Tag Archives: genetic disease

Small Population Breeds and Issues of Genetic Diversity

bell-jerold-1520260577 By Jerold s Bell DVM, Clinical Associate Professor of Genetics, Tufts Cummings School of Veterinary Medicine.  Reprinted by permission of the author.

(This article was originally published in the March 2007 AKC Perspectives Delegates Newsletter.)


All genes come in pairs: one from the sire and one from the dam. Each gene in the pair is called an allele. If both alleles in a pair are of the same type, the gene pair is homozygous. If the two alleles are different, the gene pair is heterozygous. While each dog can have a maximum of two different alleles at a gene pair, many different alleles are potentially available to be part of the gene pair. The greater the number of alleles that are available at each gene pair (called genetic polymorphism), the greater the genetic diversity of the breed.

If there is no breed diversity in a gene pair, but the particular homozygous gene that is present is not detrimental, there is no negative effect on breed health. The characteristics that make a breed reproduce true to its standard are, in fact, based on non-variable (that is, homozygous) gene pairs.

The origins of the breeds have a lot to do with genetic diversity. A breed established with a working phenotype tends to have diverse founder origins, and significant diversity. Even with substantial population bottlenecks, the breed can maintain considerable amounts of genetic diversity. This was shown in a molecular genetic study of the Chinook breed, which was reduced to 11 modern founders in 1981. Breeds established by inbreeding on a limited number of related founder individuals could have a reduced diversity. Many breeds have also gone through diversity reducing bottlenecks; such as occurred during World War II. For most of these breeds, their gene pools have expanded through breeding for many generations, resulting in a stable population of healthy dogs.


The issue of high average inbreeding coefficients is one that all breeds go through during their foundation. As the population increases and the average relatedness of dogs goes down (based on a fixed number of generations), the average inbreeding coefficient for the breed will go down. The effect of initially higher inbreeding coefficients in small population breeds will depend on the presence of deleterious recessive genes that will be expressed when homozygous.

Some breeders discourage linebreeding and promote outbreeding in an attempt to protect genetic diversity in their breed. It is not the type of matings utilized (linebreeding or outbreeding) that causes the loss of genes from a breed gene pool. Rather, loss of genes occurs through selection: the use and non-use of offspring. If a breed starts limiting their focus to breeding stock from a limited number of lines, then a loss of genetic diversity will occur.




  1. Avoid the popular sire syndrome.
  3. Monitor genetic health issues through regular health surveys.
  4. Do genetic testing for breed-related disorders.


(This article can be reprinted with the written permission from the author: jerold.bell@tufts.edu)

Related article – Outcrossing Does Not Equal Gene Pool Diversity

Photograph courtesy of Susan Roy Nelson is not intended to illustrate any point in the article, it is presented for your viewing pleasure only.