I think you just have to look at the heads in the Images folder of the game. If you have 2 parents at different levels of the image, I think that means your kid could end up looking like any head between them---with a few deviant exceptions due to strange chance genetics.

For example:


My parents are circled in black. I had 2 girls with the big hair, 1 with the short hair, and the boy with glasses.

I also adopted 1 so she would not be part of genetics.

And this guy... is just unexplainable.