Are Mixed-Breed Dogs Really Healthier Than Purebreds?

Source: Photo by Pets Adviser from Brooklyn, USA; Creative Commons Attribution 2.0 License

For as long as I remember it has been argued that mixed breed dogs are healthier than purebred dogs because of "hybrid vigor". The genetics behind this are really quite simple. For certain genetically caused diseases, all you need is one defective allele in order for the disease to show up. (Think of an allele as one of two alternate forms of a gene.) However, in many other instances, we are dealing with a "recessive" disease which will only appear if you get two of the defective alleles (one from the father and one from the mother). So if you have a line of related individuals that tend to have that allele and they interbreed, the chances that you will get two copies of the defective gene are greatly increased, meaning that many of the offspring will have that genetic disease. On the other hand, if you have breedings from outside of this lineage (hybrids) the chances are much less. Since pedigreed dogs are interbred with other dogs that can trace back to the same ancestry, this clearly predicts that purebreds should be more likely to suffer from inherited diseases than mixed breed dogs.

We see examples of this in human populations. Since humans tend to marry individuals who are of their same race and religion it means that we are essentially engaged in a limited form of interbreeding. Well-known examples of inherited diseases associated with genetic lineages include Sickle Cell Anemia which is most common in individuals of African ancestry. Data shows that 1 in 12 African-Americans carry the sickle cell allele, so any mating between two individuals in this group would increase the probability that their offspring might have the disease. A similar situation appears in the case of Tay-Sachs Disease, which is most common in Jewish individuals. Among Jewish individuals living in the United States the estimates are that that 1 in every 27 carries the allele. Thus if an individual appears in a medical facility showing symptoms of sickle cell anemia it is a good bet that both of his or her parents are of African descent, while an individual showing symptoms of Tay-Sachs disease will most likely have two Jewish parents.

Returning to dogs, the issue of mixed breed versus purebred dogs vaulted into public consciousness in 2008 when the BBC broadcasted a brutally heartrending documentary "Pedigreed Dogs Exposed". It depicted things like a beautiful Cavalier King Charles Spaniel whimpering in pain from syringomyelia, an inherited neurological disease, and also a Boxer with spasming seizures and several other difficult to watch examples involving other purebred dogs suffering from genetically-based diseases and malformations. The commentary in the film seemed to suggest that purebred dog breeders were basically dog abusers and that all pedigreed dogs were most likely genetically flawed. There was a huge public outcry, and the British Kennel Club suffered major damage to its reputation and many dog breeders found themselves being targeted with insults and harassment.

Unfortunately, TV journalism, even when it includes interviews with a few veterinarians, does not constitute scientific data. Fortunately, an extensive study has recently been published and it provides data that allows us to objectively compare the genetic health of purebred and mixed breed dogs. The research was conducted by Jonas Donner, chief scientific officer at Genoscoper Laboratories in Helsinki, Finland along with 14 other scientists.

This massive new data collection comes from the genetic analysis of 83,220 mixed breed dogs and 18,102 purebred dogs (representing 330 breeds). The DNA of the dogs was examined for 152 known disease variants, and it was found that there were three common targets for genetic disease that affected both purebred and mixed breed dogs: namely the nervous system, vision, and the cardiovascular system. Furthermore, they found that most diseases were shared by both mixed breed and purebred dogs. Ultimately, for statistical purposes, the researchers narrowed their analysis down to the nine most common genetic diseases appearing in dogs of all types.

The data did confirm one aspect of the hybrid vigor notion since 3.9% of the purebred dogs showed some form of genetically related disorder while only 1.4% of the mixed breed dogs did so. To give you some idea of what these numbers mean, the World Health Organization (WHO) reports that approximately 4% of the international human population have symptoms of genetically based diseases, at least to some extent. That means that humans are roughly as likely to have genetically based diseases as are purebred dogs.

There is, however, an odd quirk in these data. This was found when the researchers looked at the number of "carriers". A carrier is defined as an individual who has one copy of a defective allele, which means that they have the ability to pass the disease on to their offspring if they mate with another individual who happens to also have that defective allele. Here the researchers find that mixed breed dogs were 1.6 times more likely than purebreds to be carriers of at least one of the nine recessive disease variants included in the analysis (30.3% versus 18.4%). Again it is interesting to compare this to the WHO data on humans which estimates that for a similar set of key genetic diseases approximately 35 to 40% of humans carry at least one defective allele which is a bit worse than the rate that we find in mixed breed dogs.

One particularly interesting finding in this research directly relates to the media charges that dog breeders are deliberately perpetuating lines of genetically unsound dogs. The investigators note that breeders may actually have been producing healthier dogs over time. Thus some canine diseases known to have genetic basis have been virtually eliminated in purebred dogs but are still sorting their way through mixed breed populations. For example, Beagles were susceptible to a severe anemic disease (pyruvate kinase deficiency), which through careful screening of pedigrees and genetic testing has been completely eliminated in the breed but not in mixed breed dogs. Similarly, a genetic disorder which leads to blindness and neural system collapse in Border Collies (neuronal ceroid lipofuscinosis) and a severe combined immunodeficiency disease, originally found in Basset hounds, have been eradicated in pedigreed lines. The disappearance of these diseases in such purebred dogs is really based on the efforts of breeders. Conscientious dog breeders systematically eliminate dogs which have shown genetically based diseases from their breeding programs and also take advantage of genetic screening tests that have become available in recent years. Mixed breed dogs, are often the result of random matings that are usually not controlled, and are therefore still suffering from these diseases.

One major service that these researchers have provided for dog breeders, veterinarians, or just members of the general public who wish to know more about their own dog breed's genetic health, is that they created My Breed Data. This is a website based on their data as collected so far. It allows anybody with interest in a purebred dog to simply enter the dog's breed and to receive a complete listing of any of the genetic disorders associated with that kind of dog.

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These researchers summarize their results by saying "In conclusion, we report that risk variants for genetic disorders are prevalent in the general dog population, and confirm that mixed breed dogs may suffer from many of the same medical conditions as purebreds."

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