Patterns of inheritance
The specific pattern of inheritance has not been established for many of the disorders that are believed to be inherited. Where the mode of inheritance is not known, breeds that have an increased risk relative to other dog breeds are said to have a breed predisposition for a particular condition.
The following describes known patterns of inheritance.
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 (ie becomes apparent before breeding age is reached), the disorder can be readily eliminated by avoiding the breeding of affected individuals.
In many instances however, there is incomplete dominance. The trait may be dominant with variable expressivity, which means that if either parent is affected, all puppies have a susceptibility to the disorder but not all will be affected equally. Alternately, a dominant trait may have incomplete penetrance. If penetrance is 75% for example, only about 3 quarters of the pups who inherit the trait will express it.
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.
The bleeding disorder hemophilia is the best known of the X-linked traits, which are uncommon in the dog. Control programmes are possible because carrier females can be identified through blood screening.
The above-mentioned traits are inherited in a straightforward manner. Many others are inherited in a more complex fashion. In fact, most traits that are selected for in the dog are the result of the interaction of many genes. Modifying genes may influence how other genes are expressed. As mentioned above, a trait may be dominant, but with incomplete penetrance so that it is not always expressed. Epistaxis occurs when alleles at one locus mask the action of another pair of alleles.
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.
This database is funded jointly by the Animal Welfare Unit at the Atlantic Veterinary College, University of Prince Edward Island, and the Canadian Veterinary Medical Association.
1998 Canine Inherited Disorders Database.