In my case stuttering runs in my family as far back as I know, though with widely varying severity. My great grandfather stuttered, my grandad has a bad stutter like me, a bunch of other relatives have mild stutters or grew out of theirs (my dad), and even my two youngest brothers stutter. Just because you're the only one in your family who stutters doesn't necessarily mean your stutter isn't genetic. Genetic conditions can be dominant or recessive, and if it's recessive then it's not surprising there are no other stutterers in your family. Or it could be a random mutation in your case. Sometimes, too, other family members will inherit a genetic condition but it doesn't manifest in the same way or is basically "dormant". (There's a genetic eye condition that runs in my extended family. One of my uncles is blind from it, one other has low vision because of it, several have the mutation but are asymptomatic, and the rest of the family members don't have it.)