There isn't one gene persay, as far as I know, but there are a lot of indications to suggest that homosexuality is in part controlled by other biological factors. These include how many sons a mother has had before (hormonal changes in the womb environment), exposure to testosterone in the womb, identical twins are more likely to both be gay than fraternal twins, many gays say that they first had an inkling they were attracted to the same gender in childhood, when the environment factor is still in flux, etc. Do I believe homosexuality is a choice? No. After hearing countless stories (and witnessing friends go through it) of the attempts to 'be normal,' to supress homosexual tendencies, the fallout it often leaves in its wake when one comes out - it's so much emotional pain that I don't think most sane people would voluntarily take that upon themselves as a 'choice.' That said, I don't believe homosexuality is strictly genetically controlled, but I do believe biology has a large role to play in determining who we're attracted to.
This article gives a good overview:
http://www.nytimes.com/2005/05/17/op...rssnyt&emc=rss