I'd imagine this is something where there's lots of discussion and dispute, especially since "runs in the family" could be understood as being genetic or learned.
It's easy to see how dysfunctional family dynamics can repeat across generations, if kids grow up in an environment that's royally messed up but it's all they know so they assume it's normal and then repeat the behaviors when they are adults. It doesn't have to be anything to do with genetics, just the kids lacking the awareness that the only thing they know is not normal. I can imagine that something like PTSD could end up repeating itself, even if with variations, through environmental means even if it can't possibly be genetic. If, say, Dad is a war veteran who is always on edge because of his war-derived PTSD and as a result is very volatile with the kids it's easy to see how they could end up with a form of PTSD of their own, even though the trauma was totally different and the effect on them equally different. And going back to the notion that they would most likely regard Dad's volatility as being normal, who knows to what extent they would end up with their own kids walking on eggshells around them because they didn't want to be snapped at for the smallest things?
