Mixed Breed Dogs May Have Unanticipated Health Problems. When you get a purebred dog, you have a reasonable expectation of the types of health problems that it is likely to develop. If the breeder requires health certifications for his breeding stock and performs certified health checks on their puppies you'll have an even clearer idea of the health problems that may afflict your dog or whether your dog is certified free of specific breed-related health problems.
When you get a mixed breed dog, you have no such expectations regarding the dog's potential health issues. Mixed breed dogs are usually healthier than purebred dogs, but they are not guaranteed to be. Although the chances are much less, mixed breed dogs may still develop chronic infections, hip dysplasia, kidney problems, or other health issues normally seen in purebred dogs. With mixed breed dogs, you have very little way of knowing the specific types of health problems that your dog may develop or whether your dog is descended from generally healthy or unhealthy parent dogs.
Mixed Breed Dogs May Still Be Inbred. The chances of the offspring suffering from the results of inbreeding are significantly reduced when a purebred is bred with another breed or a mixed breed dog, but not completely eliminated. If your mixed-breed dog is only a couple of generations removed from having a purebred parent, it may still suffer some of the health problems associated with inbreeding.
If a dog is descended from a purebred dog, it may still be carrying disease-causing genes from its purebred ancestry. If it mates with another dog with the same disease-causing genes, the offspring could develop the same purebred-related health problems, even though your dog isn't a purebred.