Canine genetics is difficult to understand for both breeders and pet owners. It is an area of science that is continually being studied and researched. As time goes on, and more research is completed, the more we are starting to understand how specific conditions and traits are passed on through generations. That being said there is still much to be learned regarding genetics in both dogs and humans.
It is important to know that not all conditions and diseases in the Cavalier King Charles Spaniel are genetic, however many of them are. Breeders do their best to not breed from dogs that are clearly showing health problems. Things get complicated when you breed two completely health tested cavaliers who produce offspring that are clearly affected by an inherited genetic condition. What happened? What went wrong?
The problem is that not all genetic conditions are passed on to offspring in the same way, and some diseases and conditions haven't had enough research completed to clearly define a path of inheritance.
Some of the modes of inheritance that have been defined thus far are:
Only 1 copy of the gene, which may be inherited from either parent, is required to produce the trait. The parent with the dominant trait will pass the affected gene to approximately half its offspring, and the trait will be apparent in both the parent and the affected progeny. These conditions are uncommon because, as long as it is of early onset (i.e. becomes apparent before breeding age is reached), the disorder can be readily eliminated by avoiding the breeding of affected individuals.
This is the most common mode of inheritance for genetic conditions in dogs. Progressive retinal atrophy (PRA), which causes blindness in many breeds, is such a trait. To be affected, the animal must inherit 2 copies of the gene (genotype pp), 1 from each parent. Dogs with the genotype PP (normal) or Pp (carrier) will be clinically normal but the carrier will pass the affected gene to approximately half the offspring. As long as carriers (Pp) are mated to normal animals (PP), the offspring will be unaffected but some will remain carriers. If 2 carriers are mated, some of the offspring (approximately 25%) will be affected.
As long as the frequency of a gene for a recessive disorder remains low in the population, the particular gene may be passed along for many generations before by chance 2 carriers are mated and affected individuals are born. However, the gene frequency may become unusually high due to breeding of close family members, or because of the "popular sire" effect, where a sire with a harmful recessive gene is mated frequently because of desirable traits.
Because the recessive gene is carried in the population in outwardly normal animals, it is very difficult to eradicate these traits. However the incidence can be reduced by identification of carriers through test matings or through various tests that have been developed, and the conscientious use of this information in breeding programmes. Veterinarians, dog breeders, and breed associations must all work together for substantial progress to be achieved.
In these traits, the gene is located on the X chromosome. Males have 1 X chromosome from their mother, and 1 Y chromosome from their father, which carries little information other than maleness. Females have 2 X chromosomes, 1 each from their mother and father. So, if a mother who is a carrier for a harmful recessive gene (Xx) passes the recessive gene (x) to her daughter, the daughter will be an unaffected carrier, but her sons who receive that gene will be affected.
Polygenic traits are controlled by an unknown number of genes. The gene expression is influenced by a variety of factors including gender, nutrition, breed, rate of growth, and amount of exercise. These traits are quantitative traits - that is, there is a wide range within the population. Such traits include height, weight, character, working abilities, and some genetic defects. Heritability varies within different breeds and within different populations of a particular breed.
Because it is virtually impossible to determine the exact genotype for such traits, it is difficult to control defects with a polygenic mode of inheritance. The best attempts at control are based on a grading scheme for identification of the defect and a breed policy of recording and publishing the results for as many dogs as possible. Canine hip dysplasia is a polygenic trait that remains a problem in most large breeds of dog, despite efforts to control this condition dating back to the 1960s. Breed organizations and veterinarians in various countries have developed control programmes that rely on radiographic evaluation and a central registry of dogs. Thoughtful selection by breeders, using this information, has greatly reduced the incidence of hip dysplasia in those breeds in particular countries.
© Canine Inherited Disorders Database (2000, December 15)
How are Defects Inherited Retrieved December 6th, 2008
Conditions that are considered inherited in the Cavalier King Charels Spaniel are:
- Retinal Dysplasia
- Mitral Valve Disease
- Luxating Patellas
Conditions in the Cavalier King Charles Spaniel that are most likely inherited but are awaiting further research:
- Episodic Falling Syndrome
- Fly Catcher's Syndrome
- Elongated Soft Palate
Other conditions that can affect all breeds of dogs but are not genetic:
- Corneal Ulcers
- Diabetes Mellitus
- Inflammatory Bowel Disease
Overall genetics plays a very important part in breeding a healthy cavalier. However it is also a factor that can not be fully controlled by human choice. The best intentions do not always turn out as planned. Inheritied genetic conditions will present itself in the breed from time to time. It is important to have a good relationship with your breeder so that if your cavalier does develop a genetic condition you are able to inform your breeder. Your breeder will then be able to make changes to their breeding program in hopes of not having future puppies with the same condition.
For further understanding of Canine genetics there are some great resources on the internet:
Canine Inherited Disorders Database
DNA Markers - Lhasa-Apso.org
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