To a degree, inbreeding serves to improve and perfect a breed of dog by pairing specimens with excellent traits. However, it can also limit the available gene pool, which causes problems as too-closely-related animals are used over generations for breeding in a single line, such as fathers bred with daughters, or siblings bred together. Such inbreeding can foster generations of dogs who become sickly, who are infertile or who develop potentially fatal weaknesses in their immune systems.
Fertility Issues
The cautious breeder can recognize when inbreeding has gone beyond its limits when female dogs begin to repeatedly deliver unhealthy, disfigured or small litters, or when the infant mortality rate increases substantially. Also, deeply inbred male dogs tend to have lowered fertility, which makes successful breeding very difficult. One unusual litter may not in itself be attributed to inbreeding, but if they continue time after time, genetic predisposition is usually the cause of the problem.
Breeding for Special Traits
Selective breeding means that the breeder has a dog with unique traits and wants to re-create those traits using familial animals. This requires breeding only dogs that carry the unique trait in their genes. This necessarily limits the possible gene pool, which can ultimately lead to inbred bone deformities and degenerative disorders such as arthritis. Other serious problems include heads being too large for natural births and lowered immunity, which makes specimens more susceptible to disease.
Patellar Luxation and Hip Dysplasia
Patellar luxation is a genetic condition involving dislocation of the knee cap that mostly affects small dogs such as Boston and Yorkshire terriers, Chihuahuas, pomeranians and miniature poodles. It affects both knees in half the cases, resulting in crippling pain. Dysplasia is the state of the hip joint being loose, which causes the hip to pull out of joint easily. This usually leads to osteoarthritis or degenerative joint disease. These conditions cause a dog pain and disability.
Behavior Difficulties and Intelligence
Although not always, aggression can often be a serious issue with inbred dogs. This results in biting for no reason, attacking people, chasing cars or people or other animals, and playing too roughly. Inbred dogs also often have much lower intelligence than less-inbred members of the same breed. These dogs can be difficult to train, due to their lowered intelligence, which -- added to their natural tendency toward aggressiveness -- can make them undesirable as pets.