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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.

 

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Outcrossing Does Not Equal Gene Pool Diversity

In previous articles we’ve talked about the shrinking population of the purebred dog and specifically about how much smaller the Gordon Setter population is today – over 70% fewer Gordons than twenty years ago. The current bottleneck in the number of Gordon Setters available for breeding calls for us, as responsible breeders, to evaluate each mating more carefully to determine if it will accomplish our own goals while also considering the impact our mating will have on the breed gene pool. As breeders in today’s world we are not only charged with improving the breed, we are also called upon to ensure that our breeding activity has a positive impact on the preservation of the breed gene pool.  The good news my friends, is that all of this can be less painful to accomplish than you might have thought.

For topics like this I call on experts for advice, and I am grateful to Jerold S. Bell DVM, Clinical Associate Professor of Genetics, Tufts Cummings School of Veterinary Medicine for the guidance he’s offering. Jerry’s article Small Population Breeds & Issues of Genetic Diversity is the resource used for this article and is quoted here, and reprinted entirely elsewhere on the blog with his permission. (Click the title above to link to that article.)

Has the Gordon Setter population reached a level where we should consider it a “small population breed”? Perhaps not, yet..who’s to say? The point here is that the population of the Gordon Setter has shrunk dramatically (Honey, I Shrunk the Kids!) and as it is now substantially smaller, breeders must be aware of how important our breeding choices become when viewed in terms of the health of our breed gene pool. Just as there are fewer Gordon Setters, so too are there far fewer breeders bearing the responsibility for their preservation. With fewer breeders we find that many of the older lines are harder if not impossible to find today.

Jerold S. Bell DVM –  *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.

Putting a lesson in genetics aside for another time, let’s talk today about genetic diversity in our breed gene pool.  Quoting Jerold S. Bell DVM  * 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. 

In this statement then, as a group of dedicated breeders, we find a key to issues or symptoms, whose frequency of expression within the breed need monitoring. An increase or spike in these symptoms throughout the breed population, that goes beyond normal expectations, should be a cue that breeders need to seriously consider if we are experiencing limited genetic diversity in our breed gene pool. As a group we must be willing to share our breeding experiences with a wide audience of our peers. Additionally, we must understand that GSCA Health and Genetics committee surveys are also a vital indicator of the breed health, especially as it pertains to breed gene pool diversity.

As we talk about gene pool diversity, we may find some breeders who discourage linebreeding and promote outcrossing (outbreeding) as the way to protect genetic diversity in the breed. While this does sound like an easy, and maybe even an obvious answer, outbreeding would not provide the complete solution.

Jerold S. Bell DVM – *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.

I believe as a whole, that Gordon Setter stud dog owners have worked hard to manage stud dogs properly to avoid the “popular sire syndrome”. This is not an easy task to manage as so many variables, including emotions come into play. Hats off to all who have kept a diligent and watchful eye on our breed through proper stud dog management.

As I look back at what Dr. Bell has written, I realize that our breed is fortunate to have had many breeders, both past and present, who have contributed much to preserve the Gordon Setter; sometimes they contributed matings that improved specific aspects of the breed and sometimes they contributed by using breeding practices that preserved genetic diversity. Moving forward, our breed needs us to continue to attract and mentor a diverse group of breeders who also possess an understanding of the principles of gene pool diversity. As we have seen, there is simply not one step or one action to preserve diversity, instead there is a collection of various actions, that when understood and followed by the individual breeder, with each breeder working alongside the many other breeders – it is when we work as a collective group that we accomplish that one common goal – preservation of the purebred Gordon Setter…oh, and don’t forget there is still improvement of the breed to consider!

Jerold S. Bell DVM writes:  *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. (as new genes cannot be added to a closed registry this refers to preserving genes that might otherwise be lost by selection of only a few sires out of the many available)
  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.

Small Population Breeds & Issues of Genetic Diversity by Jerold S. Bell DVM

Photograph by Susan Roy Nelson shared for your viewing pleasure, is not intended to illustrate any point in the article.

 

 

 

 

Natural or AI & Pre-breeding requirements – Survey for Gordon Setter Breeders

Our readers have expressed an interest in learning more from each other 30050_1465798247214_7808514_nabout how their fellow Gordon Setter breeders manage their valuable stud dogs and brood bitches.

If you own a Gordon Setter stud dog or brood bitch and have been involved in breeding in the past 5 years please take two minutes to complete this survey regarding the use of AI or natural breeding and pre-breeding screenings. (It truly only takes a minute or two to answer these 10 short multiple choice questions.)

Click Here to be taken directly to the survey titled Natural or Artificial Insemination – Breeding survey Gordon Setters.