I agree with Busker, mostly. Crossdressing is not genetic as it would run in families and be double present in twins. Perhaps epigenetics could explain crossdressing.
I feel that crossdressing begins before birth and perhaps is a result of incomplete masculinization of the early fetus' brain (hypothalamus). Perhaps it is a mild form of transexualism. If crossdressers are different from transexuals...then there should be examples of transexuals who are crossdressers...and examples of transexuals who are not crossdressers. CD, TS, both or neither.
