Dog breeding is fraught with controversy: from puppy farmers to qualzucht (breeding animals that are likely to suffer due to poor conformation). For anyone wishing to know more about this topic, a basic understanding of genetics is essential, and this week’s guest post aims to help. Written by Lazhar Ichir from Breeding Business, here is a good grounder in canine genetics.
In order to understand the importance of keeping a large gene pool in a dog breed or in a bloodline, we must start by mastering a few genetic concepts.
First, a gene is a unit of heredity holding a piece of information that will be transmitted from a parent to its offsets. A gene is generally present as two copies in each animal; these are called alleles, they are variant forms of a given gene.
One or several genes will be used to define several features either as deterministic as in something that is, or is not (such as coat color), or in most situations, genes will define a dog’s characteristic on a continuum or as a virtual gradient (eye color, height, muscularity, etc).
Clear, Carrier and Affected
Two concepts that are extremely important in canine genetics are recessive and dominant traits. A trait (can be a specific eye color, medical condition, etc) may be dominant (the stronger trait) or recessive (the weaker trait).
In a litter, each puppy inherits two copies of a given gene, one from the father and one from the mother.
Some traits are recessive and require several copies of multiple genes to be identical while other traits are dominant and require only one copy to be expressed.
For example, in dogs, four out of the three factors that result in blue eyes are due to coat color alone, with no effect from “blue eye genes.” This includes some types of white and merle coat types. Also, black coats in dogs can be dominant, as in the Labrador retriever, or recessive, as in the German Shepherd.
So the definitions that we need are:
- Clear — the dog’s two copies of a given gene are cleared of the version we are screening for
- Carrier — the dog is carrying a copy of a gene’s version we are screening for
- Affected — the dog’s two copies of a given gene are affected by the version we are screening for
When it comes down to health issues, the ideal way to go is to breed clear dogs to make sure the bad version of the gene disappears from the living gene pool.
What Is a Gene Pool Then?
A gene pool is the comprehensive inventory of all sets of genes in a specific population. For dog breeders, the two populations which make most sense would be the breed and the bloodline.
A purebred breed is a closed gene pool because every member of that pool (ie. the breed) is descending from the same small group of founding Sires and Dams.
A purebred dog breed always originates from a few specimens selected for their look and aptitudes. These then got bred, and all dogs belonging to that breed even thirty years later descend from this very small group of dogs.
Image credits: Todd Segar — https://www.tes.com/lessons/nOlWvhEBaSZPNA/gene-pool
The Main Issue With Small and Limited Gene Pools
Some of these dogs who originated a breed were carrying several genes that can potentially cause genetic disorders when bred to another dog carrying the same copy. It can take several generations for such conditions to appear; but they eventually will.
Once this happens, and if breeders aren’t screening their dogs, you end up with affected dogs breeding other affected dogs and thus propagating the medical conditions. It’s easy to see when it results in e.g. obvious blindness at birth; it is then common sense not to breed such dogs.
It’s not the ‘affected’ dogs that contribute to the problem, since those are easy to identify and weed out; it is the carriers, which are unaffected, but still pass the deleterious genes on that contribute to the diseases proliferating.
And most of the time, health problems in dogs are on a continuum, making it harder for them to be noticed.
Because the gene pool is limited and closed to the specimens from other breeds, it is difficult to get out of a crisis. Breathing issues in French Bulldogs could be fixed promptly if we were allowed external blood but because we are limited to other French Bulldogs (with similar issues), progress takes a lot of time.
In fact, entire breeds have been altered and improved by “outcrossing” and “backcrossing” to other breeds. For example, the Dalmatian was outcrossed to the Pointer to eliminate high uric acid, and the Boxer was bred to have a naturally bobbed tail by being outcrossed to Pembroke Corgis.
Other Consequences of Closed Gene Pools
The loss of genetic diversity in a purebred breed or bloodline from a limited studbook is obvious over time. Genetic impoverishment is also a huge cause of cancer and weak immune systems because what makes a species strong is its variety — survival of the fittest! But in a closed gene pool, you are facing an inevitable sad ending. And it’s what we have been witnessing in recent years in several purebred breeds.
Inbreeding is the mating of two closely related dogs (father to daughter, brother to sister, etc) and it has been shown that the closer the two partners are the worse the consequences are for the resulting puppies: smaller puppy count in the litter, higher puppy mortality, handicaps, etc.
Inbreeding is used to fix “desirable” traits in dogs since the two parents are alike, but it also fixes a bunch of bad traits and medical conditions along the way.
Image credits: Breeding Business — https://breedingbusiness.com/definition-meaning-inbreeding-dogs/
There is no miracle when we talk about canine genetics: either the gene pool opens up to welcome more genetic diversity and regain overall good health; or the selective breeding overseen by dog breeders has to be extremely strict and comprehensive.