Stuttering is very genetic, they even have identified some of the genes that increases risk of stuttering. Just one example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5370225/ "Evidence for genetic factors in persistent developmental stuttering has accumulated over the past four decades, and the genes that underlie this disorder are starting to be identified. The genes identified to date, all point to deficits in intracellular trafficking in this disorder."