Mongrelism - Health

Health

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. 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. If the dog breed is popular, and the line continues, over hundreds of years, there will be more diversity due to mutations and occasional out-breeding; like an island with a few new birds—they will diversify. 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 order, because it comes with a different, desired, physical trait.

Populations are particularly vulnerable when the dogs bred are closely related. Inbreeding among purebreds has exposed various genetic health problems not always 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 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. However, some deleterious recessives occur across many seemingly unrelated breeds, and therefore merely mixing breeds is no guarantee of genetic health.

"Hybrids have a far lower chance of exhibiting the disorders that are common with the parental breeds", but crossbreeding two poor specimens together does not guarantee the resulting offspring will be healthier than the parents because the offspring could 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.

Purebred and mixed-breed dogs are equally susceptible to most 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”. Data from Denmark also suggest that mixed breeds have higher longevity on average compared to purebreeds. A British study showed similar results but a few breeds (notably Jack Russell Terrier, Miniature Poodles and Whippets) lived longer than mixed breeds.

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 was 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.