Answer:
Although it is certain that homosexuality is not a choice or preference, scientist have not yet determined if any gene causes homosexuality.
In 2005, the University of Illinois discovered that homosexuality was not due to one specific gene or a combination of several genes nor chromosomes.
In the case of identical twins, if one twin is a homosexual, the other twin is homosexual under 40% of the time, far lower than would be if genes played a role.
There is no solid exidence that there is a homosexual gene, and it is only suspected.