when they inherit the same - as yet unidentified - genetic factors linked to homosexuality in men. This fertility boost more than compensates for the lack of offspring fathered by gay men, and keeps the “gay” genetic factors in circulation.

The findings represent the best explanation yet for the Darwinian paradox presented by homosexuality: it is a genetic dead-end, yet the trait persists generation after generation.

“We have finally solved this paradox,” says Andrea Camperio-Ciani of the University of Padua. “The same factor that influences sexual orientation in males promotes higher fecundity in females.”

Relative differences
 

Camperio-Ciani's team questioned 98 gay and 100 straight men about their closest relatives - 4600 people in total. They found that female relatives of gay men had more children on average than the female relatives of straight men. But the effect was only seen on their mother’s side of the family.

Mothers of gay men produced an average of 2.7 babies compared with 2.3 born to mothers of straight men. And maternal aunts of gay men had 2.0 babies compared with 1.5 born to the maternal aunts of straight men.

“This is a novel finding," says Simon LeVay, a neuroscientist and commentator on sexuality at Stanford University in California. “We think of it as genes for ‘male homosexuality’, but it might really be genes for sexual attraction to men. These could predispose men towards homosexuality and women towards ‘hyper-heterosexuality’, causing women to have more sex with men and thus have more offspring.”

Camperio-Ciani stresses that whatever the genetic factors are, there is no single gene accounting for his observations. And the tendency of the trait to be passed through the female line backs previous research suggesting that some of the factors involved are on the male “X” chromosome, the only sex chromosome passed down by women. “It’s a combination of something on the X chromosome with other genetic factors on the non-sex chromosomes,” he says.

Immune system
 

Helen Wallace, of the UK lobby group GeneWatch, welcomes the new research that moves away from the controversial single-gene theory for homosexuality. “But it’s worth noting that the data on the sexuality of family members may be unreliable, so more studies are likely to be needed to confirm these findings,” she says.

Even if the maternal factors identified by Camperio-Ciani’s team are linked with male homosexuality, the research team’s calculations suggest they account for only about 14% of the incidence.

Their findings also support earlier findings that when mothers have several sons, the younger ones are progressively more likely to be gay. This might be due to effects changes to the mother’s immune system with each son they carry.

But Camperio-Ciani calculates the contribution of this effect to male homosexuality at 7% at most. So together, he says, the “maternal” and “immune” effects only account for 21% of male homosexuality, leaving 79% of the causation still a mystery.

This leaves a major role for environmental factors, or perhaps more genetic factors. “Genes must develop in an environment, so if the environment changes, genes go in a new direction,” he says. “Our findings are only one piece in a much larger puzzle on the nature of human sexuality.”

Journal reference: Proceedings of the Royal Society B: Biological Sciences (DOI: 10.1098/rspb.2004)

Andy Coghlan

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Homosexuality is biological, suggests gay sheep study

10:51 05 November 02   NewScientist.com news service  

A study of gay sheep appears to confirm the controversial suggestion that there is a biological basis for sexual preference.

The work shows that rams that prefer male sexual partners had small but distinct differences in a part of the brain called the hypothalamus, when compared with rams that preferred to mate with ewes.

Kay Larkin and colleagues from Oregon Health and Science University found the difference was in a particular region of the hypothalamus - the preoptic nucleus. The region is generally almost twice as large in rams as in ewes. But in gay rams its size was almost identical to that in "straight" females.

The hypothalamus is known to control sex hormone release and many types of sexual behaviour. Several other parts of the hypothalamus showed consistent sex differences in size, but only this specific region showed differences that correlated with sexual preference.

The differences are almost identical to those identified by the neuroscientist Simon LeVay in his studies of the brains of gay men. His work has always been considered controversial, partly because the brains he studied were mostly from men who had died of AIDS. So it was not clear whether the differences were related to the disease or to sexual preferences.

Hormone converter
 

But the findings in sheep are an important confirmation of LeVay's work, says Jacques Balthazart from the University of Liege in Belgium.

Sheep are particularly interesting, he says, because besides humans, they are the only animal where the males may naturally express exclusively gay sexual preferences. As many as one in 10 rams can be gay.

Larkin's team also found that the hypothalamic region had a rich supply of the enzyme aromatase, which converts testosterone into oestrogen. It is in this form that the hormone interacts with the brain. This may help support one theory that sexual orientation, in part at least, may be related to the hormones present during fetal development, says Balthazart.

But Larkin suggests there may also be the influence of genes at work, at least in predisposing the animals to homosexuality. This is because selective breeding seems to have been responsible for the high proportion of gay sheep compared with other animals.

Larkin presented the research on Monday at the Society for Neuroscience meeting in Orlando Florida, US.

Helen Phillips, Orlando

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Doubt cast on 'gay gene'

Friday, April 23, 1999 Published at 03:01 GMT 04:01 UK

There is no evidence for the "gay gene", a study claims to have found.

Previous research suggested that male homosexuality is passed on from mother to son.

But the new work, covering more people, aims to show that the particular genetic features implicated are no more common in gay men than would be expected.

"Because our study was larger than the original one, we certainly had adequate power to detect a genetic effect as large as was reported in that study," said the team from the University of Western Ontario in the journal, Science.

"Nonetheless, our data does not support the presence of a gene of large effect influencing sexual orientation," they concluded.

However, both the studies targeted only one part of the X chromosome. The authors of the new study say that: "These results do not preclude the possibility of detectable gene effects elsewhere in the genome."

Equality needed

Griffith Vaughan Williams from the UK Campaign for Homosexual Equality says conflicting research results in this field are beside the point: "The most important thing is, that however I and other homosexuals are created, we are treated as equals."

When the first study was published in 1993, there was concern it might lead to pre-natal screening and abortion of foetuses carrying the gene.

The new study used DNA from 52 pairs of gay brothers. These were recruited via advertisements in two Canadian gay news magazines. Mr Williams expressed surprise that so many homosexuals were willing to help in such experiments.

The researchers looked to see if the gay brothers shared more of the candidate genetic markers than would be expected. Any pair of brothers will share about half their DNA on any particular chromosome.

They found that 46% of the 52 pairs of brothers shared three key markers. In the previous study, which considered five markers, those scientists reported that 83% of 40 pairs shared the markers.

Conflicting results

"It is unclear why our results are so discrepant from the original study," say the scientists in Science. This is strong language for a scientific journal, implying the scientists believe that mistakes were made in the first study.

However, none of the team would speak to the BBC to confirm this. The new study has been criticised by the lead author of the old study, Professor Dean Hamer, over how the subjects were selected.

The 1993 study was the most powerful piece of evidence for a strong genetic factor in male homosexuality.

But other studies, such as one showing that identical twins are more likely to both be gay than non-identical twins, means that the nature-nurture debate will continue.

Queer Resources Directory: www.qrd.org/qrd/origins/

The origins and causation of homosexuality

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Scientific Inquiries into Sexual Orientation (hier gevonden)