Let me provide you with an example from my paternal side. I have a stutter, so did my father but he was able to overcome it to a great extent though some minor stutters sometimes occur when he speaks. His father, which happens to be my grandfather, also had it. Again, the brother of my grandfather, who is alive and about 90 years old, still stutters as well reportedly. So, this makes it four people in one family including myself. Plus, one of my uncles doesn't have stuttering but cluttering. His speech may usually become completely incomprehensible though he has no difficulty in articulating the sounds unlike us. I heard that even a speech disorder other than stuttering can be a risk factor for the stuttering to be encountered across generations, so this may be it. Genetics definitely play a role.