You’ve heard it, as all pet owners have: “Mutts are healthier than purebreds.” But is that true? As it turns out, things aren’t really that simple. While in some instances mixed-breed dogs may have a better chance of good health, in other cases, that’s not so. As with any individual’s health– human, dog, or any other animal– it often all comes down to good preventive care and the luck of the draw.
“Hybrid Vigor” is a term often used to explain the supposed superior health of mixed-breed dogs. This is another term for heterosis. Heterosis describes a superior result achieved by crossbreeding, or outbreeding, significantly different plants or animals. If offspring display characteristics superior to both parents, heterosity has occurred.
According to one hypothesis (and the one which I think makes the most sense), heterosis occurs because of heterozygosity in crossbred offspring, which prevents pairs of damaging recessive alleles from matching up to cause defects and health problems.
Let’s imagine a hypothetical pairing of two unrelated dogs of vastly different breeds: A Husky and a Labrador Retriever. Let’s say the Labrador Retriever carries two copies of a recessive gene that predisposes dogs to tumors. In reality, cancer inheritance is much more complex than a single gene, but for the sake of argument, our Lab has the combination of alleles I’ll represent as tu/tu, and will develop a tumor later in life. The Husky, on the other hand, carries two copies of a recessive gene for zinc deficiency, or zd/zd. The Husky will not manufacture zinc properly and will need a zinc supplement for life.
Now, if these two unhealthy dogs were bred together, provided that neither carried the damaging recessives carried by the other dog, all the puppies would be heterozygous for both defects, or Tu/tu and Zd/zd, meaning neither tumors nor zinc deficiency would be expressed. With a bit of luck and good care, the puppies would likely display heterosity by remaining healthier than either parent.
Not all outbreeding creates heterosity. In fact, in some cases, the opposite can occur. When offspring resulting from crossbreeding are inferior to their parents, outbreeding depression has resulted from the pairing. You may be familiar with a similar term, inbreeding depression, which describes the undesirable results of breeding between closely related individuals. Close inbreeding is more damaging in general than outbreeding or crossbreeding, but both types of breeding can have unpleasant and unwanted results.
Outbreeding depression can happen when a crossbreeding causes the loss of desirable recessive traits, or if two individuals who are very physically different are paired and resulting offspring receive incompatible traits from each parent. For example, if a Basset Hound were to be bred to a Mastiff, the resulting puppies could end up with traits predisposing them to serious health problems. If the Basset passed on its long back but the Mastiff passed on long legs and a deep barrel chest, the resulting puppies would be likely to suffer from slipped disks and other serious back problems.
Another factor working against the claim that mutts are healthier is a lack of record-keeping. Most mutts don’t come from bloodlines where careful track is kept of the health of every dog in the line. Purebreds may have more known health problems in their background, but sometimes this is only because better records have been kept.
No dog is guaranteed to be healthy for life. The most conscientious breeder could produce a dog that lives a short and unhappy life due to congenital or hereditary defects, and a shelter dog with no known family history might live 20 healthy years. However, either mutts or purebred dogs can be healthy, happy pets who live long lives. Here are a few tips for maximizing your chances of owning a healthy dog: