Im not a geneticist, so I could be misspeaking to a point. I reference Dennis Drayna in a previous comment. He estimates at least 50% of cases to be hereditary (based on what I heard last) (or it could be 50% genetic. I forget!) With that being said, there can be new (de novo) variants present in the child, but not in the parent. So in this case stuttering is genetic, but NOT hereditary. Edit: I found a 2020 article that cites 60% are hereditary. 20% of cases come from the one of the same 4 genes. I suppose you can make the argument that if you believe stuttering is developmental/ neurological (and not psychological, or injury etc.) All stuttering would be genetic. Unless there are non genetic causes for developmental / neurological issues. Im not a neurologist. There are obvious outliers and exceptions