Inbreeding is commonly used by dog breeders as a way to produce exceptional examples of a breed. Breeding a mother to her son is often done to preserve desired traits. On its own, such a pairing likely won't cause a problem. But if the inbreeding continues for generations, it can cause unwanted and often serious side effects.
Dogs can develop low fertility when they have been inbred too much. Males are more commonly known to suffer from infertility because of inbreeding, according to the Dog Breed Info Center. Low fertility in males or females makes producing healthy litters less likely.
Another side effect of inbreeding can be a high rate of puppy mortality. Puppy mortality, along with small litter sizes, are known as inbreeding depression. Because the mother and father of the puppies are too closely related, the puppies won't be as healthy as those of unrelated parents.
A common effect of inbreeding is abnormalities or mutations. Genes that can trigger deformities are more common when animals are inbred because recessive gene disorders often show up in the puppies when dominant genes aren't present to cancel them out. Those genes can move on to each generation and become more widespread. The abnormalities could be anything from extra toes to dwarfism to a predisposition to certain conditions, such as hip dysplasia.