I've gone through one analysis of studies from Colombia & Yale and another from Cambridge.
The former leans toward environment and says that the data is inconstant with a simple genetic influence model", that "there's substantial support for the role of social influences ", and that "its consistent with a general model that allows for genetic expression of same-sex attraction under specific, highly circumscribed, social conditions." "
The latter takes a step toward genetics and concludes that, "…it seems reasonable to conclude that male homosexuality, or, at least, some 'types' of male homosexuality, are under some degree of genetic control, although various problems with this data prevent more precise conclusions from being drawn.
I’ve seen papers and books that look at some of this data and lean much more strongly one way or the other, but these two seem to be the most academic and without obvious bias.
There is a genetic deletion called 22q11. One out of three kids with this deletion will develop schizophrenia by age 22. Harvard researchers are working to narrow the scope, since 22q11 can have verious breaking points.