Is creating perfect genetics impossible in this game?

The game doesn't use "proper" genetics. For example, if you create 2 sims with blond hair, a child will have blond hair. If you change their hair color to brown, a child could then have brown hair, even though the sims original hair color was blond.

The Sims 2 is the only game to have a good genetics system in place. The game recognizes the original hair color assigned to the sim regardless of your sims hair color changing.