poor Will. Both his own biological mother and his biological paternal grandmother have been bat crazy women, i.e clearly and attestedly mentally ill.
if Will were a real person and not a novel character, this sort of genetic burden would probably mean that he cannot be anything else than mentally ill. That Will is a resourceful, relatively well adjusted boy in the novel, requires the use of artistic license.
Regarding biological heritage, as I understand it, Will is not directly descended from Tonto in any way, so it cannot be genetical if he resembles Tonto.