This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
Chondrodysplaysia was discovered around 1930-1940s. This disease is
neither "dwarfism" as it is commonly referred to nor is it dysplaysia
(in the true sense of the word). This debilitating disease is actually
a birth defect causing the dog's upper foreleg to become overly
massive, short, and twisted and appears in Malamute and related
breeds. Malamute breeders were appalled by this condition when it
appeared and immediately set out to eradicate it.
Steps were taken to locate these recessive genes. By breeding an
unknown dog to a known CHD, the pups were then rebred to CHD dogs and
percentages were calculated. Most Malamutes today have been CHD rated.
The percentage is the actual likelihood of CHD showing up in a
breeding. Malamute breeders tend to agree that 6.25% (one
great-great-great grandparent is a carrier) is the upper limit of
acceptablity in a CHD rating.
Puppies are CHD rated now by taking the CHD factors of both parents
and averaging them together. Example:
Dog 1.75% Bitch 2.01% --------------- (1.75 + 2.01)/2 = puppies 1.88%