Genetics

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The canine genome was sequenced early in the 21st century, but genetic health testing has become more and more relevant ever since. Most breeds have genetic predispositions to at least a few genetic health problems, but the more we learn about canine genetics the more we are able to control for these health issues and breed away from them. For the most part, ethical breeders who follow their breed's health testing guidelines are able to guarantee that their puppies will not suffer from any of their breed's genetic health issues. However, there are a few exceptions:

Dobes and Danes

Dobermanns and Great Danes are tall, athletic, deep-chested dog breeds who both have a tendency for heart problems, particularly DCM. DCM, or dilated cardiomyopathy, is a genetic health condition that involves the weakening and expanding of the left ventricle of the heart (which pumps oxygenated blood to the majority of the body) and ultimately results in heart failure. There are two identified genes that contribute to DCM, and testing for these and breeding away from them has been successful in other breeds, but Dobermanns and Great Danes still frequently suffer from DCM even when they are clear of both genes. This means that there is some other gene common to these breeds that causes this disease, but it is yet to be identified. As a result, even the most ethically bred Danes and Dobes cannot guarantee longevity, and these breeds require additional health testing to give their dogs the best chance at a full life. Ethical breeding of these dogs includes an annual echocardiogram and 24-hour holter test to ensure that any breeding dogs are free of heart problems prior to breeding them. It is also not unusual for these dogs to begin breeding at a later age than most breeds in order to be more confident that they will not produce heart problems in their puppies.

Bernese Mountain Dogs

Bernese Mountain Dogs are prone to several types of early onset cancer. Most cancers in most breeds can be largely eliminated via DNA testing, but even the most rigorously tested Bernese Mountain Dogs have no guarantees of longevity. However, this issue can be somewhat attenuated by selecting lines with a history of longevity, and ethically bred Bernese Mountain Dogs have a vastly greater life expectancy than BMDs from puppy mills and backyard breeders. Their overall life expectancy (from all sources) is 4-6 years, but their life expectancy from ethical breeders is 7-9 years. Within the breed community, there is a saying that goes "3 years a puppy, 3 years a good dog, and 3 years an old dog," indicating that after age 9 every day is a blessing. 

Cavalier King Charles Spaniels

Cavs tend to have more health issues and a shorter lifespan than most small breeds, but their breed club is getting closer and closer to a definitive solution. The main issue in the breed is a heart condition called mitral valve disease. If this breed were not riddled with health issues, it would be among the most widely regarded and recommended dog breeds (especially for people with children), but for now they have a bit more work to do in order to ensure longevity.