I'm not sure if I am missing the joke here, but obviously because the game doesn't use a chromosome genetic inheritance system. It is very simply Sim + Sim = Sim with ideally mixed traits, but they have a wider pool of possibilities to pull from.
Implementing something like this would be akin to implementing a sex mechanic on top of gender, and I don't really see what we would really gain from going that detailed. Especially when we have gender customization the way it is.