There have been diagnosed PRA in dogs bred in UK, USA, Norway, Holland, Iceland, Sweden and Denmark, but of course most PRA-cases are found in those countries, who thoroughly test their dogs.

In Denmark and Sweden the breeders can't get their puppies’ registered unless both parents have been tested clear of PRA within the last 12-month before mating. If a dog is diagnosed PRA, then it will be excluded from breeding as well as it's parents and offspring.

Back in '92 Pia Johansen, Gold Wings, started gathering eye-test-results. Hoping to map the gene in order to prevent breeding dogs who were carrying the gene.
Lene Kiilerich, Emira's, has now by great effort turned these data into a huge database containing over 30.000 papillons/phalenes - whereof over 3000 test-results.

The risk of the litter becoming affected or carries can be calculated if you know the incidence of homozygotes (affected) dogs in the population, in Scandinavia as testing has been done for 10 years our stock are 100% tested by now.

On this background, and the numbers I have achieved by the CERF, the average PRA risk is 4% and the single carrier risk is 32%

Imagine a 5 generation pedigree - there are 62 individual dogs. Assuming there are 7 dogs who are known to be carriers, 41 is not tested, making 21 being cleared as affected. Noone is known to be affected
A puppy out of this litter will have a risk of 6% of being affected and 39% of being carrier. click for Pedigree

If the great-grand father turns out to be affected the risk will be like this: 11% for PRA / 45% for single carrier. There are now 9 carriers and one affected.
click for pedigree