Breeders learned that by mating related individuals of the desired type, the resulting quality and uniformity of the offspring improved As people began to learn basic genetics in the early part of this century, they deliberately sought to fix desired traits, particularly in production livestock, by breeding near relatives. This practice continues to the present day. If the offspring measure up, he will be kept for stud. If they don t, everybody goes to market. This drastic culling serves its purpose in livestock, but it is impractical and unacceptable in companion animals such as dogs.
Nature goes to great lengths to discourage inbreeding.
Related animals rarely mate, which prevents genes for diseases and defects from coming together with any great frequency. Wild animals have a variety of behaviors which will eliminate or severely restrict inbreeding. In wolves, the species most closely related to dogs, only the alpha pair will breed. Pups stay with the pack for their first year. After that time they must find a place, often low-ranking, within the adult hierarchy.