As soon as a breed of dog or cat becomes popular, careless overbreeding increases. Inattention to selecting the hardiest, healthiest animals for breeding causes health defects in the pet population.

Eye Problems

Dry eye (which can cause corneal scarring) is prevalent in Lhasa apsos, pugs and Shih Tzus. Dachshunds bred for a dappled coat are often born blind or with undeveloped or no eyes.

Hearing Loss

The Orthopedic Foundation for Animals reports that genetic deafness occurs in 80 dog breeds, primarily Great Danes, Dalmatians, collies and Old English sheepdogs. This defect is closely linked to white pigmentation.

Joint Problems

Careless breeding results in abnormal hip development (canine hip dysplasia) in large dog breeds like the golden retriever. Small dogs, such as the Chihuahua, are prone to dislocating kneecaps (patella luxation).

Breathing Problems

Breeding dogs and cats for excessively flat faces causes respiratory problems, especially in the Pekingese and bulldog and the flat-faced Persian cat.

Birthing Difficulty

Over 92 percent of Boston terriers are born through cesarean section, with bulldogs close behind at 80 percent, according to Telegraph.co.uk. This is due to selectively breeding for extremely large heads.

Degenerative Disk Disease

Some dogs are genetically predisposed to spinal injuries, especially dachshunds, due to their elongated spines. This often leads to paralysis in adulthood.