Effects of selection on genetic health of purebred dogs
Natural selection directs the emergence of new species and the eradication of inherited traits that reduce the animal’s chances of survival. Artificial selection (the formation of dog breeds) is directed by people and their breeding decisions that are not always in line with the principles of natural selection (better survival). Purebred dogs are genetically isolated populations within a species with specific traits developed through artificial selection. From the perspective of genetic diseases artificial selection can reduce, maintain, or increase the frequency of disease genes. Most breeds originated from a limited number of animals and consequently limited genetic material. The effect of the formation of geographically separated subpopulations within the breed, the overuse of popular males, and line breeding further limit the heterogeneity of genetic material, leading to an increased risk of developing genetic diseases.
Unwanted genetic conditions or diseases are the result of planned or unplanned breeding errors. Excessive selection for certain body traits has led to conditions that cause breed specific health problems (brachiocephalic syndrome, excessive amount of skin). The transfer of disease genes can be random or linked to the selection of desired traits. An example of a genetic disease linked to the selection of a specific desired trait is hyperuricosuria in Dalmatians. A recessive disease mutation is responsible for abnormal urine metabolism. Due to the linked inheritance of this mutation with the desired dotted pattern of coat, it has become fixed in Dalmatians (present in all dogs). With the project of planned mating with a healthy pointer and further planned breeding, we already have quite a few registered Dalmatians with a characteristic spotted coat and without a disease mutation (1).
In the past, breeding plans were limited only to traits that were expressed in animals. In the recessive mode of inheritance, there is a problem of elimination of carrier animals that do not develop disease symptoms but pass the causative mutation on to their offspring. In such cases diseased animals can appear in each generation. Late onset diseases which develop clinical signs at an old age are also a problem, as these animals cannot be excluded from breeding in time. Progress in the field of genetics in recent years has enabled the development of genetic tests, which are of great help to today's breeder. Based on the result of a genetic test mating can be planned with the aim of disease eradication. Without direct selection against predispositions to genetic diseases, no improvement in breed health is expected.
A genetic test is an extremely powerful tool that can be very helpful in making informed breeding decisions. For its proper use, several factors need to be considered: the presence of a specific disease in a breed, frequency of occurrence, penetrance (proportion of animals with a mutation that develop the disease), mode of inheritance and type of mutation (loss of function / increased risk). It is necessary to be aware that the genetic test gives us information about a specific disease and not about the general health of the dog, therefore the result of a genetic test must be placed in a broader context when making breeding decisions (general health of the dog, behavioural characteristics, dog function).
The fact is that advances in genetics and the development of various genetic tests have a major impact on modern dog breeding. If in the past breeding strategies were formed based on visible traits, today it is possible to plan and predict consequences of breeding with the help of genetic tests and to avoid undesirable traits in offspring.
Genetic homogeneity is not necessarily a problem as it is the result of species and breed formation, but it can be a problem if it leads to homozygous alleles on disease genes. Appropriate selection focused on health improvement ensures the long-term existence of dog breeds. Healthy and heterogeneous genetic material of purebred dogs is responsibility of breeders. Modern genetic tools give a wealth of information that can be used to pursuit breeding goals focused on health and functionality of dogs.
(1) Schaible, Robert H. (1981). "A Dalmatian Study: The Genetic Correction of Health Problems". The AKC Gazette.