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Link to the Dog breeding article Dog breeding is the practice of mating selected dogs with the intent to maintain or produce specific qualities and characteristics. When dogs reproduce without such human intervention, their offspring's characteristics are determined by natural selection, while "dog breeding" refers specifically to the artificial selection of dogs, in which dogs are intentionally bred by their owners. Breeding relies on the science of genetics, so the breeder with a knowledge of canine genetics, health, and the intended use for the dogs attempts to breed suitable dogs.

Terminology
The female parent, a bitch, of puppies is referred to as the dam and the male parent, a stud, is referred to as the sire. A litter is those puppies born from the same pregnancy. A whelp is a newborn puppy and giving birth in dogs is called whelping. Dogs commonly give birth in a whelping box, a simple box or pen provided to the dam to help shelter and contain the puppies.

A person who intentionally mates dogs to produce puppies is referred to as a dog breeder. Line breeding is the planned breeding of dogs with their relatives. This is done to strengthen the appearance of specific desired traits in offspring. Line breeding is differentiated from inbreeding by excluding pairings between parents and offspring, and between full siblings. Outcrossing is the planned breeding between two unrelated dogs, used to increase genetic diversity in a breed and decrease genetic issues or abnormalities inherited from line breeding or inbreeding.

Estrus cycle
"See also: Canine reproduction"Dogs reach puberty between 6-24 months old. Female dogs will start having an estrus (also spelled "estrous") cycle. A dog in estrus, also known as being "in heat", can become pregnant during this 3-21 day period. There are 4 stages to the estrus cycle in dogs:
 * Anestrus: 60 to 200 days long. During this time the uterine lining repairs itself from the estrus and diestus periods.
 * Proestrus: 3-17 days long. The follicle secretes estrogen and there may be some discharge from the dog's vulva.
 * Estrus: 3-21 days long. Estrogen levels peak, which causes LH levels to peak 1-2 days after estrus begins. Ovulation occurs 1-2 days after the LH peak. The vulva discharge may change to a reddish or dark brown color.
 * Diestrus: 60-100 days long. Progesterone levels are high after estrus. Both pregnant and non-pregnant dogs go through diestrus and non-pregnant dogs may show signs of a pseudopregnancy.

Breeding and gestation
"See also: Canine reproduction"

History of dog breeding
Humans have maintained populations of useful animals around their places of habitat since pre-historic times. Over millennia, domesticated dogs have developed into distinct types, or groups, such as livestock guardian dogs, hunting dogs, and sighthounds. To maintain these distinctions, humans have intentionally mated dogs with certain characteristics to encourage those characteristics in the offspring. Through this process, hundreds of dog breeds have been developed. Artificial selection in dog breeding has influenced behavior, shape, and size of dogs.

It is believed when human civilization moved towards agrarian societies, dogs were selectively bred for smaller size and more docile behavior. These traits made it more comfortable for humans and dogs to live together. Dogs may also be an example of neoteny selection, as dogs tend to retain juvenile characteristics, like fuzzy fur and drooping ears, into adulthood. It has been seen that these traits can even prompt an adult female wolf to act more defensively of dog puppies than of wolf puppies. The example of canine neoteny goes even further, in that the various dog breeds are differently neotenized according to the type of behavior that was selected. Other researchers believe that because this comparison is based on the gray wolf, which is not the ancestor of the dog, that this comparison is invalid. Further research indicates that the concept of neoteny as a means of distinguishing dogs from wolves is baseless.

With the development of breed clubs and kennel clubs during the mid-19th century, dog breeding became more rigorous and many breeds were developed during this time. Dog breeding became more systemic to preserve unique mutations such as shortened legs, a shortened face, new fur colors and textures. The increased popularity of dog shows that judged a dog’s the look more than its working ability led to different goals when breeding. Dogs bred for show rather than performance tended to develop more exaggerated and extreme features to meet breed standards. One example of this change in breeding goals is the pronounced sloped back in the modern German Shepherd breed, compared to the straight back of working pedigrees. The Shar Pei is an example of how differing breed standards can influence the direction breeders take a dog and which traits are exaggerated. The Western Shar Pei has been bred to have a meatier mouth and wrinklier skin than the traditional Chinese Shar Pei it was bred from.

Initially, the ownership of working and purebred dogs was a privilege of the wealthy. Nowadays, many people can afford to buy a dog. There are various reasons for people to breed dogs. Some people may have personal reasons, such as wanting to own or give away puppies, and are not interested in pedigree. Breeders may also breed dogs for profit, for show, because of an interest in a particular breed, or to correct some issue and improve the soundness of a breed. Responsible breeders take into consideration the temperament, as well as the health and appearance of the mating pair before breeding.

Breeders of purebred dogs can register the birth of a litter of puppies to a dog registry associated with a kennel club to record the parentage of the litter in stud books. Such registries maintain records of dogs’ lineage and are usually affiliated with kennel clubs like the AKC (American Kennel Club). Maintaining correct data is important for purebred dog breeding. Access to records allows a breeder to analyze the pedigrees and anticipate traits and behaviors that may be passed onto offspring. Requirements for the breeding of registered purebreds vary between breeds, countries, kennel clubs and registries. Breeders also have to abide by an organization's rules to participate in its breed maintenance and development programs. The rules may apply to the health of the dogs, such as joint x-rays, hip certifications, and eye examinations; to working qualities, such as passing a special test or achieving at a trial; to general conformation, such as evaluation of a dog by a breed expert. However, many registries, particularly those in North America, are not policing agencies that exclude dogs of poor quality or health. Their main function is simply to register puppies born of parents who are themselves registered.

Criticism
Some dogs have certain inheritable characteristics that can develop into a disability or disease. Inbreeding and line breeding decrease the genetic diversity of offspring and increase the risk of inheriting and developing health issues. This is an issue in purebred dogs Canine hip dysplasia is one such condition. Some eye abnormalities, some heart conditions, and some cases of deafness have been proven to be inherited. There have been extensive studies of these conditions, commonly sponsored by breed clubs and dog registries, while specialised breed clubs provide information of common genetic defects for their breeds. As well, special organizations, such as Orthopedic Foundation for Animals, collect data and provide it to breeders, as well as to the general public. Conditions such as hip dysplasia can impact some breeds more than others.

Some registries, such as American Kennel Club, may include a record of the absence of certain genetic defects, known as a certification, in an individual dog’s record. For example, the German Shepherd Dog national breed club in Germany is a registry that recognizes that hip dysplasia is a genetic defect for dogs of this breed. Accordingly, it requires all dogs to pass evaluation for absence of hip dysplasia to register their progeny, and records the results in individual dog‘s pedigrees.

There are BBC documentaries titled "Pedigree Dogs Exposed" and "Pedigree Dogs Exposed – Three Years On" that claim health problems in dogs from inbreeding. Problems such as breathing in the Pug breed and Pekingese breed, spinal problems in the Dachshund breed, and Syringomyelia in the Cavalier King Charles Spaniel breed. It has been concluded that "findings imply that when selective breeding was done by humans, it squashed the snouts of certain dog breeds, it also morphed their brains".

Some scientific researchers argue that advances in artificial reproduction technology for the purposes of dog breeding can be helpful but also have "detrimental impacts" when overused instead of natural selection principles. These scientists call for a deeper understanding of natural selection, leading to a more naturalistic approach in dog breeding.

Inbreeding depression
Inbreeding depression is the reduced survival and fertility of offspring of related individuals. A study of 42,855 dachshund litters found that as the inbreeding coefficient increased, litter size decreased and the percentage of stillborn puppies increased, thus indicating inbreeding depression.