By breeding those dogs to mates without the same gene, they could produce pups that might be carriers, but free of the disease. If the gene pool within a breed is too narrow, this approach may not work. Therefore, some breeds may have to crossed with other pedigrees periodically to improve the lines' health without sacrificing their distinctive traits, Schaffer said. Miniature bull terriers, for example, are far less genetically diverse than Jack Russell terriers, even though miniature bull terriers have been recognized by the American Kennel Club as a breed far longer than Jack Russell terriers, Schaffer said. Jack Russell terriers may serve as a good cross for similar or related breeds.
Nobody deliberately wants to create a dog that's unhealthy.