Using Relative-Risk Pedigree Analysis in Breeding



Pedigree Analysis - courtesy of Purina and Jerold S. Bell D.V.M.

Using Relative-Risk Pedigree Analysis in Breeding

Responsible breeders are selective about choosing the best dogs to breed. Beyond considering physical characteristics, temperament and colours, breeders try to avoid passing on genetic disorders. Unfortunately, breeders don’t always have the tools and information necessary to make educated decisions.

It is more difficult to predict affected and carrier dogs for some genetic disorders than others. Polygenic traits, meaning two or more pairs of genes involved in heritability, can be extremely challenging for scientists to develop a genetic test. Hip dysplasia is an example of a polygenic disease for which no genetic test has been developed. It is easier to predict affected and carrier dogs for disorders having an autosomal recessive mode of inheritance. In these cases, both parents of an affected dog are carriers, even though they may appear normal. As the number of carriers increase, so will affected dogs. Fortunately, it is possible to evaluate pedigrees for recessive trait risk and use this information to make informed breeding decisions.

Jerold S. Bell, D.V.M., clinical assistant professor of genetics at Tufts University School of Veterinary Medicine, says that when there is no genetic test for carriers of an undesired genetic trait, the most objective tool for selection against recessive disorders is a relative-risk pedigree analysis.

Several factors are important for relative-risk pedigree analysis to be effective, Bell says. Relative-risk analysis is only as good as these factors:

The mode of inheritance must be proven to be recessive.

  • The pedigree information must be accurate and verifiable.
  • There must be an established open health registry database, such as the American Kennel Club Canine Health Foundation’s Canine Health Information Center (CHIC), that records the confirmed affected and carrier status of dogs.

Relative-risk analysis does not identify carriers, just risk. “Through pedigree analysis, you can lower your chance of producing carriers with each generation, but you must limit the number of breedable offspring, so as to not increase the carrier risk of the population,” Bell says.

“If a breeding female has X amount of risk of being a carrier, breeding it to a low-risk mate can cut the carrier risk up to 1/2 X in the offspring,” he says. ”However, if you breed three offspring, then you have added three times 1/2 X into the population. With relative-risk assessment, you have to combine the analysis with replacing the higher-risk parent and limiting the number of reproducing offspring.”

For a defective recessive gene to be passed on, there must be a carrier parent in each generation. However, unless the ancestral carriers have produced affected offspring or were the offspring of affected dogs, they cannot be identified. If four generations separate the carrier sire and an obligate carrier ancestor, then it is possible the shared ancestors between them could be carriers.

Bell provides these statistics:

The parent of an affected dog has a 100 percent chance of being a carrier.

  • The offspring of an affected dog has a 100 percent chance of being a carrier.
  • The clinically normal full sibling of an affected dog has a 67 percent chance of being a carrier.
  • The clinically normal full sibling of a carrier has a 50 percent chance of being a carrier.

This information enables a breeder to assign risk factors to dogs within a pedigree and determine the risk of producing a carrier or affected dog in the next generation. The goal of this analysis is to plan matings that have carrier risk below the average of the breeding population. This also will help to lower the carrier rate for the breed.

It also is possible to find out what the average carrier rate is for a particular trait in a particular breed. "For an individual breeder, this is going to be more difficult, though not impossible,” Bell says. “For a breed-wide relative risk pedigree analysis program, it would not be as difficult. There are statistically sound methods to compute average risk without having to account for all breeding dogs."

For example, a breeder can make a crude calculation of the average coefficient for a breeding population by using catalogs from national breed specialties for the last two or three years. These specialties are normally held in different parts of the country each year, so participants in the shows theoretically provide a good cross section of dogs in the breeding population. The pedigrees of these dogs can then be evaluated based on the information available for a specific recessive trait.

Even without a breed average, a breeder can still calculate risk factors for several different prospective breedings, then compare the risk factors and use this information along with other selection factors of importance.

Here are formulas to calculate a dog's relative risk.

To determine affected risk:

1/2 of the sire's carrier risk x 1/2 of the dam's carrier risk = dog's affected risk

To determine carrier risk:

1/2 of sire's carrier risk + 1/2 of dam's carrier risk – computed affected risk = carrier risk

It isn't always easy to get accurate information about dogs for pedigree evaluation. Unfortunately, not all breeders are forthright with genetic information for fear that admitting a carrier or affected dog may blacklist one's bloodline or kennel. However, reporting this information to open health registry databases is the only way these genetic problems can be reduced or eliminated.

“This is why open health registries are important to the overall genetic health of the breed,” Bell says. “The stigma of genetic disorders should not prevent us from being informed and working together for the betterment of the breed. While tests for carriers allow breeders to test their own dogs, and not rely on knowledge of pedigree background, most recessive disorders do not have tests for carriers.”

Open reporting of health information, through databases such as CHIC, ultimately will provide the information necessary to perform objective relative-risk pedigree analysis.

Breeders and owners must take an active part in the screening processes for genetic traits in order for the incidence of these traits to be reduced or, if possible, eliminated
from purebred dogs.






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