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Study using hundreds of twins homes in on gay gene

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A genetic analysis of 409 pairs of gay twins has provided the strongest evidence yet that gay people are born gay. The study clearly links sexual orientation in men with two regions of the human genome that have been implicated before, one on the X chromosome and one on chromosome 8.

The finding is an important contribution to mounting evidence that being gay is biologically determined rather than a lifestyle choice. In some countries, such as Uganda, being gay is still criminalised, and some religious groups believe that gay people can be "treated" to make them straight.

"It erodes the notion that sexual orientation is a choice," says study leader Alan Sanders of the NorthShore Research Institute in Evanston, Illinois.

The region on the X chromosome picked out by the study, called Xq28, was originally identified in 1993 by Dean Hamer of the US National Institutes of Health in Bethesda, Maryland, but attempts to validate the finding since have been mixed. The other region picked out is in the twist in the centre of chromosome 8. Known as 8q12, it was first signposted in 2005.

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However, it is important that the findings be put in context. Inevitable headlines like "Gay gene discovered" or "It's not a choice" over-egg the results. Just because there is a genetic link to homosexuality, it does not necessarily guarantee one will end up gay. The genes, if and when they are identified, may only predispose one to the possibility of being gay, should the required environmental, nutritional or other unknown factors be present at critical stages of development.

I was pleased to see this in the article. It makes it clear that the genes are only one part of the equation. Previous twin studies involving identical twins (this one only used non-identical twins, both of which identified as gay) indicated that is one twin was gay, the other twin was also likely to be gay, but it wasn't certain. That strongly implied that there is a genetic component to homosexuality, but it's not just the genes.

Interestingly, the actual abstract of the paper doesn't mention twins, it merely says brothers. That means that they've only researched male homosexuality. It would be interesting to see if a similar study on lesbians came up with the same or different results.

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Importantly this study identifies the gene being on the X chromosome. One of the arguments used by the religious persecutors of homosexuality is that it cannot be genetic as such a trait would not be passed on given that homosexuals do not breed. A somewhat fallacious argument to start with given that until recently most homosexuals would in fact marry and breed due to social pressure. However, if the gene is on the X chromosome then it would be carried in the female line and would be passed down via the maternal line. This makes quite a bit of sense in my family tree as I am aware that one of my maternal cousins is also gay, as was a maternal uncle and great uncle.

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One of the genes identified was on the X chromosome. The other one wasn't.

I have to admit that I had similar thoughts, though, because this ties in with another study which indicated that male homosexuality was linked to female fertility. The theory was that the gene that produces homosexuality in males also increases the fertility in women, so they have more children. When viewed as a totality, the gene is pro-survival because it means more off-spring, even allow for the fact that some of those off-spring may not have children themselves. After all, the limiting factor on population growth is the number of women, not the number of men.

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