The mode of inheritance and the specific genes involved in these diseases are yet to be fully identified. Most of these conditions have complex causes and are the result of many different contributing factors. The aberrant combination of genes which predispose the dog to the disease must be inherited and then the dog is subjected to environmental stresses which exacerbate the condition. Most of these conditions are polygenic – caused by the cumulative effects of a number of genes. Some DNA tests are now available for mutations associated with single-gene traits. The contributing environmental factors in hip and elbow dysplasia are the dog’s weight, the amount and type of exercise, and the puppy’s growth rate. These all collaborate in the expression of the disease. Other diseases such as epilepsy are suspected to have a genetic basis, but the mode of inheritance has not been identified. The club recommends that no dog with a serious genetic defect should be bred from. The minimum requirements to be listed on the GRCV’s puppy list and stud dog list are for both parents to have a hip and elbow grading certificate, a heart certificate and a current Australian Canine Eye Scheme (ACES) eye certificate (eye certificates are renewed annually). An initiative introduced by the National Golden Retriever Council is for all Golden Retrievers born after 1 January 2002 to have their Hip Grading Certificates lodged with DOGS Victoria prior to the registration of their progeny.
Subaortic stenosis (SAS) is a congenital heart disease and in some case an inherited problem in some large breed dogs that ranges in severity. Mildly affected dogs may only require monitoring while more seriously affected dogs can benefit from medication and/or minimally invasive surgery. The heart is made up of two sides left & right. The right side receives deoxygenated blood from the body and then pumps it to the lungs to be oxygenated. The left side receives oxygenated blood from the lungs and then pumps it through the body to be used by important organs. Before leaving the left side of the heart, blood must pass through a special gate called the aortic valve. In dogs with Subaortic Stenosis, the area below the aortic valve (subaortic) is inappropriately narrowed (stenosis). The stenosis is caused by a ring of fibrous tissue that forms in this region. Depending on how much fibrous tissue develops, the stenosis may be mild, moderate or severe. With this abnormal narrowing, the heart has to pump harder due to impeding blood flow, causing increased resistance and continuously generates a much higher pressure. As a result, the heart muscle becomes thickened. Thickened heart muscle is more difficult to oxygenate, and abnormal and potentially lethal heart rhythms can develop when heart muscle doesn’t get enough oxygen and may result in heart failure. The extent of the symptoms and the rate at which they develop depends on the severity of the narrowing inside the heart and will vary between individual dogs.
This abnormal muscle in the heart causes abnormal electrical conduction and the heart’s normal electrical rhythm is disrupted. The pumping and filling actions are coordinated electrically, when this coordination is lost, fainting spells or even sudden death while exercising can result. Most dogs with SAS do not survive beyond three years of age without treatment, although dogs with milder cases can have normal life spans but a significant number will have fatal collapse prior to middle age. Those that live to middle age may eventually develop congestive heart failure. A dog with SAS is always predisposed to electrical arrhythmia, heart failure, and infection of the abnormal aortic valve. A Veterinarian Cardiologist will perform a complete examination. A heart murmur is typically detected when affected dogs are puppies, but occasionally is heard after one year of age. Occasionally, dogs will have abnormal lungs sounds due to left-sided heart failure. Any dog/bitch with a SAS (known as a heart murmur) should not be bred from.
Breeding dogs and bitches over 12 months of age should have their heart checked by a Specialist Veterinary Cardiologist and a certificate issued certifying that the dog is clear of cardiac disease. A heart certificate is valid for the life of the dog, unlike the ACES certificate for eyes with has to be renewed every 12 months. In subaortic stenosis, the left ventricular outflow tract just below the aortic valve has a scar-like narrowing or “stenosis”.
DNA (deoxyribonucleic acid) carries the unique “blueprint” for every living organism and is present in almost every cell of each organism, except red blood cells. In the case of dogs, each DNA molecule contains 78 chromosomes, and these chromosomes carry genes – specific sections of DNA that make up the detailed “building plans” for all the features, or physical traits, in the body. Dogs have about 20,000 genes located among their 78 chromosomes (compared to 46 in humans).
Uses of DNA testing
Genetic testing, or DNA testing, can be used in various ways including:
Confirming parentage – A technique known as “genetic fingerprinting” can be used to provide a DNA profile for any individual animal. Since offspring receive half of their DNA from each parent, DNA profiles can be used to confirm parentage. However, these tests are not based on identification of actual genes; instead, they use other DNA sequences known as markers. While DNA profile tests can be used to positively identify a dog, and to confirm parentage, they do not provide any information about hereditary diseases, physical traits, or even the breed of the dog.
Detecting inherited diseases – Genetic testing is a vitally important tool that can be used by breeders to reduce the probability that their puppies may be affected by an inherited disease
Tests for Golden Retrievers – Contrary to some ill-founded beliefs, there are no DNA tests available for Hip Dysplasia, Elbow Dysplasia, most heart conditions or Hereditary Cataracts of the eyes. The DNA tests that are available for health-related conditions in Golden Retrievers include:
Congenital Ichthyosis in Golden Retrievers – Congenital Ichthyosis is a skin condition in which the outer layer of the skin does not form properly and results in scaling (similar to dandruff). The condition often progresses to large patches of thickened, black, scaly skin.
Progressive Retinal Atrophy (PRA1 and PRA2) in Golden Retrievers – PRA is characterised by bilateral degeneration of the retina resulting in progressive vision loss leading to total blindness. More than one form of PRA affects Golden Retrievers and causal mutations in three distinct genes have been identified; two of those mutations lead to PRA1 and PRA2.
Congenital Myasthenic Syndrome (CMS) in Golden Retrievers – CMS is a group of inherited neuromuscular disorders that are characterised by progressive muscle weakening that worsens with exercise.
Degenerative Myelopathy (DM) – DM is an inherited neurologic disorder of dogs characterised by gradual muscle wasting and loss of coordination typically beginning in the hind limbs.
Neuronal Ceroid Lipofuscinosis (NCL) in Golden Retrievers – NCL results from the accumulation of granules in the neurons of the brain and spinal cord. This progressive neurological disorder manifests as behavioural changes coupled with a loss of coordination and blindness.
Progressive Rod-Cone Degeneration (PRA-prcd) – PRCD is an inherited form of late-onset progressive retinal atrophy (PRA) that has been identified in many dog breeds.
Sensory Ataxic Neuropathy (SAN) in Golden Retrievers – SAN is a progressive neurological disorder characterised by involuntary muscle movements and abnormal posture resulting from degeneration of the nerves controlling muscle movement. It affects both sexes but is only inherited maternally.
DNA Testing by Breeders
Dogs that inherit a faulty gene from either of both of their parents can in turn, pass it on to their own offspring and if the mutation is associated with a known health condition, then it can be of serious concern to breeders. Some conditions are inherited in a dominant manner while others are referred to as recessive. A dominant condition is one that can occur when a dog has only copy of a faulty gene. If a dominant condition has a serious effect on the health of a dog it is unlikely to be passed on to offspring because the dog is likely to die at an early age or be too ill to reproduce if it does survive to maturity. Therefore, health conditions that are both serious and dominant are usually rare.
A recessive condition is one that only occurs when a dog has two copies of a faulty gene – one from each parent. Dogs that have one copy of a faulty gene are unlikely to show any signs of the disease associated with the mutation but can pass it on to their offspring. Most of the DNA tests available for all dog breeds, including the Golden Retriever, are for recessive conditions. Hence, breeders can use DNA testing to reduce the risk that their puppies may inherit a recessive condition – provided the condition is one where a gene that causes the condition has been identified. It is important to remember that every dog is probably carrying genetic mutations associated with many recessive conditions. DNA testing of dogs including Golden Retrievers is at an early stage of development and reliable tests are available for only a small proportion of the known mutations. It is also important to be aware that there are likely to be many more recessive mutations that are currently unknown. Therefore, DNA testing by breeders is important but it does not eliminate the risk that a mating may result in offspring that are affected by an inherited disorder.