What is PRA?

PRA (Progressive Retinal Atrophy) is an inherited eye disease found in many breeds of dogs with varying ages of onset. There are various types of PRA but the one most commonly seen in Cocker Spaniels is prcd-PRA also known as GPRA (General Progressive Retinal Atrophy).

For more information about Progressive Retinal Atrophy (prcd-PRA) please click visit our PRA page.

DNA Testing Methods For PRA

In late 2003/early 2004, the American company OptiGen developed a DNA test for the most common form of PRA found in Cocker Spaniels - prcd-PRA. The test at the time was not a mutation test, it was a "marker" test that detected changes in coding sequences of genes located near to and inherited with (linked to) the prcd gene.

Dogs tested using the original "marker" test, will have test results showing A, B & C. The "marker" test was subsequently 'improved' & dogs tested under the new "marker" test will have A1, B1 or C1.

In June 2005, the American company OptiGen found the gene mutation responsible for prcd-PRA and ceased using the "marker" test. Dogs tested using the gene mutation test will have Normal/Clear, Carrier or Affected results.

DNA testing gives a reliable result, dogs that are 'carriers' or 'affected' (with or without obvious physical symptoms) can be identified BEFORE they are used for breeding & BEFORE the disease becomes visually clear via a standard KC/BVA eye screening examination.

The OptiGen prcd-PRA test is performed on a small sample of blood from the dog. Blood can be drawn from dogs of any age, cocker puppies need to be approx. 4 weeks old (in order they are ‘big’ enough to micro-chip & so that the blood draw is easier).

Once the sample is processed, the DNA is analysed for the gene mutation. The result of the test is a genotype - a fingerprint - that allows separation of dogs into three groups for Cocker Spaniels - Genotype: Normal/Clear, Carrier & Affected.

The OptiGen prcd-PRA test can also be performed on a DNA sample collected via a cheek swab (Buccal Cheek Swab), however OptiGen do not recommend this method due to the risk of cross contamination. If using the cheek cells method, it is advisable that puppies are fully weaned and independent from their mother before sampling (in order that their mouths are not harbouring traces of their mothers DNA).

Our personal feelings are that the risk of cross contamination is too common with DNA samples derived from cheek cells & we personally prefer to continue with testing via blood samples. The risk of cross contamination (from other dogs in our household) is totally removed and we can be sure that the results for our dogs are from their DNA & not their household companions!

How PRA is inherited

prcd-PRA is inherited in an Autosomal recessive pattern in the cocker spaniel. This means the gene mutation responsible for prcd-PRA is located on an Autosome (that is, a chromosome that is not a sex chromosome) and PRA disease results when the gene mutation is passed to the offspring by both the mother & the father.

As prcd-PRA is inherited as a recessive trait, this means a mutant gene must be inherited from each parent in order to cause disease in an offspring. A 'carrier' has one disease gene and one normal gene, and is termed “heterozygous” for the disease. A 'normal' dog has no copies of the mutant gene and is termed “homozygous normal” – both copies of the gene are the same. Moreover, a dog with two disease genes is termed “homozygous affected” – both copies of the gene are abnormal.

Due to the risk of producing 25% or more prcd-PRA 'affected' puppies in a litter, carriers should not be bred together & neither should a 'carrier' and an 'affected' be bred together as at least 50% of the litter may be 'affected' with prcd-PRA, at least one of the parents should be "Normal/Clear".

For more information about OptiGen testing for prcd-PRA please visit OptiGen's website.

Cost Of Testing

The current cost to DNA test a dog is (approximately £10 for the vet to collect the blood sample) and approximately £100 per dog (when using a 20/20 clinic & requesting the test online) to OptiGen for the DNA test. This is a generous saving compared to the prices when the test first became available, the cost at that time was approximately £140 per dog. Please note, when you order the test you will pay in Dollars as OptiGen is an American company.

Results

Notes:

prcd-PRA is inherited as a recessive trait. This means a disease gene must be inherited from each parent in order to cause disease in an offspring. A 'carrier' has one disease gene and one 'normal' gene, and is termed “heterozygous” for the disease. A 'normal' dog has no disease gene and is termed “homozygous normal” – both copies of the gene are the same, a dog with two disease genes is termed “homozygous affected” – both copies of the gene are abnormal. Although prcd-PRA is inherited, it can be avoided in future generations by testing dogs before breeding.

For an explanation of Autosomal inheritance visit this page.

With Mendelian inheritance, there is the "Law of Independent Assortment.” In independent assortment, the chromosomes that end up in a newly-formed gamete (new puppy) are randomly sorted from all possible combinations of maternal & paternal chromosomes.

Because of the “Law of Independent Assortment” and because ratios are calculated in percentages, unless the same two parents produce 100 offspring together (probably impossible!!) the difference between expected outcomes and actual results can vary hugely due to the small sample size i.e. a litter containing 6 pups as opposed to 100 puppies!

If you mate, a "clear" to a "carrier" the expected ratio of "clear" to "carrier" offspring is 50/50, however it is possible that the entire litter could be "carriers"! Mate the same parents again & the next litter could be 25% "clear" and 75% "carriers,” mate the parents again and the entire litter could be "clear"! The inheritance of genes is a random process and in a small litter, all could be "carriers" or all could be "clear" (when mating "clear" to "carrier").

Although prcd-PRA is inherited, it can be avoided in future generations by testing dogs before breeding. Identification of dogs that do not carry disease genes is the key. These "clear" dogs can be bred to any mate - even to a prcd-affected dog which may be a desirable breeding prospect for other reasons.

Normal/Clear (A1) dogs, meaning statistically 'Normal' for prcd-PRA will never develop this disease or pass it to any offspring. To date, no known normal/clear (A1) dog has developed prcd-PRA or produced prcd-PRA-affected offspring. (Source OptiGen)

Normal/Clear A/A1 can be bred to any dog

Carrier B/B1 should be bred only to A/A1 (Normal/Carrier)

Affected C/C1 should be bred only to A/A1 (Normal/Carrier)

The test makes it possible to breed from Carriers (B/B1 to B/B1) as long you test any puppies before breeding from them. Don't be afraid of your Carriers (B1´s), they ARE sound and will NEVER develop prcd-PRA - breed on with them in a controlled way i.e. ensure you mate them to a Normal/Clear (A1) partner & then test any pups who will be subsequently bred from.

To see Powerscourt prcd-PRA test results please visit our health testing page.

Please visit our Cocker Spaniel Health Tests page for a list of testing schemes available for cocker spaniels.