The field of genetics has progressed rapidly in recent years. Perhaps you’ve seen headlines about these top genetic topics in 2013. These stories show the importance of genetics and how it affects us as individuals and as a society. To understand the impact, though, one may need a review of Genetics 101: dominant vs. recessive disease traits.
In order for our bodies to work properly, our DNA must be coded in specific sequences. DNA sequences are grouped into units called genes, which tell our bodies what to make to build cells and metabolize nutrients. We are all a unique combination of re-shuffled genes from previous generations. Everything from eye, hair and skin color, muscle, bone, etc. is coded by genes. A mutation in a gene usually causes something to change and many of these changes can lead to disease. There are thousands of genes, and in humans, thousands of genetic disorders that result from mutations.
One way to classify genetic disorders is to group them by how they are inherited. With the exception of the sex chromosomes, X and Y, each of us has two copies of our genes. One comes from our mother, the other from our father. Recessive conditions need both copies of the same gene to have a mutation in order to show symptoms. Those who have only one copy of a mutation of a recessive condition are called carriers. Carriers have no symptoms, and the disease does not affect their health, but, it can affect the health of future generations. If a breeding pair are both carriers of the same genetic disorder, they have a 25% chance of having offspring with that disease, 50% chance of having carrier offspring (no symptoms), and 25% of having offspring that are not carriers or affected.
Dominant disorders work differently than recessive conditions. It only takes one copy of the gene with the mutation to have the disease. With dominant disorders, there is a 50% chance for the offspring to be affected like the parent. Sometimes, there is a new dominant mutation in which the mutation was not inherited from a parent, but rather, new in that individual. Both dominant and recessive disorders may show symptoms at birth, or they may not develop until later in life; these are called “adult onset”.
The concepts of dominant and recessive can easily be applied to dog breeding. For example, polycystic kidney disease in Bull Terriers is caused by a dominant mutation in the PKD1 gene. Symptoms may be variable, and may not be diagnosed before breeding. Kidney cysts do not usually cause pain but may lead to kidney infections, frequent urination, poor eating and weight loss. In some dogs, the disease will progress and lead to kidney failure. A breeder may choose not to breed a dog at all with this dominant mutation in PKD1, knowing there is a 50% risk of passing this on, which may affect many pups in a single litter. Remember, it only takes one copy of this gene from either parental line for this disease to potentially show up in the next generation.
Degenerative myelopathy is a progressive neurological disorder found in dozens of breeds of dogs that is inherited in a recessive manner. It’s very similar to Lou Gehrig’s disease in people. Degenerative myelopathy may be misdiagnosed as arthritis in the early stages. Dogs eventually become unable to walk. An affected dog may not be diagnosed before breeding because of the later onset of symptoms. Because this is a recessive condition, it requires two copies of the mutated gene to be affected. Dogs with one copy of the mutation have no symptoms. It is important to identify carriers when breeding. Owners may not want to breed two carriers of degenerative myelopathy together, because there is a 25% chance of pups being affected. It is possible that several pups in a single litter could be affected. Although they may choose to breed a carrier with a non-carrier, because there is not an increased risk of having affected pups, this will keep the disease mutation in the breeding lines, which may be undesirable.
Genetic testing technology is increasing at an exponential rate. Understanding some basic genetic concepts may help owners breed “smarter” with the potential of healthier dogs. A healthier dog means a better quality of life for the dog and the owner.
I find genetics fascinating. I breed Papillons, so I feel blessed they do not have many health issues. However I do have some concerns regarding patella's. I am not clear if this issue is a dominate or a recessive trait? I understand with PRA and Cryptorchids that both parents have to carry the genes for those issues to result in their progeny. However I am not sure about patella's? I also am curious to know about umbilical hernia's. I am thinking with my experience that umbilical hernias it is a dominate gene because both times that I had 2 different papillons that produced them, when taken out of my breeding program, I stop getting puppies with umbilical hernia's. However about 3 years ago, I choose to get a umbilical hernia fixed in a male and so far he has not produced any puppies with it. Also I am curious to know how cataracts work? If a dog has cataracts and never produces puppies with it, then would that means he is not a carrier or that it is a recessive gene? Also if neither of that dogs parents have cataracts, then is one to presume that the cataract in the dog was either a fluke or that both of his parents are carriers? Any comments on these matters would be helpful. Thank You. :0)
Thank you for your comment. I'm Dr. Casey Carl, a veterinarian and the associate medical director at Paw Print Genetics. The inheritance of luxating patellas hasn't been worked out though it is generally believed to be an inherited condition. It is much more common in smaller dogs than large breeds. In most cases of inherited luxating patellas, the groove which the patella normally slides in is shallow, allowing the patella to pop out of place. It is likely that the inheritance of this condition is complex involving multiple genes and environmental influences all playing a role. That being said, I would personally avoid breeding affected dogs or dogs that have previously had affected puppies to help prevent the condition.
You are correct that cryptorchidism and most PRAs are inherited in a recessive manner. However, there is also at least one known dominant form of PRA as well (Mastiff and Bullmastiff).
Your observations about umbilical hernias are interesting. Umbilical hernias, like luxating patellas have an unknown inheritance. It is likely that umbilical hernias also have multiple genes contributing to the condition. Interestingly, there appears to be a link between umbilical hernias and cryptorchidism in some breeds, in which puppies will be born with both conditions. If truly dominant, I'd expect that about 50% of the puppies born to your male that you had surgically corrected to also have hernias. Since you haven't seen any, it is suggestive of a recessive inheritance (which is what most researchers believe). However, some dominant diseases show "incomplete penetrance" meaning that not every dog with the disease causing mutation will develop the condition.
Cataracts can be placed into two general categories; acquired and inherited. Acquired cataracts can occur for a variety of reasons including certain viral infections and chronic exposure of the eyes to UV light (most commonly seen in old dogs). This form is not predictable or inherited.
Among the inherited forms of cataracts, there are two known mutations. Both of the mutations occur in the HSF4 gene. One form is recessive and one is dominant. To my knowledge, there has not been a mutation identified in the papillon for cataracts. However, if a dog has inherited cataracts and has not had puppies with it, the disease would most likely be recessive. If bred with a carrier or another dog affected with cataracts, you would likely see puppies with the condition. If a dog is born to two parents with normal eyes, it is most likely that either both parents are carriers of the mutation and don't show clinical signs or the cataracts were actually an acquired form instead of inherited.
I hope I helped answer your questions! If you have any additional questions, feel free to email me at email@example.com.
Casey Carl, DVM