Are mixed breeds really healthier than pure breeds?

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.

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