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.