
Most of these diseases are recesive, though I think that PHPV is diminant, so one parent would have to be affected.
With recesive traits if you mate two carriers you will on average get 1 in 4 affected, half carriers and 1/4 clear.
Testing ensures that affected animals are not used for breeding.
Two affecteds will produce all affecteds, an affected to a clear will produce all carriers. Affected to carrier half Affected and half carrier.
Carrier to clear will produce no affecteded offspring, but half will be carriers.
Therefore testing for the disease in recesive traits can only tell you which animals are affected, and of the ones that do not have the trait some invariably will be carriers. This may never come to light, unless they have the misfortune to be mated to another carrier. This is most likely to happen if the unknown carrier is widely used.
Carriers with diseases that can be checked for by DNA can strill be useful to the breed, for as long as they are only ever mated to clear partners, and all their offspring is also tested, then you can use their good points, and keep the gene pool more varied.
Sadly with most problems when a dog or bitch is found to be a carrier (by producing affected offspring) they need to be withdrawn from th4e gene pool, and a big question mark then hangs over their relatives, some of which may be carriers, but most are likely to be clear.