
With something like DNA tested affected, you could breed to clear and produce a litter of healthy carrier pups, who then in turn could be mated to clear, and produce statistically half clear, so thereby getting rid of the gene, but not the genetic input of the affected animal, so preserving it's good traits,and keeping genetic diversity.
I can see no ethical reason to use a clinically affected animal where there is no DNA test, as you cannot be sure any animal is clear ofthe gene only clinically unaffected, so coudl be a carrier,a dn risk producing more affected offspring.
So I see no difficulty for the KC to rule that clinically affected animals for any conditon cannot be used, unless a definitive DNA test exists that allows their safe use, ie avoiding producing affected offspring.