Example: Progressive retinal atrophy
Progressive retinal atrophy (PRA) is an inherited condition that is seen in a number of different dog breeds and affects the rod and cone cells that make up the dog’s retina. In all the breeds that have been investigated in sufficient detail, the mode of inheritance has turned out to be a single recessive gene.
In the Canine Genetics Laboratory exercise, we denote dominant form of the gene in question P, and the recessive form p. Because the defective gene is recessive, only dogs with two copies of the recessive gene will be affected, i.e. those dogs denoted pp. Dogs that are Pp carry the recessive gene but are not affected by it. PP dogs do not carry the defective gene at all.
Different breeds of dogs actually suffer from different forms of PRA, but the end result is the same; the rod and cone cells eventually die and the affected dogs become totally blind. Owners of affected dogs usually first notice a loss of night vision, especially when the affected dog is in unusual surroundings; the condition eventually progresses to produce a loss of vision under all light conditions. There is as yet no known cure for PRA.
There are broadly two different types of PRA in the dog: rod/cone dysplasia and rod/cone degeneration. Breeds like the Irish Setter and the Miniature Long-haired Dachshund suffer from rod/cone dysplasia. In this case the rod and cone cells develop abnormally and begin to degenerate even before they are fully mature, leading to a very early age of onset in affected dogs, usually within the first few months of life. In breeds that suffer from rod/cone degeneration, like the Labrador, the Golden Retriever and the Cocker Spaniel, the rod and cone cells develop normally and only begin to degenerate later in life leading to a much later age of onset of the disease, usually anywhere from 3 to 4 years of age onwards.