Any carrier has a statistical chance of passing on the same carrier status to 50% of his or her progeny who are themselves out of the ‘clear’ mate. These carriers remain hidden within the breed population until one day two carriers are bred together. Then, statistically 25% of the resultant progeny may inherit the gene to be affected by PRA and may go blind in early, or later, adulthood. Some of the littermates will carry the gene hidden in their genetic make-up, to possibly hand it on to the next generation. So here are the results of five possible matings with a recessive mode of inheritance:

1. Both parents affected
all progeny affected.
2. One parent affected – one carrier parent
50% progeny affected – 50% progeny carriers.
3. One parent affected – one parent free
all progeny carriers.
4. Both parents carriers
50% progeny carriers – 25% progeny free – 25% progeny affected.
5. One parent free – one carrier parent
50% progeny carriers – 50% progeny free.

From this it follows that both parents – sire and dam – and all progeny of an affected animal, must be at least carriers. A 2% incidence of the disease in the population means 24% of that population will be carriers. If the incidence is 10% then the carrier state is 45%!
No affected dog should be used for breeding or if the DNA status is known, then carriers can be bred to others of known status and offspring DNA tested. This way breeds can still be salvaged where there may be a high incidence of the carrier or affected state. The ideal, which is now available, is a blood test which can be done on all present breeding stock, to determine whether they are carrying either none, one (carrier) or both (affected) genes for PRA or any other genetic disorder (e.g. cataract). New-born pups could also be tested so that one could get an early insight into whether they may have a vision problem later in life (they could be euthanased or sterilised if they were only carriers).
This DNA test is now available in NZ with the DNA collected from an individual by a check swab or blood with the tests done in Australia.