Why it’s Important for Pet Owners to
Understand the uses of Genetic Testing

Cynthia O’Connor, DVM

This is an exciting time with amazing advances in veterinary medicine. The area of molecular genetics has become one of the hottest areas of research bringing the development of genetic testing to dog breeders. So why as a pet owner is this information important? Many pet owners are looking for a happy, healthy pet to add to their family. Because they are not interested in breeding, many mistakenly feel that they do not need to learn about genetic testing. Many of these people also fall into the trap of impulse buying, by purchasing a dog in a pet store or via an internet puppy broker. Many impulse purchases lack research into the health and temperament of the pedigree and these new owners can only hope that the dice will roll in their favor with a happy, healthy pet. Genetic testing offers the opportunity for many people to become more informed consumers. After all, would you consider buying a car, sight unseen, with no history? Then, why would you not put in the same, if not more effort, into investigating a new addition to your family that you will trust to sleep in your child’s bed?

 To understand the use of genetic testing and their use for breeders and pet owners, we first need to remember some basics about genetics. The term hereditary or genetic is used to describe a disease caused by a DNA mutation that can be passed from parent to offspring. The term congenital is used to describe a disease that is present at birth. It is important to remember that a congenital disease can be genetic; however, not all congenital diseases are genetic. In fact, toxins, infections, and other environmental factors can cause congenital diseases. It is also important to remember that not all genetic diseases are identifiable at birth. A good example of this is progressive retinal atrophy (PRA) in Portuguese Water Dogs (PWDs). Dogs affected with PRA generally do not show signs of blindness until much later in life.

 Once a disease is recognized as genetic, knowing how it is passed from generation to generation is the next step in preventing and managing genetic diseases. There are several inheritance patterns recognized in dogs. We are probably most familiar with autosomal recessive traits. An example of an autosomal recessive trait in Portuguese Water Dogs is storage disease (GM-1 gangliosidosis). PWDs that test normal do not have a copy of the mutated gene. PWDs that carry one affected gene and one normal gene do not show any signs of the disease, however, they may pass that affected gene onto their puppies. Puppies that receive two affected genes, one from each carrier parent, will develop the disease. These are important points to keep in mind for breeders as well as pet owners. The goal of breeders is to create healthy puppies while maintaining breed genetic diversity. Breeders can utilize tests such as this to prevent producing an affected puppy, while still utilizing the genes of exceptional carrier and affected animals. It is important for pet owners to understand that for autosomal recessive diseases such as this example in PWDs, a carrier dog is a very healthy pet. This does not always hold true for other modes of inheritance, so when evaluating a disease and the use of the genetic test, it is important to understand how the disease is inherited.

 Genetic (DNA based) tests come in two varieties. The first type is a mutation based test. These detect the exact DNA defect or mutation that is the cause of the disease. Results are often given as Normal, Carrier, or Affected. Examples of mutation based tests available for PWDs include GM-1 and prcd-PRA.

 The second type of genetic test is the linked marker test. Markers are segments of DNA that have an identifiable physical location on a chromosome whose inheritance can be followed. A marker can be a gene or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of a gene that has not yet been identified, but whose approximate location is known. The results of these tests are often given as a pattern such as A, B, or C. A good example of a linked marker test in PWDs is the newly announced test for juvenile dilated cardiomyopathy (JDCM) discussed in this issue, which will hopefully be available to breeders by late October.

 The obvious advantage of tests such as the new JDCM test is the ability to immediately reduce puppy loss from this disease based on careful breeding practices over several generations. There is nothing more devastating to a pet owner than taking home a beautiful new PWD puppy only to have this puppy die suddenly from this disease at such a young age. The emotional toll that this tragedy takes on the entire family as well as the breeder is immense. With genetic based tests like this, breeders can identify healthy carrier animals so that they can make careful breeding decisions in order to avoid tragedies such as this for pet owners. Pet owners, in turn, can use these tests to help find breeders who are working to identify and prevent diseases like this in their dogs.

 Unfortunately, breeders can not look into a crystal ball and predict that a puppy they produce will never get sick. However, an educated breeder can discuss with a new puppy buyer the most common inherited diseases in their breed and their line. A buyer should be concerned when during a discussion like this, a breeder states that they have no health concerns in their dogs and so have no need to perform genetic testing. This should hint to the buyer that the breeder is either not very forthcoming with health information, or does not know about the health history of their lines well enough. A puppy buyer, in turn, needs to understand the basics of these diseases and how they are inherited. For a puppy buyer, it is important to remember that for a simple autosomal recessive disease, such as juvenile dilated cardiomyopathy in PWDs, a carrier is a perfectly healthy pet. Buyers also need to understand that for a disease that is inherited in this way, a breeder who breeds a carrier to a normal dog will not produce the disease and may be making this decision to help preserve the long term genetic health of their line and the breed as a whole. In fact, it has been shown that by quickly eliminating animals that are carriers or affected by a genetic disease, the genetic diversity of the breed is reduced and the incidence of other diseases is increased. Breeders utilize genetic tests as one factor in making well informed breeding decisions and puppy buyers can also utilize them to become well educated consumers in their effort to find a healthy new addition to their family.

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