My doctor told me that bunions are caused by joint instability in the foot, ankle, knee, and even hip (in the sense that they're all connected and work together to determine how you walk.) She showed me how my foot (which has a flat arch) is pronated, the shin bone is positioned too far over to the side over one of the ankle bones, etc. I'm not sure I have all the terms down correctly, but you get the gist of this. I've always noticed that my shin bones sort of "curve" inward instead of being a straight continuation of my thigh bone...just a minor thing which is not noticeable to anyone unless they're looking for it...just to illustrate for you how the alignment of all the bones can be slightly out of whack, not enough to cause noticeable problems but enough to affect foot mechanics. Then, after about 30 - 40 years of pounding on the foot the "wrong" way, and/or exacerbating the problem with the wrong footwear, bunions develop.

As to why the bones get out of alignment in the first place, that's something that apparently can develop before age 6. There are, in fact, extremely few people with perfect bone alignment. But if you have inherited a tendency to have lax joints, flat feet, etc., this can affect your posture and gait and bone alignments. This is all established during that early developmental period, and after age 6 there is really not much more you can do about it. It's impossible to consciously decide to "walk differently" after that to correct the problem.

So, while there is no gene for bunions, per se, one can inherit the factors that predispose to getting bunions. Choice of footwear plays a secondary role. But you can certainly get bunions without ever having worn "bad" shoes, despite the myth to the contrary.

Hope this helps.