Selective breeding, also called artificial selection, is carried out by mating animals or crossing plants with desired characteristics or traits to produce offspring in which those traits are more dominant, according to Encyclopedia Britannica. The breeder attempts to isolate the genes responsible for the desired trait in subsequent generations so the quality becomes more pronounced and fixed.
In plants, selective breeding can produce higher crop yields, faster growth rates and imperviousness to disease. In animals, selective breeding enables such practical traits as higher milk yields in cows as well as aesthetic characteristics, such as changes in appearance. Examples of selective breeding are the many breeds of dogs. All of the purebred stocks of the approximately 400 registered dog breeds are maintained by selective breeding. Various genes bred into dogs across multiple generations account for their large or small size, head and body shapes, silky or wiry hair, length of legs, variations in color and predilection for certain activities. Purebred animals are those whose ancestry can be documented, proving descent from similar stock.
Though selective breeding has the advantage of locking desired characteristics into animals, it also tends to lock in less desirable traits, such as genetic tendencies towards certain diseases. For example, large dogs, such as Great Danes and Saint Bernards, are prone to hip dysplasia, bone tumors in their legs and heat prostration. Breeds such as Pekinese and bulldogs tend to have breathing problems due to shortened air passages because of their set-back noses. Tiny breeds have difficulties with dislocated kneecaps, heart problems and keeping warm. Often the difficulties these breeds experience are directly related to the characteristics breeders desire and try to selectively implant.