The breeding of a mother dog with her son is called inbreeding. This is a term that is not well understood and is often confused with what is known as "line breeding." Although inbreeding is the method that was originally used to develop the dog breeds we know today, it is now practiced only in special circumstances and should always be managed by an experienced dog breeder.
Inbreeding is the mating of a pair of dogs that are directly related to each other. This usually means a mother and son, father and daughter, or brother and sister -- whether they are from the same litter or not. Inbreeding is often used to perpetuate a specific attribute. A mother dog with a particularly unusual coat color may give birth to puppies that are the same color. By breeding the mother to one of her sons the next time, another family can be established that has the same color. This sets the stage for line breeding of the best specimens, once more dogs with the same color are available with which to breed.
Line breeding is considered the best way to keep good family genes going. This involves mating two dogs that have relatives in common, such as an uncle and a niece, or a grandmother and grandson. This is a complex process in which some dogs are used over and over again in different combinations to retain certain characteristics, such as good temperament or working ability.
Line-breeding may also be used to correct faults. For example, if a female dog has weak knees but is perfect in other ways, she may be mated with a male that has the same good attributes but has strong knees. In this way the breeder keeps the good qualities in the family going, while trying to "fix" the bad qualities.
Breeding for Quality
Inbreeding is done one litter at a time. Each time a litter is bred, the breeder must evaluate whether the mix was a good one. Usually one or two puppies are selected from the litter based on their quality and the breeder keeps them to use again later, possibly mating the mother back to a different son. With line breeding, it is possible to get puppies that are better than both parents.
Problems to Avoid
Breeders must ensure that dogs with genetic faults such as hip or aggression problems are not used for in- or line breeding, and especially not mated with other dogs that have similar problems. The breeding of "tea-cup" Chihuahuas is often an example of irresponsible breeding, because in order to get tiny dogs the breeders have mated the smallest dog with the smallest female. Since the tiniest dogs are frequently the "runts" of the litter, this can result in a number of genetic disorders and other problems. If a timid mother dog breeds with a son that is also timid, the puppies may all be very timid by nature.