For many years, there has been a theory that crossbred dogs were healthier than purebreds. This “hybrid vigor” was thought to be due to the presence of a wider variety of genes in a mixed breed dog, which reduced the likelihood of genetic disease. People selectively bred crossbreed pups to take advantage of this, and many people specifically chose a “mutt” because they felt they were less likely to get sick.
However, a study conducted by the University of California, Davis, veterinary school suggested that this is not true and in fact, a mixed breed dog may be just as likely to suffer from genetic health conditions as a pure breed. That makes sense because the genes a pup is born with are completely random – they are just as likely to get undesirable genes from their parents as they are to get the best from them.
The study evaluated the medical records of 90,000 dogs that were examined at the University veterinary teaching hospital. Of these, over 27,000 dogs were diagnosed with one or more of 24 genetic diseases including orthopedic conditions, eye conditions, heart disease, bloat and allergies. The 24 conditions were specifically chosen for three reasons. First, they were common in the dog population. Second, they caused sufficient problems to make the dog's people seek veterinary care for them. Third, they could be accurately diagnosed. The prevalence of 13 of these 24 medical conditions was the same in crossbred dogs as in purebreds.
Keep in mind that there are often limitations to such studies and the results can be skewed for a number of reasons. This study looked at dogs that were presented to that one single vet school in one country. There is no way to say for sure that these dogs are typical of the entire dog population of the United States or indeed the world. There are lifestyle factors that also affect how a disease develops, and not all pet parents have the budget to have them treated at a university vet hospital.
Breeders of purebred dogs do genetic health testing on their animals before symptoms develop and that can influence the incidence of a disease. Elbow dysplasia is a good example. Grade 1 dysplasia does not cause obvious symptoms but will show up on a pre-breeding x-ray. This can result in a dog being diagnosed with the condition even though they are not limping at all. In comparison, mixed breed dogs will only be checked for elbow dysplasia when they are clinically lame. This can result in more purebreds than mixed breed dogs being diagnosed with this particular condition.
Not all of these diseases are due to genes. Some of these affected dogs will develop disease just because they were unlucky and not because of their genetic makeup. This study did not differentiate between the causes of the disease.
The message to take home from this study is that hybrid vigor does not necessarily exist. It does not matter what breed of ancestry the bad combination of genes come from, so that cute mixed-breed puppy might be just as likely to develop a genetic disease as a purebred dog with a long pedigree.