from the AKC Website

There is a simple principle to bear in mind in selecting breeding partners: Mate animals that complement one another. Choose a dog whose bloodlines will strengthen your dog's weaknesses and emphasize her good qualities. For example, if your dog's coat is not as good as it might be, then find a partner with a good coat, from a line of dogs with good coats. Of course, practicing this common sense maxim can be very complex, because you must weigh all the factors that contribute to the dogs' traits and appearances. This is an area where research and the advice and experience of other breeders are invaluable.

Two vital factors to keep in mind as you make your selection are temperament and health. Temperament is a hereditary trait in dogs, although it can be influenced by other external factors. Selection over many generations eventually produced breeds with the correct temperament to pull sleds, follow scent on trails or retrieve game. The inheritance factors of temperament are complex. However, you should never consider breeding a dog with a questionable temperament. You impose a major disservice on both human and canine communities if you produce another generation of skittish or bad-tempered animals. As far as health goes, you must be aware that dogs are subject to many hereditary defects, some of which are potentially crippling or fatal. If you breed, you carry the responsibility of ensuring that the dogs you produce are not affected by the major known hereditary diseases occurring in your breed. Do not take this warning lightly. Consider how devastated you would feel if the beautiful eight-week-old puppy you place in a loving home develops a crippling hip problem at one year of age. Ignorance is no excuse for having contributed to this tragic situation.