A study of gay men in the US has found fresh evidence that male sexual orientation is influenced by genes. Scientists tested the DNA of 400 gay men and found that genes on at least two chromosomes affected whether a man was gay or straight.

A region of the X chromosome called Xq28 had some impact on men's sexual behaviour - though scientists have no idea which of the many genes in the region are involved, nor how many lie elsewhere in the genome.

Another stretch of DNA on chromosome 8 also played a role in male sexual orientation - though again the precise mechanism is unclear.

Researchers have speculated in the past that genes linked to homosexuality in men may have survived evolution because they happened to make women who carried them more fertile. This may be the case for genes in the Xq28 region, as the X chromosome is passed down to men exclusively from their mothers.

...While genes do contribute to sexual orientation, other multiple factors play a greater role, perhaps including the levels of hormones a baby is exposed to in the womb. "Sexual orientation has nothing to do with choice," said Bailey. "We found evidence for two sets [of genes] that affect whether a man is gay or straight. But it is not completely determinative; there are certainly other environmental factors involved."

There is a widespread but mistaken view about genes as a computer program running a robot, implying absolute genetic determinism of body traits. I think it was Richard Dawkins who came up with the alternative imagery of a cooking recipe: the end effect results from the interplay of the ingredients and also the circumstances set by preparation and cooking.

_{*Lunatic*, n.
One whose delusions are out of fashion.}