Dog Breeding – Inheriting problem genes

Overall, about one out of every 100 puppies will have a demonstrable congenital defect. Some undesirable hereditary traits commonly seen in dogs include undescended testicles, inguinal and umbilical hernias, abnormally short or absent tails, congenital heart defects, canine hip and elbow dysplasia, luxating patella, wobbler syndrome, malocclusion and incorrect bite, bleeding diseases, congenital deafness, entropion and ectropion, Collie eye anomaly, progressive retinal atrophy, glaucoma, congenital cataracts, idiopathic epilepsy, and behavioral disorders such as inherited aggression and shyness. This list is by no means complete.

A number of undesirable traits are carried by recessive genes. Traits controlled by recessive genes are not expressed unless both genes in the gene pair carry that trait. A recessive gene can be carried down through many generations of offspring, causing no problem until it is combined with a like recessive gene. Simple recessive disorders are relatively easy to control if there is a genetic test for the carriers. If there is no test, control is difficult.

Disorders caused by dominant genes are much easier to control, because all carriers of the genes are clinically affected. Breeders can choose not to breed such individuals and their offspring, thus eliminating the problem from their breeding stock. Sometimes these problems do not show up until later in life, however, and by then a dog may have been bred. So even problems caused by dominant genes may survive in a population.

However, simple dominant or recessive genes do not cause the majority of hereditary problems in purebred dogs. Most of the traits that breeders are interested in are polygenic – that is, they are controlled by multiple genes, the majority of which are unknown. Hip dysplasia, for example, is considered to be a polygenic trait because defects in muscles, ligaments, and bones must be present for dysplasia to occur.

Genetic disorders for which there is no specific genetic test can be identified and their mode of inheritance determined through test breedings and pedigree analysis. Knowing which animals in a pedigree were affected by an inherited disease makes it possible to predict whether a specific individual is at high or low risk for transmitting the disease. This helps greatly in selecting breeding stock. For example, in progressive retinal atrophy, which in some cases is controlled by a simple autosomal recessive gene, both parents must carry a defective gene for a puppy to be affected. Therefore, if a puppy does have PRA, it can be deduced that both parents were at least carriers and may even have had the disease. Other offspring of either of the two parents, and any siblings or half-siblings of the affected puppy, would be highly suspect of carrying the defective gene.

For genetic disorders caused by more than one gene, there is no way to tell whether a dog who tests normal for the disease is a carrier. The probability that the dog is a carrier increases if some of his immediate relatives have the disease. It is important to know whether his litter-mates, his sire and dam, and their litter-mates were affected. The breadth of the pedigree is more important than its depth. In other words, you want to know more about the health status of the extended family, as opposed to looking many generations into the past.

One problem is that even when the frequency of a disease is high in immediate relatives, some offspring who test normal will be carriers and some will not. If there is no genetic test for the carrier state, some genetically normal dogs who may be superior examples of the breed may not be used for breeding, because of their family history. In addition, in breeds where the gene pool is rather small, eliminating normal dogs from breeding can adversely affect genetic diversity.

Health information on the pedigrees of normal and affected animals is difficult to obtain, but may be available through open registries such as the Institute for Genetic Disease Control in Animals (GDC), registries sponsored by breed clubs, or in specific cases through the records of dedicated breeders who are willing to share that information. OFA now offers dog owners the option of making test results open.

In an open registry, the results of a dog’s tests are available to anyone who makes an inquiry. This provides a real service to the public, but many breeders are understandably reluctant to come forward with information that might reflect badly on their breeding programs. Further education and acceptance of the fact that hereditary problems are a concern for all should result in more widespread sharing of this information. Enlightened breed clubs are already moving in this direction.

Certification by the Orthopedic Foundation for Animals (OFA), Canine Eye Registry Foundation (CERF), or the GDC indicates that a dog shows no clinical signs of having the diseases for which he or she has been tested (see appendix D). OFA now has checkoffs for owners to allow their dog’s information to be made available to the public. AKC pedigrees now show OFA and CERF certifications. A pedigree that contains such certifications indicates a low probability of transmitting the disease. As a general rule, the greater the number of normal relatives in a dog’s pedigree, the less likely it is that the dog will carry and transmit a heritable disease.

Carrier dogs can sometimes be identified by a blood-based DNA test, such as the test used for PRA in Irish Setters. However, these tests are available only for certain diseases in certain breeds. The canine genome has now been mapped, so that researchers will be able to isolate defective genes and sites that affect certain traits more easily. As more is known about the canine genome (and this will take several years), newer methods should become available to test for and treat genetic diseases. Already the number of genetic tests available is growing.