Review of Health & Genetics Seminar at the RRCUS National Specialty - May 2002

By Mary Teeling
RRCUS H&G Committee

Our speaker was George A. Padgett, DVM, of Michigan State University of Veterinary Medicine addressing the topic of “Genetic diseases, disease frequency  and gene frequency of the Rhodesian Ridgeback.”

Dr. Padgett spoke specifically on Rhodesian Ridgeback diseases based on an evaluation he did of Ridgeback disease information taken from the RRCUS health surveys of 1996 and 2001. Survey forms had been completed by Ridgeback owners about the health problems of their own dogs, then compiled by the RRCUS Health & Genetics Committee.

Here are some highlights of the seminar:

There are 58 diseases known in the Rhodesian Ridgeback breed.

There are about 6.6 defective carrier genes in each Rhodesian Ridgeback, so essentially all  Rhodesian Ridgebacks are “carriers” for something.

45.8% of Rhodesian Ridgebacks have a genetic disease.

Interesting Note: Mixed-breed dogs have more genetic diseases than purebred dogs. There are 215 known diseases in mixed breed dogs, with 71 percent of them having defective genes. The idea that a mixed-breed dog is likely to have less genetic diseases than a purebred is a misconception.

Matadors, or males that are used frequently for stud, have a profound genetic influence on the breed, since dozens or even hundreds of their puppies are put into the population and some ultimately breed. If recorded, a matador’s traits and diseases, along with those of his get and respective dams, can be invaluable information when matching a female for breeding.

Since hidden carrier genes and mutations can build up over generations, controlling disease in the population can be reduced if information about individuals, the diseases, and genes causing these traits are made available.

Here are some main Ridgeback diseases evaluated from the RRCUS health surveys. Listed with them are 1) mode of inheritance, 2) disease frequency in percents (or the number of dogs affected per 100 Ridgebacks) and guesstimate gene frequencies (the percent chance that your dog is a carrier of that gene):

• Ridgelessness (complex inheritance); disease frequency 10.6%, with a gene frequency of 43.9 %
• Hypothyroidism (recessive gene); disease frequency 5%, with a gene frequency of 34.7 %
• Dermoid Sinus (recessive gene); disease frequency 4.7%, with a gene frequency of 33.9%
• Dermatitis (undetermined); disease frequency 3.6%, with a gene frequency of 30.6 %
• Mast Cell Tumor (polygenetic); disease frequency 3.5%, with a gene frequency of 30.5 %
• Hip Dysplasia (polygenetic); disease frequency 2.2%, with a gene frequency of 25.2 %
• Excessive Aggressiveness (undetermined); disease frequency 1.8%, with a gene frequency of 23.2 %
• Dental Abnormalities: (undetermined); disease frequency 1.4%, with a gene frequency of 20.8 %
• Cataracts (undetermined); disease frequency 1.0%, with a gene frequency of 18.0 %
• Cryptorchidism (recessive/undetermined); disease frequency 1.0%, with a gene frequency of 18.0 %

Control of genetic disease requires knowledge of an individual’s phenotype (genes that are express as actual appearance) and genotype (genes not express but will be passed on to offspring). Males and females that are affected with a disease can help define those related dogs that are carriers of the genes causing the disease, even though they appear normal. It requires honesty and integrity on the part of the breeders and owners to determine the males and females that show and/or carry genetic defects, and, most important, to acknowledge and record this vital information as a tool for others.

This does not mean that dogs that carry for genetic disease should not be bred. On the contrary: Dr.  Padgett emphasized and re-emphasized the fact that every Rhodesian Ridgeback is a “carrier” for something. Our jobs as breeders is to know what the 6.6 genetic diseases are that our Ridgeback carries for and to breed to non-carriers for those diseases.  Each time a carrier is bred to a non-carrier the percent chance that a puppy will be a carrier is reduced by 50 percent. This is how we reduce and, yes, eliminate genetic disease in purebred dogs.

DNA mapping of all dog breeds genes, also know as The Canine Genome project, will be the ultimate tool in helping breeders identify diseased genes, once the technology evolves. Presently, diagnostic tests (such as X-rays, blood tests and examinations) are the tools used to identify affected dogs that phenotypically express a disease. Relatives of affected dogs (and all family members in the pedigree) can then be looked at to see which are likely to be genotypic carriers of that gene for the disease/trait.

The important aspect is that test results be reported to an open registry that accumulates information on phenotypically normal dogs and affected dogs (such as the Canine Health Information Center, or CHIC). That way, there is a biological and genetic base from which to determine carriers of a problem gene.

CHIC is a coordinated effort between the Orthopedic Foundation for Animals (OFA) and the American Kennel Club, and works with parent breed clubs that wish to establish testing criteria for their breed. CHIC has expanded to include other certifying agencies besides OFA if parent breed clubs which to use them. Penn-HIP is one such example.

While full and open disclosure of tests results is strongly encouraged by RRCUS, it is still the owner’s decision.  Regardless, when a dog has completed its required testing a CHIC number is issued identifying the dog as being genetically tested. Breeders can use this tool to match a sire and dam when planning a mating, and to calculate and reduce the frequency of genetic disease to offspring, creating healthier future generations and, ultimately, the Rhodesian Ridgebacks as a whole.