
It is only in the last 2 devades that the canine genome has been mapped, and mutated versions of genes causing inherited diseases identified.
The most common inherited diseases have been various eye diseases.
Most DNA tests developed have been for Reccesively inherited diseases, especially useful for those late onset diseases not clinically apparent until a dog has already been bred from.
The BVA/KC,/ISDS scheme is a Clinical exam, and all that was available to try and reduce the incidense of eye disorders.
The problem of course was that this could only identify clinical signs, so could not identify carriers, or affected dogs of late onset conditions until the symptoms appeared.
Even so the Clinical eye exam reduced the number of affected dogs bred hugely.
Of course clinical testing is still vital to identify emerging conditions, and of course not all eye conditions have definitive DNA tests for all breeds.
An unfortunate effect though was discarding known carriers (parents of affected individuals) from future breeding, even though they were healthy, and had other useful traits, thus adding to reduction in gene pools.
DNA testing allows a breeder to retain the breeding potential of dogs through the use of DNA tests for recessive conditions.
This means as long as one parent isclear for a given recessive condition the puppies produced will all be unaffected. Some will be Carriers, but any used for breeding should be tested and if Carriers mated to clear partner.
Eventually there will be a larger pool of clears of various bloodlines (that otherwise would have been excluded), and fewer Carriers will be used, unless they are otherwise exceptional or useful.
As we go on our breeds will have more DNA tests.
For example my very healthy robust breed has 3, so any individual could be Clear/Carrier for some, all or none, and need a mate to complement.