What is a Genetic Disease?

Many breeds of dogs carry untreatable diseases and disorders from parent to puppy. The puppy inherits the disorder or predis-
posing factors from its parents, and this is commonly known as a genetic disease. It is important to note that it is not only pure-
bred or pedigreed dogs that are affected by genetic disease.

 

Every one of the billions of tiny cells that make up the animal has a nucleus which contains deoxyribonucleic acid or DNA. This DNA is made up of genes containing the 'blueprint' or information relating to the way the dog looks and acts. These genes are carried on a chromosome, of which the dog has 78, or 39 pairs on which approximately 100,000 genes are located. All animals have two copies of each gene, one copy inherited from each parent.

The building blocks of genes are nucleotide base pairs. There are only four base pairs (A, T, G and C) which are arranged in different orders. For the gene to be expressed correctly the base pairs need to be in the correct sequence. Many disorders are inherited but some occur because the sequence becomes altered and this change is called a mutation. The mutation could be the substitution of one base (e.g. an A) for another (e.g. a G), or a deletion or insertion of one or more bases.

The mutation causes the gene to be expressed incorrectly and this leads to the development of the genetic disease. For example von Willebrand's disease Type I is caused by a mutation in the gene that controls the manufacturing of a blood clotting protein. The gene is not processed correctly and an insufficient quantity of this protein is manufactured to ensure sufficient blood clotting.

Congenital disorders are malformations with which the animal is born. These are not necessarily genetic in origin but could well be due to environmental factors affecting the mother during gestation.

How do breeders avoid genetic disorders in their breeding?

It is important that breeders avoid inherited disorders and dis eases by good breeding practices. This will include educating themselves on known disorders within their chosen breed, how these disorders are inherited and how to avoid breeding with carriers of these disorders. Depending on the mode of inheri-
tance, phenotypically correct dogs (dogs that do not appear to be affected by the disorder) may carry it to their progeny. It is therefore important to identify these carriers before breeding with them.

Not all diseases are hereditary. For instance, the formation of cataracts in the eyes could be hereditary and may be a known disorder in a certain breed; however, cataracts can also be caused by external factors such as injury or diabetes.

How are diseases inherited?

There are several different modes of inheritance including simple recessive, incomplete penetrance, autosomal dominance, polygenic inheritance, and others. Testing is currently available for several diseases linked to recessive modes of inheritance.

Canine Genetic Disease Testing

Over the past several years a great deal of research has been undertaken in various countries to establish the modes of inheritance of various inherited diseases. It is now possible to test dogs for certain autosomal recessive disorders, which are caused by a mutation in only one gene. The recessive gene is
the mutant gene and two copies of this gene need to be pres ent before the animal presents the symptoms of the disease. It must therefore be remembered that an animal may carry the disorder without being phenotypically abnormal, and only when bred to another similar carrier (thus resulting in two copies of the mutant gene) it becomes apparent in some of the progeny.

What diseases/disorders can be identified by testing?

Several disorders can be tested including Von Willebrand's Disease Type 1; Collie Eye Anomaly; Heritable Cataracts in cer tain breeds; Familial Nephropathy, a renal disorder in English Cocker Spaniels; prcd/PRA – eye disorder leading to blindness in over 27 breeds; and several others.

Disorders like Hip Dysplasia, allergies or cancer cannot (yet) be diagnosed with a simple genetic test. It is caused by a number of genes that sometimes even interact to cause the disorder.

As a breeder, why should I test my dog?

For most genetic diseases there is no cure. Dogs affected by Von Willebrand's Disease Type I could bleed to death and PRA affected dogs could go blind at a young age. Nobody wants to have dog suffering from a disorder and much less wants to sell a puppy that has this disorder. The only way to manage this disease is by selective breeding.

Ideally all mating should be between two clear dogs. Unfortunately dogs with excellent features for other traits might be a carrier or even affected by a genetic disorder. A breeder should use these test results as a tool to plan a mating.

References:
Canine Inherited Disorders Database – http://www.upei.ca/~codd/ intro.htm
Inqaba Biotechnical Industries (Pty) Ltd – www.inqababiotec.co.za

 

All About Dogs July/August 2010 18