I study biopsych (like neurology, except not medicine-related), so I've thought about this question a lot. In short, the question can't be answered until it has been empircally studied, which nobody has come even close to doing yet. However, we can theorize about it.

I should say that if someone has been kinky since pre-puberty, that means absolutely nothing. A baby's experiences within the first year can mold their behaviour for life.

From what I've learned thusfar, I must say I'd be extremely surprised if evidence arose that kinkiness was genetic. I actually suspect that many (most?) cases of kinkiness come from some sort abuse or at least involuntary power exchange in the past. This doesn't mean that it is bad or less special or that you should be ashamed of it. I also doubt it would explain every case of kinkiness. There are a multitude of ways that bdsm could be explained psychologically, so there's little need to fall back onto a gene theory unless there's actually evidence either way.