There's always a normalization step unless two animals go in and two come out. In sexual reproduction it's meiosis for example. In your example you're just pre-normalizing. I may still be wrong about the (2/3):(1/3) ratio though.

Edit: If centaurs reproduced sexually with humans though they’d have 50% centaur offspring and 50% fully human. That’s assuming half their diploid chromosome count comes from each species and they can somehow compatibly align. Lulz.