does consistent line breeding if done correctly as you've instructed ensure the continuation of traits like puffy cheeks and stubby bills in little call ducks?
I LOVE JULIE!!! (But so does everyone.)
It does in the ones that survive it. There really isn't such a thing as good line breeding over many generations. You get less and less vitality in the lines because of the undesirable recessives, and that's the least of what can go wrong.
It's the same thing if you're breeding dogs, cats, whatever. It's the same reason the French royals had so much hemophilia in their males years ago when they weren't allowed to marry outside the royal families. If you match two animals that have an undesirable gene, it's probably not going to come out well for the progeny.