I saw something interesting about it, it s called "transgenerational epigenetics". It s very very long to read, but i think this could be a summary : "A trauma would be transmitted from generation to generation via so-called epigenetic mechanisms. This would modify the expression of one or more genes in the victim of the trauma and could be transmitted to the next generation." I know it s subjective, but to me being a stutterer was linked a lot to my émotions. (Or traumas). I think "transgenerational epigenetics" could explain this hereditary aspect of the stuttering