Purebred dogs come from a much more limited gene pool than do mongrels, crossbreed or random-bred dogs. Purebred dogs are usually only bred with other dogs of the same breed. This limited gene pool can enhance many undesirable characteristics in the puppies, because the chances of both parents carrying mutated or recessive genes are high. Mutated and recessive genes tend to contain health problems, although just how is not entirely known.
Health Problems Due To Physical Build
The most obvious detrimental affects are health problems that are directly caused by the exaggerated shape of a particular dog breed. For example, the Cavalier King Charles spaniel often grows a head that is too small for its brain. Pugs, Pekingese, Boston terriers and other flat-faced, or brachycephalic, dogs have troubles breathing because their noses and airways are too small for them to breathe properly. Dachshunds and dogs with long backs and short legs often have spinal troubles due to their long spines.
Health Problems Due to Genetics
Many illnesses or severe medical conditions do not develop until after a purebred puppy is brought home. These illnesses are passed on through genes. Illnesses and medical conditions known to contain a strong genetic component include epilepsy, progressive retinal aphasia and hip dysplasia. Some of these conditions are unusual but not deadly, such as blue Doberman syndrome or color mutant alopecia, in which dogs of blue or fawn coloring such as Doberman pinschers, Italian greyhounds of Irish setters lose their hair. Some dog breeders are trying to remove these inherited traits by simply neutering any dog with the health problems. For example, there is a national group called the Orthopedic Foundation for Animals that certifies dogs that are free of certain inherited conditions like hip dysplasia.
Purebred dogs were often bred to perform a specific function, such as herding livestock or fighting other dogs. These traits have not been bred out over the generations. If a dog was bred to fight other dogs, which would be any bulldog breed, then they may need to be supervised when they are around other dogs. Herding dogs and hunting dogs were bred to be on the go all day, so they need a lot of exercise or they could have mischievous tendencies. Herding dogs in particular will round up people or children in a bossy or threatening manner because that is what they have been bred to do.