Hybrid Vigor is a very useful tool to create healthy individual dogs, to create a healthy breed it has to be combine with serious health and performance based selection. Hybrid vigor can hide negative traits but only testing and selective breeding can eliminate them from a bloodline.
In order understand how hybrid vigor works you first need to understand a little about genetics. A gene is positioned on something called its locus. All genes have two copies – one inherited from the mother and the other from the father. Different combinations of genes produce different results, and some genes are dominant over others. Some loci are dominant over others, and sometimes genes sitting on different loci are needed for certain genes to show themselves.
The next thing you need to know is all “purebred” dogs are inbred to make them ‘breed true’, or produce offspring that look just like their parents- that’s how breeds are created. In an effort to guarantee pointless traits like the angle of a dog’s forehead relative to the angle of its muzzle, or a solid white coat, a dog would be selected who carried these traits and inbred to its own children and siblings to create a population which also carries these traits. Unfortunately, this inbreeding allows potentially negative recessive traits to surface by removing potentially positive dominant traits from the bloodline.
All pure/inbred breeds carry negative recessive traits. That’s really the key to why hybrid vigor works, these negative recessive traits are recessive.
Negative dominant traits show themselves in any dog who has a copy of that gene, and so are easy to bred against. People just don’t breed sick or deformed dogs. Recessive genes “hide” behind dominant ones. So you either need two copies of a recessive gene or it and an even more recessive gene for these negative traits to surface.
For example: Canine Dwarfism is a negative recessive trait. If both the sire and the dam are carriers, that means they each have a copy of the dwarf growth gene and a copy of the normal growth gene on the same locus. Since the dwarf gene is recessive and the normal gene dominant, both dogs are normal size. There is a 50% chance that either dog will pass one gene or the other on any locus. That means in a litter, on average:
25% will get the normal gene from the sire and the dwarf gene from the dam making them carriers as well
25% will get the normal gene from the dam and the dwarf gene from the sire making them carriers as well
25% will get the dwarf gene from both the dam and the sire making them dwarfs
25% will get the normal gene from both the dam and the sire making them healthy
That means 75% of this litter is unbreedable, and 25% probably won’t live very long. The worst part is, with conditions we don’t have genetic tests for, there is no way to tell the difference between the normal dogs and the carriers. So these problems just get passed on from generation to generation.
Now on to hybrid vigor. Hybrid vigor is another name for the Heterosis effect. Heterosis just means that two genes sitting on a locus are different from one another. If a dog has the black and the brindle gene on its K locus, it shows heterosis. That dog will be black because black is dominant, and will carry for brindle. Some of this dog’s offspring will be black, and some will be brindle. This is not what show breeders want. They want a “good” stud to produce dogs just like himself. In an effort to get a dog who will only make black dogs they resort to inbreeding to get dogs who have two copies of the black gene sitting on the K locus. The problem is while they are destroying genetic diversity in the coat color the same thing is happening on other loci, allowing negative recessive traits to show themselves. This leaves the poor show breeder wondering why the healthy dogs he started with are starting to produce crippled, deformed, sickly, dying dogs.
However, when you cross two different breeds with different problems, it pushes the negative recessive traits back under the dominant genes from the other breed. This is hybrid vigor. This means negative genes for allergies, joint problems, cancer, heart problems, eye problems, etc just “vanish”. This creates a dog with less problems in their health and temperament. Hybrid dogs live longer, go to the vet less, make better mothers, produce more milk, and are more flexible in their training. For more information you can see this study on the effects of inbreeding both positive and negative. http://cal.vet.upenn.edu/index.php?page=effects-of-inbreeding
So don’t listen to the show breeders when they try to propagate hatred for “mutts”, and push the greatness of their inbred dogs.
Having owned and worked with hybrid dogs, I will never own another “purebred” dog unless I intend to cross breed it. All I hear when someone says their dog is a purebred with papers from champion lines, is that their dog is the inbred offspring of beauty pageant winners, and they have proof.
For more information collected by the BBC please see Pedigree Dogs Exposed.