I'm pretty sure that saying applies to stuttering. If I remember my thesis research correctly, genetic predisposition (or physical trauma) is the underlying "cause" but whether or not stuttering actually develops is environmental. If nobody in your family stutters but you do, one of your relatives was still predisposed to stuttering, it was just never triggered by their environment. So the genes are still there. Somebody with an equally triggering environment as you but no genetic predisposition will not develop stuttering.