Breeding Away from Inherited Diseases

…where DNA tests have been developed for Simple Recessive Diseases
Karen Hedberg BVSc  2006

Of the 400 odd diseases in the dog that are inherited or have significant inherited components, more than half are believed to be single gene defects, and around 70% of these, are inherited as in a simple autosomal recessive mode of inheritance.

Because so many of the canine genetic disease conditions are as a result of single gene defects, there is great hope that eventually there may be many DNA disease specific tests available to dog breeders.

Similar diseases within related breeds often have the exact same genetic defect, but unfortunately this is not always the case. For example what causes PRA (progressive retinal atrophy) in one breed, may not involve exactly the same gene in another breed, which may have the PRCD PRA(progressive rod and cone degeneration) form of the disease.

Each test as it is developed has to be checked against known affected dogs and their related siblings/parents etc, to give the very high degree of accuracy necessary (>98%) before the test can be released for that breed.

Once a DNA disease specific test has been developed within a breed, it can be used to clear the problem from a breed. The problem can usually be cleared within several generations without any loss of important breeding stock. Where the DNA test is for conditions such as PRA, where the consequences of breeding an affected dog is blindness at a relatively early age, any such test is a an enormous step forwards for the health and welfare of the breed.

Simple autosomal recessive gene DNA tests.

With a simple autosomal recessive gene, the DNA test will clearly differentiate between affected, carrier and normal individuals. The beauty of this type of genetic test, is that you do not need to lose any valuable animals from your breeding program. Outstanding examples of the breed that are otherwise sound can be retained even if they are affected once this type of test is available. All you need to remember to do is to always breed ‘up’.

Normal/clear dogs are usually given the ‘A’ pattern designation.

Carriers (heterozygous) are given the B’ pattern designation.

Affected dogs are given the C’ pattern designation.

Where ‘A’ = normal dominant allele and ‘a’ = affected recessive allele.

One allele comes from each parent.

When mating these dogs, mating two normals ie. A to A will result in all A dogs, no testing required.

With normal/clear A to carrier B dogs, 50% will be A and 50% will be B dogs – no affected dogs produced, only need to test those puppies that are retained to sort out the A from the B dogs, but this can be delayed if necessary until nearer breeding age (as no affected progeny).

With normal/clear A to affected C dogs, no testing is necessary, as all progeny are automatically B ie. carriers.

What you should try to avoid as much as possible is putting a carrier or an affected dog to anything but a normal/clear (A) dog.

B to B matings will produce ¼ normal, ½ carrier and ¼ affected progeny, therefore all progeny must be tested prior to sale (an expensive undertaking).

Equally, B to C matings are very high risk as there will be 50% carriers (B) and 50% affected (C) progeny – again, all progeny must be tested prior to sale.

Affected or C dogs can only be safely bred to a normal dog (A’s), as any other mating will produce significant numbers of affected progeny.

The most important point of using this type of DNA test correctly is that one can avoid producing any affected animals provided you know the DNA status of the parents for that disease. Once all dogs in your kennel are clear or normal for the condition, no further testing is needed, provided you keep using normal animals.