Not all polygenic characters can be classified into grades; some, such as epilepsy, are all-or-nothing traits in that the dog is either normal or abnormal. Such traits, known as threshold traits, are difficult to work with but can respond to selection. Anyone purchasing a dog in a breed in which polygenic defects are common should ensure that parental stock are evaluated. It would, for example, be foolish to purchase breeds like German Shepherd Dogs, Labradors, Golden Retrievers or the giant breeds like Newfoundlands and St Bernards without seeking hip scores from the British Veterinary Association/German Shepherd Dog League or their equivalent in other countries.
A dog is acquired for its virtues rather than for its defects, but virtues are different things to different owners. Sound character is the most important trait of any dog whether i t is acquired as a family pet or as a potential show or working dog. In working breeds it should be possible to obtain a dog of sound character and working ability as well as beautiful looks, but in some breeds working and show strains have tended to diverge, polarizing in two quite different looking animals.
Most of the aspects governing so-called beauty, character and working ability are inherited. They are, with a few exceptions such as coat color, largely polygenic in their mode of control. The dog is the result of genetic make-up and environmental influences. The latter may start in the womb of the mother and be carried on throughout the first years of life. A black dog will be a black dog regardless of nutrition, but a genetically large dog will achieve its potential only if it is correctly reared. A dog of inherently sound character may end up aggressive or afraid of particular influences if it is not correctly socialized in puppy hood, and if the owner fails to establish a human pack order.