Breaking the canine genetic code

Scientists work to build a genetic map



Introduction

All the world’s creatures have a code locked in their cells, a code that spells out their inherited characteristics. Everything from eye color to body shape and behavioral adaptations are quantified and qualified in the code. Appearance, diet, temperament, and health are embedded in the

DNA, a marvelous spiral-bound double helix of deoxyribonucleic acid and protein that carries the key to life. Dinosaurs and mastodons had DNA, plants have DNA, and amoebas and mosquitoes and snakes and monkeys and humans and dogs have DNA.

DNA is arranged into chromosomes, twisted ribbons of genetic material that carry the destiny of each individual, breed, race, and species. A gene is the DNA-plus-protein at a particular spot on the chromosome. Chromosomes occur in pairs, each of which has a gene for a particular trait at the same location. The genetic code is inherited when one chromosome from each pair is contributed to the offspring by each parent.

The number of chromosomes differs among species. Dogs have 78 chromosomes or 39 pairs; humans have 46 chromosomes or 23 pairs; horses have 64 chromosomes or 32 pairs, etc. Chromosomes come in different lengths and can have 100,000 genes or more in complex organisms.

A genome is a map of the chromosomes that identifies the genes for various traits. Scientists are working to build a canine genome so that structural abnormalities and diseases can be prevented through knowledgeable breeding programs. Money for the research is coming from the American Kennel Club and various breed clubs and breeders.


Genetic diseases in purebred dogs

In recent years, purebred dogs have been described as “genetic nightmares” in the press and by those who champion the cause of mixed breed dogs in shelters. It is true that purebred dogs have many genetic anomalies ranging from those that are innocuous (long coats or unacceptable colors in some breeds, for example) or easy to repair (entropion, the turning in of the eyelid) to those that are crippling or life-threatening, such as hip dysplasia or progressive retinal atrophy.

It is not true that mixed breed dogs are free of genetic diseases due to “hybrid vigor,” a benefit of first-generation crosses between breeds that is lost in subsequent generations. Dr. George Padgett, a leading canine geneticist, wrote in Dog World in January 1997 that mixed breed dogs can have the same genetic diseases as the breeds crossed to produce them. Padgett said that his files include information on 102 genetic defects identified in mongrel dogs, more than double the number identified in the Cocker Spaniel, one of the country’s most popular breeds.

The eradication of genetic diseases in purebreds and mixed breeds is possible, according to Padgett. However, it depends on an alliance of breeders and breed clubs to support the research, open registries to make the research available to breeders, and educational efforts to reach puppy buyers and novice breeders.


Breed clubs

Breeders and breed clubs are the first line of defense in protecting the integrity and health of purebred dogs. Breeds recognized by the American Kennel Club, the United Kennel Club, and the American Rare Breed Association have national and regional clubs devoted to breed activities, and many of these clubs are involved in genetic research projects for breed-specific anomalies.

Discovery of genetic links and of the mode of inheritance (simple or complex, dominant or recessive) is the work of researchers, but they must have dogs to study. Breeders have made their dogs available for research in hip and elbow dysplasia; sebaceous adenitis (a skin disease); copper toxicosis; progressive retinal atrophy (an eye disease); epilepsy; cancer; and more – not only by providing study subjects but by submitting radiographs and other diagnostic tests to various health registries.

Padgett’s Dog World article, one of a series about genetic disease, listed several of the clubs that are deeply involved in educating members about controlling and preventing genetic disease. Breeds represented by these clubs include Alaskan Malamutes, Newfoundlands, Golden Retrievers, Great Pyrenees, Collies, Boykin Spaniels, Nova Scotia Duck Tolling Retrievers, Bedlington Terriers, Cairn Terriers, Cavalier King Charles Spaniels, Doberman Pinschers, Labrador Retrievers, Poodles, Portuguese Water Dogs, Bulldogs, Bernese Mountain Dogs, and English Springer Spaniels.

This is not a complete list; many clubs have joined the battle since the article was published. Clubs have also responded to the call for funds to bankroll research into specific diseases.


Registries

Health registries evaluate diagnostic tests as normal or diseased and make records available to breeders. Some registries are closed; that is, they provide only information on dogs that pass the tests. Others are open; they provide information on all dogs submitted for evaluation and on sires, dams, and siblings submitted for evaluation.

A family picture helps breeders select sires and dams that are not only free of disease but have produced puppies in the past that are free of disease. Unfortunately, some breeders are afraid to use open registries because they don’t want other breeders to find out about problem dogs or litters.

The major health registries are the Orthopedic Foundation for Animals; PennHip; the Canine Eye Registration Foundation, and the Institute for Genetic Disease Control in Animals.

OFA was established in 1966 to evaluate hip radiographs in an attempt to reduce the incidence of hip dysplasia. OFA is a closed registry; it lists only those dogs that have fair, good, or excellent hips. Because it charges the same fee to read all radiographs, breeders and owners often do not send in films from dogs they think will fail to make the grade.

Today, OFA also registers dogs with elbow dysplasia, slipping patellas (knee caps), autoimmune thyroiditis, and congenital heart disease in all breeds and copper toxicosis in Bedlington Terriers. OFA also works with VetGen to screen blood tests for von Willebrand’s Disease, a bleeding disorder affecting several breeds and donates money to support canine genetics programs at Michigan State University, University of Michigan, and University of Missouri.

PennHip provides and alternative method for evaluating hip health. OFA bases its decisions about dysplasia on a single radiograph, but PennHip x-rays the dog in three different positions to judge not only the presence of joint abnormalities but the amount of joint laxity (looseness), an indicator of future problems and of the propensity for passing bad hips along to offspring. All dogs are measured against other dogs in their breed and reports are available on all dogs.

Any veterinarian can x-ray a dog for OFA evaluation, but PennHip trains veterinarians to do it their way. The resulting films are not only read by PennHip radiologists, they are also submitted to OFA for additional appraisal.

CERF is a closed registry dealing only in eye problems. While many genetic tests must be done only once in a dog’s lifetime, CERF tests must be done annually. CERF officials are considering the possibility of opening the registry to record the status of all dogs submitted for testing, not only the ones that test clear.

The Institute for Genetic Disease Control in Animals (GDC) is an open registry. All dogs submitted for the diseases it monitors are placed in the data base and records of littermates and parents are provided to those who request information about a particular dog.

Norma Bennett Woolf

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