How a breed is formed: the Gene Pool
Let's imagine that these 11 dogs are the “founders” of your breed, the first dogs registered in the studbook (studbook).
All the individuals that come later will be descendants of these dogs only. In this case, we have a breed with a closed gene pool.
All the genetic variability that can ever exist in this new breed is already present in these founder dogs.
Mutations are unlikely to add useful new genetic variation, because most mutations are harmful. If a mutation is dominant and harmful, it will probably be eliminated quickly by selection.
If the mutation is recessive, it will not be expressed unless the animal is homozygous for that allele, that is, it receives one copy of the mutated allele from each parent.
In the heterozygousstate, a mutated recessive allele can stay “hidden” in the genome for many generations without causing any problem.
Therefore, unless new “founders” are later introduced into the population, all the genes you will be able to work with in your breeding program are already present in these founder dogs.
Each of these dogs carries at least some (and perhaps many) recessive genes capable of causing genetic diseases.
However, these diseases will only be expressed when a dog is homozygous for the disease-associated allele: it must receive two copies, one from each parent.
As long as these disease-related genes remain rare in the population, few animals will have the disease.
CAN THE GENE POOL GET BIGGER?
No!
Let's start with the 11 founder dogs and let them breed.
To keep it simple, let's imagine that they produce only genetically identical copies of themselves: clones.
Now we have 27 dogs, all exact copies of one of the founders.
What happened to the size of the gene pool?
Nothing.
Now we have more dogs, and more copies of the genes found in the dogs that had the most offspring, like that busy gray dog with the red tongue.
So the frequency of certain alleles is different in this population compared to the founding population.
However, the number of different alleles in the population remains exactly the same.
In this example, we are ignoring the possibility of new mutations.
What if the dogs bred normally instead of producing clones?
In sexual reproduction, each puppy receives one set of genes from each parent.
Each puppy receives a different combination of its parents' genes, so each individual has its own genetic makeup.
In addition, each parent dog may produce a different number of offspring and may be mated several times with different partners.
As a result, the frequency of the different alleles can be very different in the new population compared to the founding population.
But, again, even though there are now more dogs in the population:
The gene pool does not get bigger.
In fact, it does not matter how large this breed becomes.
It could one day reach thousands of dogs.
As long as the studbook remains closed, however:
the gene pool will never be larger than it was when the breed was formed.
CAN THE GENE POOL GET SMALLER?
Yes!
The gene pool of a closed breed cannot get bigger, but it can get smaller.
Imagine, for example, that dogs with black ears had lower fertility, higher puppy mortality or some other biological problem.
In that case, the frequency of the genes responsible for black ears would be reduced by natural selection.
Dogs with black ears would contribute fewer copies of their genes to the next generation.
Over time, through genetic drift (chance) or natural selection, the genes in these dogs could become rarer and rarer and could eventually be completely eliminated from the population.
WHAT IF BREEDERS DON'T LIKE BLACK EARS?
The same process can happen through artificial selection.
If breeders did not like black ears and all puppies with this trait were neutered or spayed and placed only in pet homes, those individuals would no longer contribute to breeding.
As a result, the alleles responsible for the trait would become less frequent in the population and could eventually be eliminated altogether.
In this case, the loss of genetic diversity would be a consequence of artificial selection carried out by the breeder.
WHEN DOES THE GENE POOL GET SMALLER?
The gene pool gets smaller when certain genes are completely eliminated from the population.
A lost gene is unlikely to be restored by a chance mutation.
The other way to recover that gene would be to bring into the breeding population an animal that carried that allele and could breed successfully.
Therefore, once a given allele is lost from a closed population, recovering it may require the introduction of outside genetic material.
DOG BREEDS WITH A CLOSED STUDBOOK
In purebred dogs, when the studbook is closed, new alleles cannot be introduced into the breed.
As a result:
the loss of an allele is permanent
and that loss reduces the heterozygosity of the genome at that gene.
THE FUNDAMENTAL PRINCIPLE
The concept presented in the text can be summed up simply:
A closed gene pool can change in size in terms of the number of individuals, but it cannot increase its genetic diversity just because the population has grown.
A breed can go from 11 founders to hundreds or thousands of dogs, but these animals still carry combinations of the genes that were already present in the founders.
On the other hand, alleles can be lost over the generations through natural selection, artificial selection or genetic drift.
That is why the number of dogs is not the same thing as the amount of genetic diversity.
Bigger population ≠ bigger gene pool.
Original source · Institute of Canine Biology: “What’s in the Gene Pool?”
