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Estimates place their numbers at 150 million animals worldwide. Although the term “mixed-breed dog” is preferred by some, many mongrels have no known purebred ancestors. The implication that such dogs must be a mix of defined breeds may stem 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, behavioral, or temperamental characteristics.

Mongrel” is still the preferred term. There are also regional terms for mixed-breed dogs. North Americans generally prefer the term mix or mixed-breed. There are also names for mixed-breeds based on geography, behavior, or food. 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 may have different genetic profiles. 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 purebred ancestry, but rather an indication that those dogs share common ancestry with certain purebreds. DNA tests to determine breed. As well, many newer dog breeds can be traced back to a common foundational breed making them difficult to separate genetically. If the founding population for the breed was small, then the genetic diversity of that particular breed may be small for quite some time.

In essence, when humans select certain dogs for new breeds, they artificially isolate that group of genes and cause more copies of that gene to be made than might have otherwise occurred in nature. Initially, the population will be more fragile because of the lack of genetic diversity. This is why some of the very “old” breeds are more stable. The problem is when certain traits found in the breed standard are associated with genetic disorders.

Then, the artificial selective force favors the duplication of the genetic disorder, because it comes with a desired physical trait. The genetic health of hybrids tends to be higher. Populations are particularly vulnerable when the dogs bred are closely related. Mixed-breed dogs are more genetically diverse due to the more haphazard nature of their parents’ mating.

However, “haphazard” is not the same as “random” to a geneticist. The offspring of such matings might be less likely to express certain genetic disorders because there might be a decreased chance that both parents carry the same detrimental recessive alleles, but some deleterious recessives occur across many seemingly unrelated breeds, and therefore merely mixing breeds is no guarantee of genetic health. Also, when two poor specimens are bred, the offspring could inherit the worst traits of both parents. Several studies have shown that mixed-breed dogs have a health advantage over pure-bred dogs.

A German study finds that “Mongrels require less veterinary treatment”. Data from Denmark also suggest that mixed breeds have higher longevity on average compared to purebreeds. In one landmark study, the effect of breed on longevity in the pet dog was analyzed using mortality data from 23,535 pet dogs. The data were obtained from North American veterinary teaching hospitals. The median age at death was determined for pure and mixed breed dogs of different body weights. Within each body weight category, the median age at death was lower for pure breed dogs compared with mixed breed dogs.

The median age at death was “8. 5 years for all mixed breed dogs, and 6. 7 years for all pure breed dogs” in the study. In 2013, a study found that mixed breeds live on average 1. Mixes that show characteristics of two or more breeds.