THE VALUE OF POPULATION GENETICS TO THE BREEDER
Dr John Armstrong
©1997
As a breeder, you are a practicing geneticist. To breed effectively you need to know something about genetic principles. (Would you sit down to a bridge game expecting to win without any knowledge of the rules?)
What is often called "Mendelian genetics" deals with the outcome of specific crosses. Population genetics deals with the distribution of alleles in a population and the effects of mutation, selection, inbreeding, etc. on this distribution. A knowledge of both is critical not only to your own success, but also to the survival of your breed.
Once-upon-a-time, many geneticists believed that there were only two alternatives for a gene - "good" alleles that functioned normally and "bad" alleles that didn't. If things were this simple, then the task of the geneticist-breeder would be simplified to one of identifying the bad alleles and trying to eliminate them from the population. Such a simplistic model could be modified to allow for different "good" alleles, but it should not matter whether you have one or another. These early geneticists expected to find little genetic variability in a population. The majority of individuals were expected to be homozygous for the good allele for most genes.
With the advent of modern biochemical and molecular tools, geneticists studying populations found far more variability (diversity) than they had expected. There are a number of possible reasons for this, and even the experts are not in total agreement on the most likely reason(s). However, geneticists have also discovered that populations lacking genetic diversity often have significant problems and are at greater risk from disease and other changes in their environment. The conclusion is that genetic diversity is desirable for the health and long-term survival of a population.
Are purebreds dogs genetically diverse? Some may regard that as a contradiction in terms. The very concept of creating a breed with characteristics that are distinctly different from other breeds implies a certain limitation on diversity. Nevertheless, within the standards for a breed, diversity should still be possible for genes that do not affect the essential characteristics that distinguish one breed from another. If, in order to maintain breed identity, one has to compromise on genes that relate to general structural soundness, good health, intelligence and temperament, perhaps this breed should not exist. However, as long as these essentials are not compromised, I see no reason why one cannot have different breeds with different appearances and different talents. For those genes that establish breed identity, there will be markedly less variability within a breed than within Canis familiaris as a whole.
The tricky bit is restricting variability for those genes that make a breed distinctive without sacrificing the variability/diversity that is necessary for good health and long-term survival of the breed. In many cases, this has not been achieved, and we are now paying the price in terms of high incidence of specific genetic diseases and increased susceptibility to other diseases, reduced litter sizes, reduced lifespan, inability to conceive naturally, etc.
Why has this happened - or do we have to accept it as an inevitable consequence of creating a breed? I don't think we do.
The principal reasons for limited genetic diversity are:
Such a database would enable breeders to identify which individuals are most likely to carry the genes from each founder. In practical terms, measures might then be considered to rebalance the breed in order to ensure that the remaining diversity is more evenly distributed end that, therefore, there is less risk of loss.
At the level of the individual breeder, it would enable him/her to make intelligent, informed choices when selecting mates.
A Lesson from the PoodlesAs a poodle owner my interests naturally started with the poodle. With the assistance of several others, we are building a Standard Poodle database. The goal is to be able to trace any current SP back to the imports into North America (most of which were in the 1930s) and beyond that where possible. For a breed such as the poodle, which is not of modern origin, establishing all the original founders is impossible. However, as these imports came from a variety of countries, they would appear to represent reasonable diversity.
Though our work is far from complete, I am able to give a breeder a profile of their dog. Armed with this information, a breeder may then be able to avoid inadvertently breeding to a "cousin" (with the attendant risks of doubling up on undesirable recessive traits). There are no guarantees. Until DNA tests are developed for the most common problems, this is often the best we can offer.
You might think that the situation I have described would suggest that there is reasonable diversity in the SP population. This, unfortunately, is not the case. Over-use of popular males from the same line during the past forty years has come close to wiping out the other lines and the majority of the population owe far too much of their heritage to a small number of ancestors. The same potential exists for degrading the gene pool in any breed.
What is often called "Mendelian genetics" deals with the outcome of specific crosses. Population genetics deals with the distribution of alleles in a population and the effects of mutation, selection, inbreeding, etc. on this distribution. A knowledge of both is critical not only to your own success, but also to the survival of your breed.
Once-upon-a-time, many geneticists believed that there were only two alternatives for a gene - "good" alleles that functioned normally and "bad" alleles that didn't. If things were this simple, then the task of the geneticist-breeder would be simplified to one of identifying the bad alleles and trying to eliminate them from the population. Such a simplistic model could be modified to allow for different "good" alleles, but it should not matter whether you have one or another. These early geneticists expected to find little genetic variability in a population. The majority of individuals were expected to be homozygous for the good allele for most genes.
With the advent of modern biochemical and molecular tools, geneticists studying populations found far more variability (diversity) than they had expected. There are a number of possible reasons for this, and even the experts are not in total agreement on the most likely reason(s). However, geneticists have also discovered that populations lacking genetic diversity often have significant problems and are at greater risk from disease and other changes in their environment. The conclusion is that genetic diversity is desirable for the health and long-term survival of a population.
Are purebreds dogs genetically diverse? Some may regard that as a contradiction in terms. The very concept of creating a breed with characteristics that are distinctly different from other breeds implies a certain limitation on diversity. Nevertheless, within the standards for a breed, diversity should still be possible for genes that do not affect the essential characteristics that distinguish one breed from another. If, in order to maintain breed identity, one has to compromise on genes that relate to general structural soundness, good health, intelligence and temperament, perhaps this breed should not exist. However, as long as these essentials are not compromised, I see no reason why one cannot have different breeds with different appearances and different talents. For those genes that establish breed identity, there will be markedly less variability within a breed than within Canis familiaris as a whole.
The tricky bit is restricting variability for those genes that make a breed distinctive without sacrificing the variability/diversity that is necessary for good health and long-term survival of the breed. In many cases, this has not been achieved, and we are now paying the price in terms of high incidence of specific genetic diseases and increased susceptibility to other diseases, reduced litter sizes, reduced lifespan, inability to conceive naturally, etc.
Why has this happened - or do we have to accept it as an inevitable consequence of creating a breed? I don't think we do.
The principal reasons for limited genetic diversity are:
- Many breeds have been established with too few founders or ones that are already too closely related.
- The registries are closed for almost all breeds. Therefore you cannot introduce diversity from outside the existing population.
- Most selective breeding practices have the effect of reducing the diversity further. In addition, the wrong things are often selected for.
- Even if the founders were sufficiently diverse genetically, almost no one knows how their genetic contributions are distributed among the present day population. Consequently, breeding is done without regard to conserving these contributions which may be of value to the general health and survival of the breed.
Such a database would enable breeders to identify which individuals are most likely to carry the genes from each founder. In practical terms, measures might then be considered to rebalance the breed in order to ensure that the remaining diversity is more evenly distributed end that, therefore, there is less risk of loss.
At the level of the individual breeder, it would enable him/her to make intelligent, informed choices when selecting mates.
A Lesson from the PoodlesAs a poodle owner my interests naturally started with the poodle. With the assistance of several others, we are building a Standard Poodle database. The goal is to be able to trace any current SP back to the imports into North America (most of which were in the 1930s) and beyond that where possible. For a breed such as the poodle, which is not of modern origin, establishing all the original founders is impossible. However, as these imports came from a variety of countries, they would appear to represent reasonable diversity.
Though our work is far from complete, I am able to give a breeder a profile of their dog. Armed with this information, a breeder may then be able to avoid inadvertently breeding to a "cousin" (with the attendant risks of doubling up on undesirable recessive traits). There are no guarantees. Until DNA tests are developed for the most common problems, this is often the best we can offer.
You might think that the situation I have described would suggest that there is reasonable diversity in the SP population. This, unfortunately, is not the case. Over-use of popular males from the same line during the past forty years has come close to wiping out the other lines and the majority of the population owe far too much of their heritage to a small number of ancestors. The same potential exists for degrading the gene pool in any breed.
