Not so long ago, the only way for a breeder to know what genes—good and bad—her dogs carried was to breed the dogs and watch the offspring. Now science has expanded the tools available to the responsible dog breeder. As more knowledge is gained, breeders will have better and better tools at their disposal to eliminate or control many serious inherited disorders from their dogs.
The mode of inheritance is the way in which a trait is passed from parent to offspring. If a trait is expressed, it occurs in the individual. (A person with blue eyes expresses the gene for blue eyes.) Genotype is an individual's genetic makeup, while phenotype is the individual's expressed traits, the ones we can experience with our senses.
Whether we want to reduce the occurrence of a problem trait, or increase the occurrence of a desirable trait, we need to know its mode of inheritance—that is, how the trait is inherited. We also need to know how to identify the trait as early as possible in puppies. Sometimes that can be done just by looking at the puppy—if we're concerned about color or markings, we can see those very early in the puppies of most breeds. Your dog's underlying genetic makeup is his genotype. The traits that we can physically see make up the dog's phenotype. Phenotype usually reflects a combination of genes and environmental factors that affect development. For instance, your dog's potential adult size is determined by his genes. But his adult size is also affected to some extent by the quality of food you give your dog, his general health during his growth phase, and sometimes other environmental factors.
The modes of inheritance have not yet been determined for many traits thought to be inherited. Interestingly, some traits that appear in more than one breed are not always inherited in the same way in the different breeds, and the traits sometimes behave differently from one breed to another. Finally, although many problems occur in multiple breeds, some occur only in a handful of breeds, and some breeds simply never inherit certain disorders.
New tests to detect problems are being developed every day, and the number of inherited disorders for which breeders can have their dogs screened is rapidly increasing. Until recently, it has been difficult (if not impossible) to eliminate diseases that are passed on by dogs who do not have the symptoms of the diseases. Now, however, DNA can be used to determine not only if a dog has some disorders, but also if he carries the genes for those disorders. As scientists learn more and expand the number of tests available, breeders will be able to screen all breeding animals and prevent transmission of many more inherited disorders.
Not all inherited disorders can be detected or prevented through testing, but you greatly increase your chances of getting a healthy pet if you insist on seeing the parents' screening test certificates. Let's look at the common inherited disorders for which tests are now available.
Dogs are prone to a number of inherited defects of the skeletal system. The most prevalent of them is canine hip dysplasia (CHD), a malformation of the hip joint. CHD cannot be detected by watching the dog move, so don't be misled by anyone who claims she doesn't need to x-ray her dogs since she can see that they're okay. Not all breeds are prone to CHD, but many are. If you've chosen a breed in which CHD occurs, be sure to see certification showing that the parents are free of hip dysplasia. Preferably, the puppy's grandparents and great-grandparents will also be certified free of CHD, as well as siblings in each generation and prior offspring if they have any. In other words, you want to see as many relatives as possible certified free of CHD.
Chew on This
Mixed-breed dogs are not necessarily healthier than carefully bred purebreds. A mixed-breed dog can, in fact, inherit the potential for all the diseases that occur in all the breeds in his ancestry. Hip dysplasia, epilepsy, allergies, and other disorders that occur in many breeds are not uncommon in mixed-breed dogs.
The OFA rates hip structure by evaluating x-rays. The dog must be at least 24 months old when x-rayed in order to be certified, although preliminary evaluations can be made earlier. Dogs that are considered free of CHD are rated Excellent, Good, or Fair. Dysplasia is also ranked at three levels of severity.
The Pennsylvania Hip Improvement Program (PennHIP) uses a different method, also requiring x-rays, and can evaluate puppies as young as four months. PennHIP provides two numbers. First, they provide a distraction index for each hip that indicates the laxity, or looseness, of the hip joint. Laxity is considered an accurate predictor of degenerative joint disease. Second, PennHIP provides a percentile score showing where an individual dog ranks in relation to all members of his breed that have been evaluated by PennHIP. The percentile ranking may change as more dogs are tested, but the laxity index will not.
Some breeders now use PennHIP, but most use OFA. A few use both systems. Several other programs are also available, a few of them limited to single breeds, so once again, you need to do some research to determine what's appropriate for your chosen breed. Whatever system is used, your breeder should show you an original certificate or copies of original certificates verifying that the parents are free of CHD. You can also use your potential puppy's pedigree (which the breeder should be happy to provide prior to selling you a pup) to verify OFA or PennHIP certification.