I don't believe anyone in transition believes that they are changing their chromosomes and that they will be, officially, purely female or male. The weekly reminders that they are biologically another gender (testosterone or estrogen shots to the ass or thigh, which obviously hurts, and you have to learn to do them on yourself) would help. It's just a matter of making themselves more comfortable in their body, as you said. Imagine you're a woman and looking down at your breasts gave you a sense of dread--what the **** are those doing there? You hate them. You hate your own body for the way it curves and you wish you had something else in between your legs--it's called penis envy. On the other hand, as a male, you would hate that thing dangling there. It reminds you that biologically, you're male, even when you can't relate to males, you're not anything like them except in the similarities of your bodies but you're everything like females and you would go to such lengths as surgery, to that intimidating penis inversion, if it would only just get others to accept the same, that you are female, and view that you in that way even if you all know, it's the elephant in the room, that there remains your chromosomes. But you're happy with your new image. You're happy with people interpreting your sex as the gender you identify with, with finally being recognized as male or female as you have always felt you were.