WHY A MIXED BREED?
  SHORT ANSWER: For the same reason you don't marry your cousin

LONG, BORING ANSWER FILLED WITH LOTS OF BIG WORDS:  Let’s begin the discussion of hybrid vigor by talking about a couple basic genetics principles. Hang in there!

 Sets of genes control every characteristic of an organism. Each parent contributes one gene, called an allele, so a set consists of 2 genes, or 2 paired alleles. A dominant allele will cause that trait to show up even if only one copy is present. An allele that is recessive needs 2 copies in order for the trait to show up. For example, in humans, brown eyes are dominant, and blue eyes are recessive. If a person has an allele for brown and an allele for blue, they will have brown eyes. The dominant brown overrides the recessive blue. For a person to have blue eyes they need to have TWO copies of the allele for blue eyes. Notice in this example that carrying the recessive trait does not influence the physical result. The recessive gene’s presence is completely hidden.

 A recessive trait, as you just learned, needs 2 copies in order for the trait to show up. If a dog carrying one recessive allele for some genetic disorder is bred to a dog also carrying one recessive allele for that disorder, some of the pups will get 2 copies and show the disorder. Those genes have become more ‘concentrated’ in the population. As one produces successive generations of a certain breed, trying to concentrate and ‘fix’ the traits that define the breed, other traits become concentrated and ‘fixed’ as well, those traits that cause genetic disorders. That’s why certain disorders are more common in some breeds than others. Nearly half of hereditary diseases found in dogs occur predominantly or exclusively in one or just a few breeds. If a dog carrying the recessive, defective allele is bred to a dog with ONLY normal versions of that allele, some pups will be carriers, but none will show the disorder. This is the basis of hybrid vigor.

 Hybrid vigor is the phrase commonly used for what is correctly called heterosis. That is, the possibility that one may obtain a better individual by combining the virtues of the parents, by preventing the concentration of undesirable traits within the group. Individuals that are members of a population share genes, that’s what makes them members of that population! In the case of dogs, these different populations are different breeds, and those genes define every characteristic that makes a dog a recognizable member of that breed. It takes differences in only 10 to 30 genes to define one breed from the next. So will crossing two breeds result in a healthier animal? Maybe, it depends on whether the two breeds have in common any genetic disorders, or defective alleles.

 Expanding on that thought, consider what happens if we cross 2 breeds who SHARE a genetic disorder. For example, Hip Dysplasia is a genetically based disorder controlled by a number of sets of genes, and is found in all foundation stock for doodles.... Poodles, Golden Retrievers, and Labrador Retrievers. So crossing these breeds would NOT result in hybrid vigor for that particular trait. The defective, recessive alleles could be contributed from any choice of parents, resulting in pups that have multiple copies of the allele, and therefore show the disorder.

In the last couple decades, several breeds have been in danger of being bred into oblivion due to the concentration of genes carrying genetic disorders. Breed clubs responded to this by doing what to some was absolutely heretical. They outcrossed with unrelated breeds. The AKC literally saved the Dalmatian from extinction (nobody wanted a breed of deaf dogs, regardless of other characteristics) by allowing breeding to non-Dalmatians. Similarly in Europe, Dutch Shepherd Dogs were outcrossed with the Belgian Tervuren, and Bernese Mountain Dogs were crossed with Newfoundlands. The choice was made to save these breeds by taking advantage of the phenomenon of heterosis...hybrid vigor.... to strengthen them.

Hybrid vigor has been clearly shown to exist in everything from fruit flies to orchids to pigs to humans. That’s why there are laws against people intermarrying, and why certain families that DID intermarry, like the Russian czars, find disorders like hemophilia among their members. It doesn’t matter what you are breeding, by maximizing the number of different alleles in the gene pool, you minimize the chances that disease-causing genes will end up paired together in any individual. Therein lies the promise of heterosis, or hybrid vigor.