I believe it comes down to visual defect vrs deadly defect. How unfortunate Hector's tail is not better! But tails can be improved. When dealing with recessive deadly genetic defects it's best to eliminate known carriers from the gene pool. Since it seems clear that Apollo is a carrier, you could do some test hatchings with each girl individually, separating them and collecting eggs from each to try to ID who are female carriers. It would be a lot of work, though.