The AMCA set up a screening and control program to address the problem of
chondrodysplasia in the Alaskan Malamute. Initial screening was done by means of
pedigree analysis. [reference 7]
Using the dog's pedigree, a statistical analysis was performed based upon the number of ancestors that were either known carriers or closely related to known carriers. The analysis yielded a number expressed as a percentage. This number represented the dog's chances of being a recessive carrier of the gene, based upon the configuration of ancestors. If the number exceeded 6.25%, the dog was considered to be in the "suspect" or "potential" category. The value of 6.25% was selected because it represented one known carrier as a great-great-great-grandparent and also corresponds roughly to a 95% confidence level (04.37%). [reference 7]
If the pedigree analysis indicated the animal fell into the "suspect" category, two recommendations were made to the owner.
Another method of testing, based on the blood profile of the animal, saw limited utilization. It was considered superior to pedigree analysis but not as good as test-breeding.
Let the symbols for the various genotypes be as follows.
In test-mating with a chondrodysplastic animal, then:
(+/?) X (dan/dan) => 1/2(+/dan) + 1/2(?/dan)
If all the test puppies in the litter are radiographically normal, then ? = +, and the genotype of the unknown is statistically identified as being clear. If however one or more chondrodysplastic puppies are detected, then ? = dan, and the suspect animal is conclusively proven to be a recessive carrier. Such an animal should be removed from all further breeding, unless it is the control dog in the test-mating of another suspect animal.
In test-mating with a known recessive carrier, then:
(+/?) X (+/dan) => 1/4(+/+) + 1/4(+/dan) + 1/4(?/+) + 1/4(?/dan)
If none of the puppies are chondrodysplastic, then ? = +, and the animal is animal is statistically identified as clear. Upon replacing the symbol "?" with the appropriate symbol "+", collection of terms leads to a more familiar form for this equation.
(+/+) X (+/dan) => 1/2(+/+) + 1/2(+/dan)
If one or more chondrodysplastics are detected in the litter, then ? = dan, and the "suspect" animal is conclusively proven to indeed be a recessive carrier. Again, replacement of symbols (? = dan) and collection of terms leads to the well-known "25-50-25" equation.
(+/dan) X (+/dan) => 1/4(+/+) + 1/2(+/dan) + 1/4(dan/dan)
It is important to note that for all test-matings, the number of test puppies must be great enough to be statistically significant before the dog can be considered clear of the gene. This means a minimum of five puppies for testing against a chondrodysplastic (confidence level = 96.88%) and ten in testing against a recessive carrier (confidence level = 94.37%). Because of these statistical constraints, more than one test litter may be necessary for a single animal. [reference 7]
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Copyright © 2003 Linda Dowdy, last revision 030406