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Doubts and questions regarding breeding carriers have been present among dog breeders since the development of modern dog breeds. With the fast development in the field of genetics and molecular biology, a number of DNA tests have become available for purebred dogs. These tests are a powerful tool in breeding quality control, and they enable planning a breeding program on a whole new level. Breeders are often surprised or disappointed when their dog tests as a carrier for specific mutation.In the following article we would like to discuss what ''carrier'' in the test certificate means exactly, whether carriers should be included in the breeding program, as well as break the stigma regarding this genetic status.
Since prehistoric times and first wolf domestication, humans have been selecting which dogs to breed based on desired traits and characteristics. Through this practice, today we have developed specific dog breed groups: hunting, shepherding, guarding, work and company breeds. You can read more about dog breeding history here.
Dogs with desirable traits will be used more often in the breeding program, while dogs lacking those traits will not be used at all. This means that the popular sire or dam will transfer their genetic material more often to offspring, while the genetic material of dogs excluded from breeding will not be passed on to future generations.
Such selective breeding led to extreme shrinkage of the pure breed dog's gene pool. A smaller gene pool increases the risk of inbreeding, i.e. mating dogs that are closer or distant relatives, which in turn also increases the risk of inherited disorders among dogs. The reduced gene pool is exactly the reason why pure breed dogs are often more prone to diseases, compared to generally healthy mixed-breed dogs.
Gene pool is a total number of all genes in a certain population.
Most of the genetic disorders are inherited in an autosomal recessive pattern. Each dog has two specific alleles (gene copies), each inherited from one parent. Alleles for a specific disorder can be wild-type (do not cause the disorder) or can be mutated (can cause the disorder in case the dog has two copies of the mutated gene – in case of recessive inheritance, or even with a single copy present – dominant inheritance).
An example of a disorder with simple (mendelian) inheritance are many inherited eye disorders, such as progressive retinal atrophy (PRA). Before the DNA tests were developed, a dog's eyes were examined by a veterinarian, but such examinations do not provide fully reliable information, since symptoms of inherited disorders often do not develop until later in life, after the dog has already entered reproductive age. This means that the affected dog will mate and pass on the genes that cause the disorder, without the owner knowing that the dog is affected.
The only way to be sure of your dog's genetic heritage is DNA testing. Responsible breeders are aware of the importance of such tests and get their dogs tested in order to maintain or improve the quality of their dogs. On the other hand, when the results show that their dog is a carrier for certain autosomal recessive disorder, owners often appear confused or disappointed. Considering centuries of inbreeding among pure breed dogs, the appearance of carriers is not unusual. Additionally, some breeders appear discouraged from using their carrier dogs in the breeding program. Although the dog carries one copy of the causative mutation, the other copy of the allele is healthy, so the carrier dogs will be healthy dogs in relation to that specific recessively-inherited disorder. When breeding carrier with a clear dog, there is 50% chance that puppies will be clear and 50% chance that puppies will be carriers, but ALL PUPPIES WILL BE HEALTHY. On the other hand, excluding the carrier completely from the breeding program will reduce the gene pool for this dog's genetic material. Continuing this practice throughout years would cause dramatic shrinkage in the gene pool, which in turn, can lead to appearing of some new, until now unknown or extremely rare disorders.
Autosomal recessive inheritance patterns
An extreme gene pool shrinkage can be seen on the example of the Norwegian Puffin Dog. At some point in the 20th century, there were only 5 dogs of this breed left. Today the breed is well preserved, but a breed specific disorder occurs in this breed, the Lundehund syndrome, with an estimated prevalence of the disorder of up to 100%. While today the carrier rate of 10% is already considered high, this is an example of extreme gene pool shrinkage.
Like many other purebred dogs, the Lundehund went through severe gene pool shrinkage.
Over the years it's been noted that excluding dogs from the breeding program was not sufficient for eliminating disorders from the breed. Phenotypical examination of a dog's pedigree provided good but not complete information about the quality of the future litter. Today's developments in the molecular diagnostics allows a completely new approach to dog breeding based on genetic heritage, unlike diagnostics based on already developed symptoms or visible phenotype. Identificaation of carrier dogs allows their inclusion in the breeding programme, while obtaining healthy litters when mating with clear dogs, and at the same time it enables avoiding mating two carriers or affected dogs which would result in affected puppies. Simultaneously, keeping carriers in a cleverly planned breeding program will maintain the stability of the genetic pool, without increasing the risk of new, unknown disorders emerging. Genetic tests for dogs offer an easy and simple way for the exploration of your dog's genetic heritage and improved quality of future litters, without the deleterious effects of gene pool shrinkage.
http://www.breedingbetterdogs.com/article/managing-carriers-pt-1
http://www.instituteofcaninebiology.org/whats-in-the-gene-pool.html