I definitely feel that a weak chromosomal push is one cause for it, but I also believe that a number of CD's are created from their environments. Mothers dress their boys as girls, women dress their boyfriends and husbands, maybe a traumatic event in someone's life. I love the idea of "born this way", but I don't think it is always the case.
Personally, I first got the urge to dress up when I was in Elementary School (I didn't understand those feelings or try until high school), and all my life people have told me I am acting a little "gay", so maybe I was born with it, but I don't know.