Does Hybrid Vigor Exist in Doodles?

 Does Hybrid Vigor Exist in Labradoodles and
Goldendoodles?

Good question.  Before addressing it I’d like to address the somewhat smug assertion by some that hybrid vigor cannot possibly exist in Labradoodles, Goldendoodles,  and North American Retrievers (NAR) because Labradoodles, NAR, and Goldendoodles are not hybrids. The premise for this assertion is a very restricted definition of the term hybrid. This is akin to claiming that the opening in the front of your trousers cannot possibly be called a fly, because the first definition in the dictionary says a fly is a winged insect.  Yes, one definition of a hybrid is a cross between species, and another definition is a cross between populations.  It is this secondary definition we are using when discussing hybridization of dogs, or crossing of different breeds.

Let’s begin the discussion of hybrid vigor by talking about a couple basic genetics principles.  Hang in there!  I’ll keep it short and simple.

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.  

This is actually a helpful way to think of recessive traits: as hidden and very sneaky.  A dog can carry such a trait and show no signs of it at all.  But if bred to a dog of similar genetic composition, that is carrying that hidden gene, some of the pups will end up with 2 copies of that hidden gene.  With 2 copies of the hidden, defective trait gene, and NO normal copies, the dog will have a genetic disorder that was not evident in the parent.

A recessive trait, as you just learned, needs 2 copies in order for the trait to show up. If a dog carrying one recessive (hidden) allele for
some genetic disorder is bred to a dog also carrying one recessive (hidden) 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 disorder.  The defective, recessive alleles could be contributed from any choice of parents, resulting in pups that have multiple copies of the defective alleles, and therefore show the disorder.

There are genetically-based disorders that are NOT found in all those breeds.  An eye disorder called prcd-PRA is found in Mini and toy poodles, and Labradors, but is very, very rare in Standard poodles and Golden Retrievers.  Similarly, Lymphoma is found commonly in Goldens and Poodles but not in Labs. Osteosarcoma is more common in Labs, but not common in Goldens and Poodles.  Narcolepsy and  von Willebrand’s Disease is found in Labs and Poodles, but not in Goldens.  So hybrid vigor will occur for some disorders and not others, depending on the type of doodle.  It is easy to see how we can avoid concentrating the genes for certain disorders by crossing a breed that carries a disorder with one that does not.

So hybrid vigor occurs for specific disorders, and it can also occur in terms of general disease resistance.  There is a set of genes called the major histocompatibility complex (MHC) consisting of thousands of potential allelic combinations. The genes of the MHC are involved in controlling disease resistance, immune function, and reproduction.  The long term viability of any breed depends on maintaining a high degree of genetic diversity in the MHC. The loss of MHC genetic diversity is responsible for a portion of the reduced “hybrid vigor” in some breeds.  These breeds are literally dying due to a lack of diversity in this complex, and researchers are working on the development of canine MHC genetic markers, so breeders can select and match these genes to maximize health.

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 saved the Dalmatian from extinction (a breed of deaf dogs was undesirable, no matter what other characteristics they had!) by allowing breeding to a Pointer, although they later rescinded the registration of the progeny.  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 does not 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.

Author: Helene Roussi