Unless the breeder is a complete idiot. Purebreds are likely to be more healthy when bred by breeders that know what they're doing because they only breed those dogs that have undergone extensive health tests and DON'T breed dogs that are related. This is how we got rid of epidemic hip dysplasia and other common genetic health problems, and why good breeders can and do guarantee that none of their pups will get hip dysplasia. Pedigrees provide genetic information on each dog going back at least four generations with each one being registered so you can look up their specific health tests... eyes, hips, bones, etc.
This careful breeding of purebreds also provides much needed information for scientists to formulate what genetic health issues might be able to be tested for in the future, odds on which animals may only be carriers of certain genetic health issues that they can pass on though don't develop that certain health issues themselves, the odds on how many of their progeny will likely be healthy or only carriers or those dogs that have a genetic health issue that they will have symptoms of.
Basic common sense says that breeding dogs with questionable genetic health issues as well as questionable family history runs the MUCH higher risk of passing on genetic health issues to their progeny.
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