There is alot of doubt in population genetics because we haven’t yet figured out how to properly detangle genetic from epigenetic (environment gene interaction including psychologist development) and epistatic factors. I am aware there are some genetic predispositions and candidate genes but we don’t know how related this is to stuttering severity etc. My stutter was predominantly psychological. It essentially disappeared when I put on an accent or read as a character in drama, even though I was way older than “grow-out” phase. There may be a treatment-resistant phenotype that has a higher genetic snp component than other phenotypes.