The "gay gene" soft science of the early 90's done by LeVay[1] and Hamer[2]) has been debunked for years now. If a "gay gene" were such a case, how come identical twins who share the "gay gene" aren't both gay?
large proportion of monozygotic twins who [did not share] homosexuality despite sharing not only their genes but also their prenatal and familial environments.[3]
The... [50% odds]... for homosexuality among the identical twins could be entirely accounted for by the increased similarity of their developmental experiences. In our opinion, the major finding of that study is that 48 percent of identical twins who were reared together [and where at least one was homosexual] were discordant for sexual orientation.[/I][4]
Also, new research taken up by Mustanski[5] in 2005 (since the entire human genome has been mapped) have found no gene with a locus score high enough to show predeliction for homosexual behavior.
A possibility exists for a gene suggestive to homosexual behavior but it is more probable that it is learned behavior.
[1] LeVay S (1991) A difference in hypothalamic structure between heterosexual and homosexual men. Science 253:1034–1037
[2] Hamer DH, Hu S, Magnuson VL, Hu N, Pattatucci AM (1993) A linkage between DNA markers on the X chromosome and male sexual orientation. Science 261:321–327
[3]Byne W and Parsons B. Human sexual orientation: the biologic theories reappraised. Archives of General Psychiatry. 50, 3:230 (1993).
[4]Quoted by Horgan, J., Scientific American: Eugenics Revisited. June 1993, p. 123.
[5] Mustanski, B.S., DuPree, M.G., Nievergelt, C.M., Bocklandt, S., Schork, N.J. & Hamer, D.H. (2005). A genomewide scan of male sexual orientation. Human Genetics, 116(4), 272-278.