I've never read or come across that. Quite the opposite. I thought perhaps you had a source. I've seen the term "clusters" of families used in stuttering and genetics studies. Specifically Dennis Drayna's research. Yes, speech is learned. I think you'd be hard pressed to find significant evidence that speech disorders are learned. Anectdotal evidence at best. I think it's important to separate the two. Maybe an SLP will chime in.