|Inbreeding is the mating together of closely or semi-closely related dogs, for example mother/son, father/daughter and sibling/sibling matings.
For breeders, it is a useful way of fixing traits in a breed - the pedigrees of some exhibition dogs show that many of their forebears are closely related. For example, there is a famous dog (shown in the 1990s) appeared in more and more pedigrees, sometimes several times in a single pedigree, as breeders were anxious to make their lines more typey. Superb specimens are always much sought after for stud services or offspring, having won the approval of show judges.
However, inbreeding holds potential problems. The limited genepool caused by continued inbreeding means that deleterious genes become widespread and the breed loses vigor. Laboratory animal suppliers depend on this to create uniform strains of animal which are immuno-depressed or breed true for a particular disorder e.g. epilepsy. Such animals are so inbred as to be genetically identical, a situation normally only seen in identical twins. Similarly, a controlled amount of inbreeding can be used to fix desirable traits in farm livestock e.g. milk yield, lean/fat ratios, rate of growth etc.
Most dog breeders are well aware of potential pitfalls associated with inbreeding although it is tempting for a novice to continue to use one or two closely related lines in order to preserve or improve type. Breeding to an unrelated line of the same breed (where possible) or outcrossing to another breed (where permissible) can ensure vigor. Despite the risk of importing a few undesirable traits which may take a while to breed out, outcrossing can prevent a breed from stagnating by introducing fresh genes into the gene pool. It is important to outcross to a variety of different dogs, considered to be genetically "sound" (do any of their previous offspring exhibit undesirable traits?) and preferably not closely related to each other.
How can you tell if a breed or line is becoming too closely inbred? One sign is that of reduced fertility in either males or females. Male dogs are known to have a low fertility rate. Small litter sizes and high puppy mortality on a regular basis indicates that the dogs may be becoming too closely related. The loss of a large proportion of dogs to one disease indicates that the dogs are losing/have lost immune system diversity. If 50% of individuals in a breeding program die of a simple infection, there is cause for concern.
Highly inbred dogs also display abnormalities on a regular basis as "bad" genes become more widespread. These abnormalities can be simple undesirable characteristics such as misaligned jaws (poor bite) or more serious deformities. Sometimes a fault can be traced to a single male or female which should be removed from the breeding program even if it does exhibit exceptional type. If its previous progeny are already breeding it's tempting to think "Pandora's Box is already open and the damage done so I'll turn a blind eye". Ignoring the fault and continuing to breed from the dog will cause the faulty genes to become even more widespread in the breed, causing problems later on if its descendants are bred together.
Inbreeding is a double-edged sword. On the one hand a (with an experienced breeder who understands genetics) a certain amount of inbreeding can fix and improve type to produce excellent quality animals. On the other hand, (with an in-experienced breeder who does'ntt know genetics) excessive inbreeding can limit the gene pool so that the breed loses vigor. Breeds in the early stages of development are most vulnerable as numbers are small and the dogs may be closely related to one another. It is up to the responsible breeder to balance inbreeding against out-crossings with unrelated dogs in order to maintain the overall health of the line or breed concerned.