Dog breeding is not a task to be taken lightly. Every person who breeds a litter should know the purpose for which their dogs were originally bred, the ancestors of the dogs to be bred for at least three generations, and any genetic diseases that the dogs might carry. It is of utmost importance that a breeder also understand the difference between line breeding, inbreeding and outcrossing, in order to avoid health problems in any puppies they produce.
Breeding by Appearance or Pedigree vs. Genetics
According to Dr. Carmen L. Battaglia, in a lecture from the Tufts Canine and Feline Breeding and Genetics Conference, "breeders have had to rely on their ability to recognize and associate names and titles with the traits and characteristics of each ancestor." As recently as a few decades ago, dog breeders would select two dogs solely based on similarly desirable appearance to produce their future generations of puppies. Alternatively, they would breed dogs with pedigrees and ancestors that were deemed desirable, whether in terms of "working" traits or for appearance. Today's breeders, however, also have access to genetic profiling and testing to ensure improved health in future generations of their chosen breeds.
Line Breeding, Inbreeding and Outcrossing
Line breeding is not as aggressive a process as inbreeding, although it's used to obtain similar results. A breeder might choose to breed a grandsire and a granddaughter, for example, or an aunt to a nephew. Line breeding will "concentrate" the desirable or undesirable genes in the same manner that strict inbreeding will; however, it is a more gradual process. By contrast, inbreeding occurs when two very close relatives are bred together: mother to son, father to daughter, or littermates to each other. Outcrossing occurs when two apparently unrelated dogs are bred together. Because most breeds have a relatively small gene pool, an outcross might be considered breeding dogs unrelated within four, five or six generations.
Genetic Disease and Coefficient of Inbreeding (COI)
The coefficient of inbreeding (COI) is a tool used to determine the probability that a particular gene came from a particular ancestor. Two completely unrelated dogs would produce puppies with a COI of zero. However, a brother to sister mating would result in a COI of only 50 percent, not 100 percent, as might be expected, as long as the siblings' parents are unrelated. It would take many generations of brother to sister matings before the COI would reach 100 percent. Although it is best to keep as low a COI as possible, neither inbreeding nor line breeding will create new genes. Neither will they create undesirable genes. However, because the genes are being distilled into a more uniform group, such breeding will expose the positive and negative genetic traits of a puppy's ancestors.
As can be seen above, then, nothing extraordinary will come from a single generation of breeding brother to sister. A slightly greater probability might exist for a birth defect or genetic issue to occur, should one already exist in the bloodline being refined; however, it is also possible that the converse would also be true. In the hands of an experienced and knowledgeable breeder, inbreeding can be a valuable tool that reveals the genetic flaws present in his or her stock, which then can be reduced or eliminated by selective line breeding.
The Truth about “Hybrid Vigor” in Dogs
Because all breeds of dogs belong to the same species, true hybrid vigor does not exist when breeds are mixed. In fact, genetic health issues become more difficult to track and may appear unexpectedly in future generations. Donna Noland, writing for "The Dog Press," cites an article that identifies 102 genetic issues that can appear in mixed breed dogs. By way of example, if a collie that carries progressive retinal atrophy (PRA) and hip dysplasia breeds with a German shepherd dog that has a mild degree of hip dysplasia, those puppies will have a probability of carrying PRA. If siblings of the same litter mate, there is a probability that the puppies resulting from that breeding will then have a greater probability of actually having PRA, as well as displaying varying degrees of hip dysplasia. It is clear, then, that if any hybrid vigor exists in dogs at all, it has a very limited effect over time and, if inbreeding is present in a mixed breed dog population, that it can have the same deleterious effect that is found in inbred pedigreed dogs displaying varying degrees of hip dysplasia.