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.
...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%.
...There are some defective disease-causing genes that mutated so long ago, that the mutation (and its associated disease) is found in evolutionary divergent breeds. The same ancestral autosomal recessive mutation for the progressive rod cone degeneration (prcd) form of progressive retinal atrophy (PRA) is shared by over 17 diverse breeds, including the American Cocker Spaniel, Australian Cattle Dog, Chinese Crested, Kuvasz, Labrador Retriever, and Toy Poodle. The list of affected breeds continues to grow as more are discovered with the same defective gene. The question is not,
"Which breeds carrie4 this defective gene during their development", but
"Which breeds did not lose this defective gene during ancestral development."