The Progressive Retinal Atrophy as an inherited form leads to a degeneration of the retina and results in blindness. The Retina which is composed of photoreceptor cells as well as nerve cells covers the back inside wall of the eye and is responsible for image formation. The photoreceptor cells can be divided into rods or cones depending on their function. The rod cells are specialised for the mesopic vision and contrast sensitivity. The con cells are responsible for color vision.
The prcd-PRA causes initially a loss of function of the rod cells accompanied by night blindness. At later stages the cone cells become affected leading to a complete blindness of the dog.
Age of onset of clinical symptoms is typically in early adolescence or early adulthood. However, the onset of the disease may vary among different dog breeds.
Trait of inheritance - autosomal recessive
Two copies for each trait exist in the genome. An animal derives one of each of these copies from the father and one from the mother. A recessive allele is overwritten by it's dominant counterpart. Therefore, autosomal recessive inherited diseases only affect animals which received two mutated alleles. Father and mother of the affected animal have to carry the recessive allele, but could be healthy themselves.
Three possible genotypes exist:
1. Genotype N/N (homozygous healthy): This animal does not inherit the mutation and contracts the disease with very low probability. The animal will never pass a mutated allele to it's offspring.
2. Genotype N/mut (heterozygous carrier): This animal posses one copy of the mutated allele with a very low probability to contract the disease. The offspring of the carrier inherit the mutated allele with 50% probability. Therefore, breeding should only take place with homozygous healthy animals.
3. 3. Genotype mut/mut (homozygous affected): Carrying two mutated copies of the allele, this animal will most probably exhibit the disease. Additionally, 100% of it's offspring inherit the mutated allele. Only breeding to a mutation free animal could be tolerated.
With 50% probability carrier animals inherit the mutated allele to their offspring. Breeding two carriers results in 25% of the offspring being affected by the disease.
In healthy populations, carrier animals enlarge the gene pool and should not be excluded from breeding categorical. Never the less, breeding of carriers with other carriers or even affected animals should strictly be avoided.