Understanding the Principles of Inheritance
Whether it’s eye color or medical conditions, understanding basic inheritance factors in dogs can go along way in helping you understand just how your pawsitively adorable new puppy ended up with its beautiful brown eyes or a life threatening genetic illness.
Although this article is written with medical conditions in mind, the genetic inheritance factors could just as easily apply to other virtues and faults used to select suitable dogs for breeding purposes.
A gene is the hereditary blueprint which determines what trait the offspring will possess. Many conditions, such as hip dysplasia and other orthopedic conditions, typically follow a pattern of inheritance where more than one gene is involved, called multifactorial inheritance. This means many factors must come into play for the gene to express itself in the offspring.
These types of conditions are also known to have polygenic traits, meaning that the expression of genes is influenced by a number of factors such as nutrition, growth rate, amount of exercise, gender, etc., and the exact genotype and mode of inheritance is often not known.
For this reason, in the case of hip dysplasia and other orthopedic conditions, there is no specific genetic/DNA test to determine if a breeding animal has the genes to reproduce the condition. Breeders can’t go get a blood test or cheek swab to say if their dog is a carrier of hip dysplasia.
Instead, potential breeding dogs are evaluated by orthopedic specialists, and if they are “clear” of having a condition themselves, there is far greater likelihood their offspring will also be clear. And even more so if their parents, and their parents before them and so forth, were also tested clear.
The same inheritance patterns apply to many cardiac conditions and eye problems as well. Their genetic inheritance is commonly known to be multi-factored, and so dogs used for breeding should be evaluated and cleared by canine ophthalmologists and cardiologists, thereby increasing the likelihood their puppies will also be clear.
Although such “clearances” in the case of multifactored inheritance cannot ever guarantee that your puppy will not end up with a genetic disorder, it does give you a fighting chance, and a far greater likelihood that the puppy you purchase will not have problems if the parents themselves (and their parents before them) do not have conditions known to affect the breed. That is the importance of getting “clearances” on breeding dogs.
The Other Side of the Coin
There are other disorders however, for which specific DNA testing is available for certain conditions in specific breeds. These DNA tests determine if a dog is CLEAR, CARRIER, or AFFECTED for diseases such as thyroid conditions, skin diseases, bleeding disorders, and musculoskeletal or neurological disorders, for example.
And part of being a responsible breeder is not only getting these tests, but understanding about the inheritance of these conditions when breeding dogs.
For those conditions which have specific DNA tests to determine CLEAR, CARRIER, or AFFECTED dogs, the inheritance factors are known as to whether a puppy will or will not be affected.
With some disorders, the inheritance factors are straight forward. The genes are either dominant or recessive. With dominant genes, all it takes is one parent to have the disorder for it to show up in the offspring.
These dogs are AFFECTED, and they will reproduce the condition in roughly half their puppies. Responsible breeding practices demand that dogs AFFECTED with an inheritable medical disorder are not ever used for breeding.
It takes TWO to Tango!
A recessive gene is a little more complicated. A dog might not actually have the disorder itself, but they can carry the gene to reproduce it their offspring. These are known as CARRIER dogs, and for puppies to be affected, BOTH parents would have to be carriers, in which case, approximately 25% of their puppies will be AFFECTED.
Recessive genes are the most common form of inheritance of medical conditions in dogs. They are hidden genes in a sense. The dog doesn’t actually have the disorder, but they may carry it in their genes. This is where DNA tests are important to determine whether a dog is a carrier of a certain disorder to ensure that two carrier dogs are not ever bred together.
Some breeders will breed dogs that are CLEAR BY PARENTAGE – meaning both of the dog’s parents were tested as CLEAR, and therefore all of the offspring MUST ALSO BE CLEAR. This is genetically true, but an understanding of inheritance factors is required when breeding CLEAR BY PARENTAGE dogs in subsequent generations.
The best breeding practices involve breeding CLEAR to CLEAR dogs – meaning neither parent has the disorder, nor do they carry the genes to reproduce the disorder. In cases where CARRIER is bred to CLEAR, puppies will only ever be carriers themselves and not be affected by the condition.
Peace of Mind
Take your time, do your research, ask the breeder about the testing of their dogs. And if they don’t test their dogs and get clearances, then you might want to consider whether you want a puppy from that breeder as opposed to a puppy where the breeder has at least attempted to reduce, or eliminate in some cases, the chances of a genetic disorder occurring in the puppy you want to bring home and love.