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Canine Genetic Diseases

The dog-human bond goes deeper than mutual admiration. The dog's genome has more genetic similarities with humans than does the mouse -- the most-often used mammal for human health studies. Causes for diseases associated with a single gene can be uncovered with current biological techniques, but finding the source of diseases associated with multiple genes is a much more daunting task.

Genetic diseases are caused by mutations — or alterations in the structure and function of genes, rather than by the environment. Genes are comprised of DNA sequences. These sequences serve as templates for messenger RNA, which directs protein synthesis. Hence, genetic defects ultimately result in the manufacture of abnormal proteins -- perhaps enzymes, biological receptors or structural proteins comprising cell membranes and connective tissues. Genes, which direct growth, development and function of the body's organs throughout life, number some 50,000 in every mammal. The genome of dogs differs from that of humans by only about 10-20 percent of the nucleotides. Thus, explained Dr. Donald F. Patterson, professor of medical genetics at the University of Pennsylvania School of Veterinary Medicine, "You can expect that all mammals will generally have the same number and kinds of genetic diseases."

A large percentage of genetic maladies documented in the dog thus far are orthopedic, ocular or neurological, perhaps because of the more obvious clinical signs they produce. But researchers are now striking major advancements in those genetic abnormalities that cause more subtle signs and challenge diagnostic methods.

In order to eliminate recessive diseases, Dr. Patterson explained, we must be able to recognize affected animalsand refrain from breeding them. But, he added, "That's not enough, because most of the genes that cause these recessive disorders are in the population in carriers. We don't know that an animal is a carrier unless it's bred to another carrier and produces affected individuals."

If, for example, the frequency of a defect were four percent and the affected individuals were removed from the breeding population, it would take over ten generations to reduce the frequency of disease to one percent. But if both the carriers and those affected were extracted from the breeding population, the disease would be eliminated in one generation. So the key to abolishing canine genetic diseases, Dr. Patterson said, is to identify carrier dogs.

Screening for Inherited Diseases

According to Dr. Urs Giger, professor of medicine and medical genetics, in contrast to the common dominant inheritance of diseases in humans, most hereditary diseases in dogs are recessively inherited. In autosomal recessive inheritance both asymptomatic parents may contribute an abnormal (mutant) gene to an offspring which will then have two mutant genes (homozygous) and is, therefore, affected. Most metabolic and eye disorders are inherited by an autosomal recessive trait. In x-linked recessive disorders, the asymptomatic dam will pass on one of her x-chromosomes to the male and female offspring, however, if the male inherits the x-chromosome containing a mutant gene, the males will be affected with e.g., hemophilia or muscular dystrophy. Finally, with complex traits such as hip dysplasia and certain cancers, environmental factors also play a role in the expression of the disease. Thus, in order to control hereditary diseases in dogs, it will be important not only to identify affected dogs, but also carriers which carry one copy of a mutant gene, but have no clinical signs.

The Importance of Testing for Inherited Diseases

Veterinarians and breeders have several tools to identify diseased animals. Since many genetic diseases are breed-specific, the signalment of an animal may suggest a disorder known to occur in that breed. Most diseases are associated with characteristic clinical signs which occur at a typical age. Neonates and juvenile dogs are more likely to suffer from a genetic disorder (this is known as the fading puppy syndrome), although a few genetic diseases may only cause signs in adulthood. Failure to thrive, growth retardation, and malformations are commonly seen, and neurologic and ophthalmologic signs may also be evident. In contrast to malnutrition, infections, and intoxications, the clinical manifestations are usually chronic progressive, however, genetic predisposition to infection and bleeding may be associated with intermittent signs. An easy way to recognize an unthrifty puppy early, is to regularly weigh puppies and compare with littermates. Routine blood and urine tests as well as imaging studies such as radiographs, ultrasound and ophthalmoscopy rule out many acquired diseases and may further suggest an inherited disorder.

There are several types of genetic diseases

Heritable Diseases of Dogs

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