Tag Archives: line-breeding

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.)

Issues of genetic diversity are a concern to dog breeders, and this can be especially so for breeds with small populations. The concern is whether there is enough genetic variation within a breed’s gene pool to maintain health and vitality. Breeders should be concerned about genetic diversity, because there are examples where damage has been done to a breed due to breeding practices. Restriction of genetic diversity can also occur in large population breeds.

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.

There are two factors that must be considered when evaluating genetic diversity and health issues in a breed; the average level of inbreeding, and detrimental recessive genes. With a small population, there is a tendency to find higher average inbreeding coefficients due to the relatedness between dogs from common ancestors. There is, however, no specific level or percentage of inbreeding that causes impaired health or vigor. The problems that inbreeding depression cause in purebred populations stem from the effects of deleterious recessive genes. If the founding population of a breed produces a high frequency of a deleterious recessive gene, then the breed will have issues with that disorder. This can be seen as smaller litter size, increased neonatal death, high frequency genetic disease, or impaired immunity. If these issues are present then the breed needs to seriously consider limited genetic diversity.

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.

The process of maintaining healthy lines, with many breeders crossing between lines and breeding back as they see fit, maintains diversity in the gene pool. If some breeders outbreed, and some linebreed to certain dogs that they favor while others linebreed to other dogs that they favor, then breedwide genetic diversity is maintained. It is the varied opinion of breeders as to what constitutes the ideal dog, and their selection of breeeding stock based on their opinions, that maintains breed diversity.

The most important factor for diminished genetic diversity in dog breeds is the popular sire syndrome. The overuse of a popular sire beyond a reasonable contribution through frequent breedings significantly skews the gene pool in this direction, and reduces the diversity of the gene pool. Any genes that he possesses – whether positive or negative – will increase in frequency. Through this founder’s effect, breed related genetic disease can occur. Another insidious effect of the popular sire syndrome is the loss of genetic contribution from quality, unrelated males who are not used for breeding. There is a finite number of quality bitches bred each year. If one male is used in an inordinate amount of matings, there will be fewer females left for these quality males that should be contributing to the gene pool. The popular sire syndrome is a significant factor in both populous breeds and breeds with small populations.

The best methods for ensuring the health and diversity of a breed’s gene pool are to:

  1. Avoid the popular sire syndrome.
  2. Utilize quality dogs from the breadth of your population to expand the gene pool.
  3. Monitor genetic health issues through regular health surveys.
  4. Do genetic testing for breed-related disorders.
  5. Participate in open health registries, such as CHIC (www.caninehealthinfo.org) to manage genetic 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.



Food for thought… why can’t I get any puppies on the ground?

Photo by Silvia Timmermann

I was talking with another breeder this week about the “state” of Gordon Setter litters, the quantity of litters being produced as well as size of the litters surviving. The conversation centered around the observation that overall, the Gordon Setter breed appears to be experiencing a decrease in fertility as well as in the viability of newborn offspring. Fewer breedings are taking, fewer puppies are surviving. As breeders I believe we all (yes, I include myself) need to hold ourselves responsible for understanding what may be causing the creation of fertility issues and for the creation of their solution. It’s been said many times by numerous authors before me, when choosing a breeding pair we must consider the fertility and nurturing qualities of the sire and dam in addition to every other trait we consider desirable. To do that, breeders need to understand pedigrees and how they speak to us about the inbreeding coefficient of the litter –  as that coefficient my friends, can drive what is known as “inbreeding depression.” To eradicate inbreeding depression we first need to learn how to identify it and when it appears we need to acknowledge that this could be at play and then we need to plan our breeding accordingly.

I’m not a geneticist and I won’t ever pretend to be, luckily though I am a voracious reader, and with all of the information at our fingertips on the internet today I can find a wealth of expert information for my own, as well as your reading pleasure. I urge you to explore with me, if you’ve not already done so.

Photo by Silvia Timmerman

First let’s understand what’s meant by inbreeding. Inbreeding would be the breeding of related dogs who may be closely or distantly related to each other. Some, including me, designate inbreeding as close relationship breeding, like mother-son, father-daughter, sister-brother, and call “less close” breeding like a nephew-aunt breeding a line-breeding. But when reading articles written by the genetic expert you’ll find that it’s all packaged together under the name inbreeding. Don’t let that confuse you. With that said, if inbreeding is used carefully as part of a breeding plan that includes balancing the benefits with the dangers it can be a powerful tool. Using inbreeding without consideration of both positive and negative effects can be destructive.

Now let’s go back to inbreeding depression and talk about how that is related to the opening subject of decreased fertility in Gordon Setters and viability of newborn offspring. Inbreeding depression is not about an increase in the number of genetic disorders in the breed, like PRA for example. It refers to a loss of what a biologist would tell you is called fitness. Fitness in this context refers to the dog’s ability to pass on its genes to the next generation. So to a biologist, if a dog dies from disease before it can reproduce it has a fitness of zero. If a bitch successfully reproduces a litter but won’t properly care for her offspring (which will die without intervention) she has a fitness of zero. To breeders like us then, if our Gordon Setters have a high level of fitness they will produce offspring that can go on to reproduce themselves and perpetuate their genes in the population, where a Gordon Setter that cannot reproduce (without human intervention) or reproduces less effectively (below average litter size for example) have a low or zero fitness. In the Gordon Setter breed we could be experiencing evidence of inbreeding depression if we are in fact seeing an decrease in fertility and a decrease in litter viability. Evidence of inbreeding depression can be indicated by singleton litters, decline in conception rates, reduced sperm count, reduced litter size, lower pre and post-natal survival rates, shorter lifespans, higher cancer rates in young dogs, allergies, and many other issues that we might be taking for granted as normal in dog breeding these days. The “fitness” of the dog should be kept in mind as we make breeding choices.

My next step is to send you to some very informative articles I found at The Institute of Canine Biology (as I said earlier I am not a geneticist and won’t pretend that I can write in-depth articles about this topic). What a wonderful resource site!

Photo by Silvia Timmermann

(Quick note – the photos included in this article are simply for your viewing pleasure, they have nothing to do with the content of the article).

Now, “don’t go throwing the baby out with the bathwater” my friends – that’s not the message here. Oh,and don’t forget to pass it on! Your comments, questions and suggestions are always encouraged in our comment section.

Sally Gift, Mesa, AZ