I don't know about a genetic predisposition, but there is the mystery of why some develop an erotic interest in feet and in ticking feet and most others don't. Just think of all the varieties of things that turn people on. How and why does each one become erotically involved with whatever turns them on. There are countless kinks in regard to erotic interests. A doctor friend of mine confided that his big erotic thrill was to wear girls' panties and jack off. Where did that come from? Some get off on hands. The possiblities are endless. But, how does each kink become erotically charged?
Well, this is just my own little explanation. As far as I remember I was 4 or 5 years old. I don't think it's appropiate to go into details here, but I still remember some occasions quite well . But looking back the most astonishing thing is that I already felt that there was something strange or even weird about my exitement. I mean, I was a little kid, not aware of the sexual aspect of it (of sexuality at all), but I was almost magically attracted 1. to girls (not boys) and 2. only feet (upperbody tickling didn't have any effect on me) I can't remember that tickling ever had just playful quality for me. I didn't understand my feelings of course and, though I might sound unbelievable, but I even felt some kind of shame. And here is my point: I believe my feelings were some kind of early form of my later fetishism. They were just there, and where did they come from?