Purebred dog refers to a dog of a modern dog breed that closely resembles other dogs of the same breed, with ancestry documented in a stud book and registered with one of the major dog registries. Documentation (so that the dog is known to be descended from specific ancestors) and registration distinguish modern breeds from dog types or landraces of dog (sometimes called natural breeds or ancient breeds) that arose under human influence over a long period of time to do a specific type of work.
Purebred dog may also be used in a different manner to refer to dogs of specific dog types and landraces that are not modern breeds. An example is cited by biologist Raymond Coppinger, of an Italian shepherd who keeps only the white puppies from his sheep guardian dog's litters, and culls the rest, because he defines the white ones as purebred. Coppinger says, "The shepherd's definition of pure is not wrong, it is simply different from mine." However, the usual definition is the one that involves modern breeds.
The earliest use of the term "pureblood" in English referring to animal breeding, according to the Online Etymological Dictionary, was in 1882 and "pure bred" in 1890. The Merriam-Webster Online Dictionary dates the use to 1852.
Purebred dogs are by definition registered members of modern breeds. Breeds of dogs may be registered either in an open stud book or a closed stud book. The term purebred dog is typically used to mean dogs registered with a closed stud book registry, but the connotation of desirability of this type of registration is disputed by owners of purebred dogs from open stud book registries.
Crossbred dogs (first generation crosses from two purebred dogs, also called dog hybrids) are not breeds and are not considered purebred, although crossbreds from the same two breeds of purebreds can have "identical qualities", similar to what would be expected from breeding two purebreds, but with more genetic variation. However, crossbreds do not breed true (meaning that progeny will show consistent, replicable and predictable characteristics), and can only be reproduced by returning to the original two purebred breeds.
Among breeds of hunting, herding, or working dogs in open stud book registries, a crossbred dog may be registered as a member of the breed it most closely resembles if the dog works in the manner of the breed. Some hunting, herding, or working dog registries will accept mixed breed (meaning of unknown heritage) dogs as members of the breed if they work in the correct manner, called register on merit..
For mixed breed (unknown heredity), crossbred (from two different purebred breeds), or otherwise unregistered purebred pet dogs there are available many small for-pay internet registry businesses that will certify any dog as a purebred anything one cares to invent. However, new breeds of dog are constantly being legitimately created, and there are many websites for new breed associations and breed clubs offering legitimate registrations for new or rare breeds. When dogs of a new breed are "visiblily similar in most characteristics" and have reliable documented descent from a "known and designated foundation stock" they can then be considered members of a breed, and, if an individual dog is documented and registered, it can be called purebred. Only documentation of the ancestry from a breed's foundation stock determines whether or not a dog is a purebred member of a breed.
A showdog is a purebred dog that participates in dog shows with its owner or handler.
The term showdog is commonly used in two different ways. For people in the dog fancy, a showdog is an exceptional purebred dog that conforms to breed type, and an outgoing, high energy character. For people who have no interest in dog shows, the term "showdog" is often used facetiously to refer to a dog whose only attributes are in its appearance. Unfortunately, most breeders of showdogs are more interested in conformation - the physical attributes of the dogs in relation to the breed standard - rather than in the working temperament for which the dog was originally bred. Raymond Coppinger says, "This recent breeding fad for the purebred dog is badly out of control.".
Dog shows (and the related sport of Junior Handling for children and young people) continue to be popular activities; a single show, the 2006 Crufts dog show alone had 143,000 spectators, with 24,640 purebred dogs entered, representing 178 different breeds from 35 different countries. The sport of conformation dog showing is only open to registered purebred dogs.
Purebred dogs represent to many commentators the attitudes of the late Victorian era, when dog breeding first became popular and when most modern breeds originated. Purebred dogs were bred from a narrow set of ancestors, and an idea developed that this somehow made them superior in both appearance and in general goodness. Englishman Francis Galton used the term eugenics to refer to his ideas for applying domestic animal breeding techniques to humans, to produce a 'pure' and 'good' elite; the idea became an intellectual fad, promoted by people as diverse as Margaret Sanger and dog writer Leon Fradley Whitney, who both promoted the sterilization of 'unfit' humans; ideas that were extended horrifyingly by the Nazis in World War II era Germany.
Purebred dog breeders of today "have inherited a breeding paradigm that is, at the very least, a bit anachronistic in light of modern genetic knowledge, and that first arose out of a pretty blatant misinterpretation of Darwin and an enthusiasm for social theories that have long been discredited as scientifically insupportable and morally questionable." Information about the way early dog shows were intellectualized is of little interest to modern breeders and owners of purebred dogs, who for the most part have never heard of eugenics. Breeders and serious fanciers are more interested in the real or imagined early history of their favourite breed's development. Reputable breeders attempt to produce the healthiest dogs the limited gene pool will allow, and buyers of purebreds primarily are interested in a puppy whose adult size, appearance, and temperament are predictable. In addition, tens of thousands of people worldwide enjoy the sport of conformation dog showing, which is restricted to purebred dogs.
Health issues of purebred dogs and genetic problems in purebred dogs has been extensively covered in these articles - Genetic disease | List of dog diseases | Canine reproduction as well as the others linked in this article. Also see articles about individual dog breeds for more on health issues of individual breeds. The BBC also recently ran a documentary on the health problems in pedigree dogs .
Most purebred breeds that exist today were created in the late 1800s from older dog types by selective breeding and rigorous culling. This created a genetic bottleneck that will at some point render breeding from closed stud books unviable. Suggestions for improvement have included outcrossing (opening studbooks) and measuring and regulating inbreeding.
Books on choosing a puppy advocate for purebred dogs, as long as they come from breeders who are willing to invest the time and money in producing healthy dogs that they are willing to guarantee. "The difference is that purebred breeders know what to expect", writes Chris Walkowicz in The Perfect Match. Stephen Budiansky in The Truth About Dogs writes, "It is true that the standard criticisms leveled against inbreeding are not always well informed from the point of view of modern genetics." He continues, "Curing the problems that inbreeding has engendered in purebred dogs will require more subtlety than either most breeders or their more vocal critics have so far displayed."
Hungarian ethologist Vilmos Csányi sees purebred dog breeders, in efforts to meet breed standards, increasing the extent of inbreeding and thereby reducing the breeds' desirable attributes; "This process appears to be unstoppable," he says. He continues with the idea that breeders could preserve their purebred breeds, while improving the health of dogs for pets, through the breeding of uniform crossbreeds (hybrids). "If we take care to maintain the parent breeds," Csányi writes, "we can always produce good hybrids." The crossing should only be in the first generation; "hybrids produced from different breeds should not be bred further", as the advantage of predictability is lost.
Humans can survive without dogs, but dogs have adapted so completely that most dogs cannot survive without humans. The majority of people do not use dogs in any kind of work, and dogs are expensive to keep. Large numbers of both mixed breed and some purebred dogs are discarded to shelters or become strays. In some countries this has resulted in eradication attempts (as was done in recent dog culls in China.)