A mixed-breed dog, also called a mutt, mongrel, tyke, cur, bitzer, or random-bred dog, is a dog whose ancestry is generally unknown and that has characteristics of two or more types of breeds, or is a descendant of feral or pariah dog populations. "Random-bred" is a genetic term meaning an animal, population, or breed that was bred or developed without planned intervention of humans; and whose ancestry and genetic makeup is generally not known. The most common term is mixed-breed, but that description is technically a misnomer. Along with the term purebred dog, the idea that such dogs are a mix of defined breeds stems from an inverted understanding of the origins of dog breeds. Pure breeds have been, for the most part, artificially created from random-bred populations by human selective breeding with the purpose of enhancing desired physical or temperamental characteristics. Dogs that are not purebred are not necessarily a mix of such defined breeds.
In contrast, the term crossbreed refers to dogs produced by the intentional crossing of two different known purebred dog breeds. Dogs interbreed freely, except where extreme variations in size exist, so mixed-breed dogs vary in size, shape, and color, making them difficult to classify physically.
There is a profusion of words and phrases used for dogs that are not purebred. The words cur, tyke, mutt and mongrel are used, sometimes in a derogatory manner. In the United Kingdom mongrel is the unique technical word for a mixed-breed dog. North Americans generally prefer the term mix or mixed-breed. Mutt is commonly used (in the U.S.A and Canada), often in an affectionate manner. In Hawaii, mixed-breed dogs are referred to as poi dog, and in The Bahamas, they call them Potcake Dogs (referring to the table-leftovers they are fed). Some American registries and dog clubs that accept mixed-breed dogs use the breed name All American. In South Africa the tongue-in cheek expression pavement special is sometimes used as a description for a mixed-breed dog. Random-bred dog, mutt, and mongrel are often used for dogs who result from breeding without the supervision or deliberate planning of humans, whereas crossbreed implies mixes of known breeds, sometimes deliberately mated.
In Brazil and the Dominican Republic, the name for mixed-breed dogs is vira-lata (trash-can tipper) because there are dogs without owners that feed on urban garbage on the streets, and often knock over trash cans to reach the food.
Slang terms are also common. Heinz 57 or Heinz is often used for dogs of uncertain ancestry, in a playful reference to the "57 Varieties" slogan of the H. J. Heinz Company. In some countries, such as Australia, bitsa (or bitzer) is common, meaning "bits o' this, bits o' that". A fice or feist is a small mixed-breed dog. In Newfoundland, a smaller mixed-breed dog is known as a cracky, hence the colloquial expression "saucy as a cracky" for someone with a sharp tongue.
To complicate matters, many owners of crossbreed dogs identify them—often facetiously—by an invented breed name constructed from parts of their parents' breed names. These are known as portmanteau names. For example, a cross between a Pekingese and a Poodle is called a Peekapoo, possibly a play on peek-a-boo, along with the Goldendoodle, a cross between a poodle and a golden retriever. As another example, one of the UK's Queen Elizabeth II's famous Corgis mated with her sister's Dachshund, and the resulting offspring were referred to as Dorgis.
Dogs that are descended from many generations of mixes are typically light brown or black and weigh about 18 kg (40 lb). They typically stand between 38 and 57 cm (15 and 23 inches) tall at the withers.
Guessing a mixed-breed's ancestry is difficult for even knowledgeable dog observers, because mixed-breeds have much more genetic variation than among purebreds. For example, two black mixed-breed dogs might each have recessive genes that produce a blond coat and, therefore, produce offspring looking unlike their parents.
Starting in 2007, blood samples and cheek swabs have become available to the public to narrow down the ancestry of mixed-breed dogs. The companies claim their DNA-based diagnostic test that can genetically determine the breed composition of mixed-breed dogs. These tests are still limited in scope because only a small number of the hundreds of dog breeds have been validated against the tests, and because the same breed in different geographical areas could have different genetic profiles. Also, the tests do not test for breed purity, but for genetic sequences that are common to certain breeds. With a mixed-breed dog, the test is not proof of pure-bred ancestry, but rather an indication that those dogs share common ancestry with certain purebreds.
The theory of hybrid vigor suggests that as a group, dogs of varied ancestry will be healthier than their purebred counterparts. In purebred dogs, intentionally breeding dogs of very similar appearance over several generations produces animals that carry many of the same alleles, some of which are detrimental. This is especially true if the dogs are closely related. This inbreeding among purebreds has exposed various genetic health problems not readily apparent in less uniform populations. Mixed-breed dogs are more genetically diverse due to the more haphazard nature of their parents' mating. "Haphazard" is not the same as "random" to a geneticist. The offspring of such matings are less likely to express certain genetic disorders because there is a decreased chance that both parents carry the same detrimental recessive alleles. However, some deleterious recessives are common across many seemingly unrelated breeds, and therefore merely mixing breeds is no guarantee of genetic health.
The declining overall health of many purebreds is also leading to a decline in the popularity of mixed-breed dogs, especially with the "designer dog" cross-breeding fads. In fact, crossbreeding two poor specimens together does not guarantee the resulting offspring will be healthier than the parents because the offspring could easily inherit the worst traits of both parents. This is commonly seen in dogs from puppy mills. Healthy traits have been lost in many purebred dogs lines because many breeders of showdogs are more interested in conformation - the physical attributes of the dogs in relation to the breed standard - than in the health and working temperament for which the dog was originally bred.
Not all damaging genes are recessive, and there are relatively few single-gene traits. Purebred and mixed-breed dogs are equally susceptible to non-genetic ailments, such as rabies, distemper, injury, and infestation by parasites.
Several studies have shown that mixed-breed dogs have a health advantage. A German study finds that "Mongrels require less veterinary treatment". Studies in Sweden have found that "Mongrel dogs are less prone to many diseases than the average purebred dog" and, referring to death rates, “Mongrels were consistently in the low risk category”. Another study reports that “The median age at death was 6.7 years for all mixed-breed dogs and 8.7 years for all pure breed dogs... For each weight group, the age at death of pure breed dogs was significantly less than for mixed-breed dogs”, and a study in Denmark finds that "Higher average longevity of mixed-breed dogs (grouped together).
Most dog breeds are a result of human selection. Existing dog breeds began as mixed-breeds, either random-bred or by deliberate crosses of existing breeds. Encouraging desirable traits and discouraging others, breeders sought to create their ideal appearance or behavior, or both, for dogs, and, additionally, to ensure that the dogs could consistently produce offspring with the same appearance or behavior.
Mixed-breed dogs can be divided roughly into four types:
There is no scientific justification for the belief that a purebred dam is in any way tainted after mating with a dog of another breed. Future matings with dogs of the same breed will produce purebred puppies.
Mixed-breed dogs can excel at dog sports, such as obedience, dog agility, flyball, and frisbee. Often, highly energetic mixed-breeds are left with shelters or rescue groups, where they are sought by owners with the caring, patience, and drive to train them for dog sports, turning unwanted dogs into healthy, mentally and physically stimulated award winners.
Until the early 1980s, mixed-breed dogs were usually excluded from obedience competitions. However, starting with the American Mixed Breed Obedience Registry (AMBOR) and the Mixed Breed Dog Clubs of America (MBDCA), which created obedience venues in which mixed-breed dogs could compete, more opportunities have opened up for all dogs in all dog sports. Most dog agility and flyball organizations have always allowed mixed-breed dogs to compete. Today, mixed-breeds have proved their worth in many performance sports.
In conformation shows, where dogs' conformation to a breed standard is evaluated, mixed-breed dogs normally cannot compete. For purebred dogs, their physical characteristics are judged against a single breed standard. Mixed-breed dogs, however, are difficult to classify except according to height; there is tremendous variation in physical traits such as coat, skeletal structure, gait, ear set, eye shape and color, and so on. When conformation standards are applied to mixed-breed dogs, such as in events run by the MBDCA, the standards are usually general traits of health, soundness, symmetry, and personality. The Kennel Club (UK) operates a show called Scruffts (a name derived from its prestigious Crufts show) open only to mixed-breeds in which dogs are judged on character, health, and temperament. Some kennel clubs, whose purpose is to promote purebred dogs, still exclude mixed-breeds from their performance events. The AKC and the FCI are two such prominent organizations. While the AKC does allow mixed-breed dogs to earn their Canine Good Citizen award, mixed-breed dogs are not permitted to enter AKC "all breed" events.
The mature appearance and behavior of purebred puppies may be more predictable than that of mixed-breeds, including cross-breeds. With purebred dogs, genetic variations are well documented and a reputable breeder has a fair estimation of what type of offspring a given pair will produce. Still, there is variation within breeds; for example, two champion sheep-herding Border Collies might produce offspring with no interest in sheep herding.
Some trainers believe mixed-breeds exhibit higher average intelligence than purebreds, but others believe mixes are no more intelligent than purebreds. Both sets feature both slow learners and dogs with high learning capacity.
Studies that have been done in this area show that mixed-breeds on average are both healthier and longer-lived than their purebred cousins. This is because current accepted breeding practices within the pedigreed community results in a reduction in genetic diversity, and can result in physical characteristics that lead to disabilities
Many people enjoy owning mixed-breeds, valuing their unique appearance and characteristics. While purebred dogs exhibit little variability of appearance within their breed, mixed-breed dogs exhibit often unique appearances. Although some dog owners prefer the status of owning a specific breed of dog or have a nostalgic attachment to a breed they wish to acquire, many others enjoy mixed-breed dogs that exhibit characteristics similar to their favorite breeds.
Studies have shown that cross-bred dogs have a number of desirable reproductive traits. Scott and Fuller found that cross-bred dogs were superior mothers compared to purebred mothers, producing more milk and giving better care. These advantages led to a decreased mortality in the offspring of cross-bred dogs.
Some owners value a dog's pedigree as a status symbol and, therefore, have no use for mixed-breed dogs; others appreciate or have an emotional attachment to the physical or behavioral traits of certain breeds; still others ignore pedigree and, instead, value a dog's personality and health.
Local animal shelters adopt out dogs of both purebred and mixed ancestry, emphasizing each dog's personality and suitability as a companion for each potential owner's lifestyle.