Crossbreed (dog)

If the population of the breed is small, its genetic diversity can become very low over a period of time. When humans select certain dogs for new breeds, they artificially isolate this group of genes and cause more copies of this gene than would have occurred naturally. Initially, the population will be more fragile due to lack of genetic diversity. If the dog breed is very popular, diversity will increase due to mutations. This is why some of the 'older' breeds are stable. A problem occurs when certain traits found in the breed standard are associated with genetic disorders. Artificial selective pressure favors the duplication of the genetic disease, because it is linked to a desired physical trait. The genetic health of hybrids tends to be better. Characteristics of good health have been lost in many lines of purebred dogs because show dog breeders are more interested in conformation (the physical attributes of dogs relative to the breed standard) than in the health and working temperament for which the dog was originally bred. Populations are particularly vulnerable when purebred dogs are closely related. Inbreeding among purebred animals has created various genetic health problems not always apparent in less uniform populations. Mixed breed dogs are genetically more diverse because of the more random nature of their parents' mating. However, 'at random' is not 'random' for a geneticist. The descendants of these crosses might be less likely to express certain genetic diseases because there might be a decreased chance that both parents carry the same harmful recessive alleles, but some harmful recessive alleles occur in many apparently unrelated breeds. Thus, a mix of breeds is not a guarantee of genetic health. Also, when two poor specimens are bred, the offspring could inherit the worst traits of both parents. This can be seen in puppy mill dogs. Purebred or mixed breed dogs are also susceptible to most non-genetic diseases, such as rabies, canine distemper, injuries, and infestation by parasites. Several studies have shown that mixed breed dogs have better health. A German study concludes that 'Mixed breeds require less veterinary treatment'. Studies in Sweden state that 'mixed breed dogs are less prone to disease than purebred dogs' and referring to mortality rates, 'mixed breeds are always in the low-risk category'. Data from Denmark also suggests that mixed breeds have higher average longevity. A British study showed similar results, but a few breeds (notably Jack Russell Terrier, miniature poodles and whippets) lived longer than mixed breeds.