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.