A recent blog post by Dr. Stanley Coren mused about “hybrid vigor” or the notion that mixed-breed dogs are healthier — or, at least, less prone to genetic diseases — than purebred dogs. I decided to read the full study he referenced to learn more.

A large group of researchers, mostly Finnish, studied a huge sample of dogs: more than 83,000 mixed-breed and more than 18,000 purebred dogs of 330 different breeds. They analyzed the dogs’ genotypes, looking for 152 different genetic markers that underlie hereditary diseases.

They quickly narrowed down the study: Of the dogs with a faulty gene, 96 percent had one (or more) of a group of 30 genetic markers or “disease alleles.” They narrowed further, selecting the nine most common markers, which all appeared in multiple purebred breeds as well as in mixed-breed dogs in the sample.

Several findings might be interesting to dog owners:

• Mixed-breed dogs are as likely or more likely to carry some genetic variations linked to diseases than purebred dogs.
• But purebred dogs are more than twice as likely to actually have a genetic disease.
  That makes sense; the number of purebred dogs who are actually bred is quite small, and many are bred to dogs from the “same lines” — relatives, even very close relatives. Within a closely related population, the likelihood of dogs sharing a recessive gene is much higher than in the broad population of mixed-breed dogs.
• Deeper study of individual mixed-breed dogs who carried rare genetic variations found that, even when the dogs exhibited symptoms of a genetic disease, these individual dogs were only diagnosed after the owners had the results of the genotyping.
  That also (sadly) makes sense: The vets didn’t suspect that the mixed-breed dogs had rare, usually breed-linked, genetic diseases.

The question of “hybrid vigor” is nuanced, since the dogs are likely to be carriers but less likely to suffer the diseases, but I’d argue that mixed-breeds are healthier.

In his post, Coren also points out that another common belief — that breeders frequently breed dogs known to be carriers or even sufferers of a disease — is unfounded. Many breed-specific genetic diseases have become extremely rare or have been eradicated — due to careful breeding. Some breed clubs forbid breeding of carriers of known genetic diseases.

The researchers are sharing their data; they’ve created the free My Breed Data database, where anyone can search for information on genetically linked diseases.