The evidence that stuttering is partly genetic is some of the most solid evidence we have in stuttering. The belief that stuttering is made worse or permanent by giving it attention or putting a child in speech therapy early is one of the most damaging misconceptions out there. There is no evidence for this. The diagnosogenic theory of stuttering has never seen substantiated. I dont know where you're getting your numbers from but it seems like you're just picking them out of the air. Listen to this episode if you want more information: http://stuttertalk.com/what-is-stuttering-ep-550/