It's possible, but even if it is genetic it could just be a random mutation. There have definitely been other royals in history who stuttered, but I don't know of any close relatives of George VI that did. In my family stuttering is clearly genetic and it only affects males, and there are a bunch of stutterers, but even so the majority of my relatives don't stutter. My great grandfather stuttered (he had no brothers), my grandad has a bad stutter like me but for some strange reason his identical twin never stuttered at all, my dad grew out of his (none of my grandad's eleven children - seven of whom are boys- ended up with severe stutters like him), my little brothers stutter but they may yet grow out of it. Stuttering is weird.