Adding to the complexity of breeding is the expansion of planned cross-breedings (designer breeds) to produce offspring. Therefore, the discussion is no longer between pure-bred and crossbred, but between purposely-bred and random-bred dogs.
There is a general misconception that mixed-breed dogs are inherently free of genetic disease. This may be true for rare, breed-related disorders; but the common genetic diseases that are seen across all breeds are seen with the same frequency in mixed-breeds. A mixed-breed dog with hip arthritis has no less a case of hip dysplasia than a pure-bred dog. The only difference is that conscientious breeders test and label their dogs as dysplastic prior to the onset of clinical signs. I do not see a difference between the relative frequencies of old pure-bred dogs versus old mixedbreed
dogs with hip arthritis requiring arthritis pain medication.
Testing for inherited hypothyroidism (for thyroglobulin autoantibodies by Michigan State University) shows 10.7% of 55,053 tested mixed-breed dogs to be affected. The average percentage of affected dogs for all pure breeds is 7.5%. This does not tell us that mixed-breed dogs are more prone to autoimmune thyroiditis: More mixed-breed dogs are tested based on clinical signs. However, these results show us that this hereditary disorder is seen frequently in both pure-bred and mixed-breed dogs. To those that feel that this disorder is not genetic, we look at the historical breed predilections for the disorder. Those breeds with the highest genetic propensity for autoimmune thyroiditis remain high over the years (example: 31.4% of English
Setters tested), and those breeds with the lowest propensity remain low (example: 1.1 % of FrenchBulldogs). Selection based on thyroid testing (and in the future direct genetic tests for liability genes) should reduce the frequency of this disorder.
The most common inherited disorders for all dog breeds according to the AKC Canine Health Foundation are: cancer, eye disease, epilepsy, hip dysplasia, hypothyroidism, heart disease, autoimmune disease, allergies, patellar luxation, and renal dysplasia. With the exception of renal dysplasia, all of these genetic conditions are routinely seen in mixed-breed dogs.