Hmm... now you've got me wondering. I had a ton of quitter eggs early this year with my calls.Oh, my, YES. Let me tell you, it sure as hell does cause issues. I lived that nightmare this year. I think you can get by just fine with parent x offspring breeding for a bit, but full siblings are problematic.
Last year, I had a ton of genetic diversity, and every egg I set was fertile, and hatched.
I only kept babies from one pairing, and the mom. Mistake. Less than 50% fertility rates, and a ton of quitters throughout. Bad news bears. Never again for me. That's why I am only keeping girls from that group, and boys from entirely different lines.
I had them paired off with the coloring I wanted.
Now I have more males in the pen and the females choose and it's been much higher fertility rates.
Maybe they know who "not" to breed with...