Understanding DNA Testing

 So, you have decided to research before buying a new puppy so that you can find one that is from healthy parents, and one of the topics that is frequently mentioned in your research is DNA testing.  What exactly does this mean?

Many illnesses and health conditions that affect dogs are linked to their genetics, and some of those conditions are breed-specific, meaning that the condition only affects that breed, and any mixes that include that breed.  To understand the statistical likelihood of a puppy inheriting a condition, you have to understand the basic biology of how DNA is contributed from the puppy's parents.  

Every dog has two of every gene that comprises their genetic makeup.  Each parent contributes one gene from each pair of chromosomes to their offspring.  Therefore, 100% of the offspring will have a gene from their sire and a gene from their dam for every pair of genes in their DNA makeup.

As genes get passed down through multiple generations, the likelihood of which genes will appear down the line can get complex, but if a breeder keeps track of the genes for each specific pairing of sire and dam, they can control or eliminate some of the known bad genes.

Genes can be dominant or recessive.  A dominant gene will over-ride a recessive gene when they are paired together.  For instance, in humans, brown eye color is dominant over blue eye color. If a child inherits a brown gene from one parent and a blue gene from another parent, the child's eyes will be brown.  However, the child can still pass along that blue gene to his or her offspring.  The same is true of the various genes in dogs. Everything from coat color, build, and genetic conditions can be passed along, even if it's a recessive trait. 

So, if we take the chart above, and instead of noting the Sire and Dam as a whole, we isolate one gene, such as the gene for Ichthyosis (ICT), we can see what the statistical likelihood is of the offspring either carrying the gene, or being affected by the condition, based on whether one or both parents carry the gene. Icthyosis is a skin condition that is common in Golden Retrievers, and can result in severe dandruff or other more serious skin problems, so a breeder who tests and seeks to eliminate or control passing this gene on, can save puppies and their owners from a lot of irritation and medical intervention. The following chart shows the probability of being affected by ICT based on whether one or both parents carry the gene.

Of course, statistical probabilities are not a guarantee of outcomes.  Any time a parent carries a bad gene, there is at least some chance that the entire litter will inherit that gene. A responsible breeder will either seek to eliminate known bad genes completely from their breeding stock, or will seek to ensure that none of the offspring carrying the bad gene will be bred.  The following list is the current known genetic conditions that Golden Retrievers should be tested for:

Ichthyosis (ICT) - skin condition.  There are two types that should be tested for.

Degenerative Mylopathy (DM) - a neurological condition, usually affecting dogs in their senior years, Loss of hind limb coordination is an early sign of the condition, and as it progresses the hind limbs become increasingly weak.

Progressive Retinol Atrophy (PRA) - eye disorder that causes degeneration of the light sensing retina at the back of the eye, resulting in progressive loss of vision. There are three types that should be tested for. 

 Neuronal Ceroid Lipofuscinosis (NCL) - a group of progressive degenerative diseases of the central nervous system.

Additionally, there are some conditions that have a genetic component, but there are not currently DNA tests for diagnosing them.  Therefore, the only way to check is to have other types of testing done on the parents. OFA recommended testing includes the following:

 Hip/ Elbow Dysplasia - This is a serious and painful condition that can lead to the need for hip replacement surgery.  The condition can be inherited, or it can be caused by environmental factors, so there is no guarantee that a dog with healthy parents will not get it, but parents without the condition are more likely to have offspring that can avoid the condition. Testing for hip dysplasia involves taking an x-ray of the hips after the dog has reached two years of age, and having the joints evaluated by an orthopedic veterinarian.  Elbows are usually evaluated at the same time.

Cardiac Testing - Basic cardiac testing is recommended for any dog that is being considered for breeding.  This involves having the veterinarian listen for any signs of abnormality in the heart. 

 Eyes - Eyes should be checked by an eye specialist on an annual basis for any dog that is being considered for breeding. 

Author
Tamara Forsyth 
Golden Hills Grove