Gay filmmaker Bryce Sage sets out to answer this question on a cross-country and around the world journey and along the way confronts his own homosexuality and family history. He explores the nature vs. nurture side of the issue and talks to animal biologists about their studies of homosexuality in other species. There is documented evidence of homosexuality in over two hundred.
Bryce visits Samoa and discovers that every family has a male member who is either gay or is encouraged to become more feminine to support familial needs. The idea is that homosexuals, like the fa’afafine (or third gender) may not reproduce themselves, but in the support of their blood (and therefore genetic) relatives, they increase their overall chance for survival.
One of the interesting things that he discovers from a recent study out of Italy is that in families where there are gay members the females have a higher rate of reproduction (between 15% and 25% more children) which suggests that when passed on to women the gay gene helps makes them more fertile. Mr Sage does not reproduce due to his homosexuality, but the rest of his family does and increases their chances of survival into future generations.
I wonder how much longer it will be before we find a genetic marker for the transgender condition.