Why you need to understand population genetics: the “quick, essential explanation”
By Carol Beuchat, PhD
1) All the useful genetic variation your breed will ever have available was present in the dogs that founded the breed. This genetic diversity is finite.
2) In every generation, alleles can be lost by chance (this is called “genetic drift”) and also through the artificial selection carried out by breeders, who select dogs with the traits they like and remove other dogs from the breeding population.
3) Because the studbook is closed, the genes that are lost cannot be replaced.
4) Therefore, from the moment a breed is founded and the studbook is closed, the loss of genetic diversity over time is inevitable and continuous.
5) You cannot remove just a single gene from a population. You have to remove a whole dog and all the genes it carries.
6) You cannot select for or against a single gene, because genes tend to be passed on in groups with other genes (this is called “genetic linkage” or linkage). If you select for a gene (or against it), you also select for (or against) all the genes associated with it.
7) Selecting for homozygosity in a few traits promotes homozygosity in all traits. Homozygosity is the kiss of death for the immune system. And as genetic variability decreases, so does the breeder's ability to improve a breed through selection, because selection depends on variability.
8) The consequences of inbreeding (in all animals) are insidious, but obvious once you notice them: reduced fertility, difficult births, smaller litters, higher puppy mortality, puppies that fail to thrive, shorter life expectancy, etc. Genetically healthy dogs should get pregnant when mated. They should produce large litters of robust puppies with low mortality. Animals that cannot produce viable offspring are eliminated by natural selection.
9) Mutations in dominant genes are removed from the population when they reduce fitness. Recessive mutations have no effect unless they are homozygous. As a result, rare mutations are not removed; they are inherited from one generation to the next, and every animal carries many of them.
10) If you produce a large number of puppies from your favorite stud dog, you are producing dozens of copies of every harmful mutation present in that dog (which had never been a problem before because they were recessive; see item 9) and spreading them through the population. Now a mutation that was once rare will become common, its frequency in the population will rise, and so will the chances of producing a homozygous puppy (with two copies of that harmful allele), and homozygous recessive alleles are no longer silent.
11) Therefore, genetic diseases caused by recessive alleles do not “suddenly appear” in a breed. The defective gene was probably there all along. Make a zillion copies of it and, suddenly, you have a disease.
12) Using DNA tests to try to remove disease genes from the breed will not make the dogs healthier (see items 2, 5 and 6).
13) The breed will keep losing genes in every generation (by chance or by selection) until the gene pool no longer has the genes needed to produce a healthy dog.
14) At that point, the breed may look wonderful (because of selection for type), but it will suffer the effects of genetic impoverishment: inbreeding depression, diseases caused by recessive alleles, increased cancer risk, etc.
15) The health of individual dogs cannot be improved without improving the genetic health of the breed. The only way to improve the genetic health of the breed is to manage the health of the breed's gene pool.
16) Population genetics provides tools for the genetic management of breeds or other groups of animals. Breeders CAN improve the health of the dogs they breed if they understand these tools and use them.
Copyright (c) 2013 Carol Beuchat
This document may be reproduced without permission, as long as it is accompanied by the copyright information.
Last revised: March 2015.
Original source · Institute of Canine Biology: “The Elevator Pitch”
