minute gene faults on certain chromosomes
First of all, because its very important to stress - there are no 'faults' as such in the DNA/RNA assemblage. In so far as we know, there are only mathematical variations that occur. "Fault" implies some sort of values-based judgment, where the natural order provides no such assessment. Nature has no good or bad only a probability of occurrence.
But, back to the quote: I think in the end everything is going to come down to this, or living world (bacteria, viruses). The DNA code, if not setting up exact specifications, at least pushes the odds in one way or the other. In other words, DNA does not make one a TS, but certain DNA configurations make it more (less) likely that the person may well develop in that manner.
In that way, it may become much harder to point to a difference between mental and medical in the future. It will be much more of a matter between DNA factors or and outside/invasive forces (injury, infection, flu).
I used to tease my Xian students there at the Super Huge State University of Science and Technology about their protests of Darwin being two major scientific advances behind other things that they would like even less. Those being advanced physics, and DNA. Now that advanced physics thing, forget about it, they couldn't read Darwin correctly (and Darwin wrote very well) I'm such not going to start handing out Einstein, Planck, and Heisenberg to them. (Much less Chaos stuff) But DNA, that's very interesting, because what happens if it turns out that so much of what we think is 'choice' and 'free will' (thereby a 'sin' or a 'not sin') turns out to be pretty much encoded into us? (By god itself, who else in their world view could possibility code DNA?, Matter of fact - currently painting themselves into a corner - the fundys here in the States use DNA as proof of "intelligent design'.) Hummm...
So what happens when most of this turns out to be - not a birth defect - but a birth variation? Then what happens when we find out how to control it? (Or at least gain the illusion of control over it).