Genetics And Breeding Strategies:
Essays For The Dog Breeder

By Dr. Susan Thorpe-Vargas
Reprinted with permission.

Appendix 3

"The Additive Relationship is the most commonly used measure of relationship. It is a measure of the fraction of genes shared by two animals and thus is an indication of how reliable one of the relative's records will be in predicting the genetic value of the other animal. The Inbreeding Coefficient of an animal is calculated as one-half the Additive Relationship between the parents." (p199, "Genetics for the Animal Sciences," LD Van Vleck, EJ Polak, EAB Altenacu, 1987)

Additive Relationship is twice the Coancestry (which is also called the coefficient of kinship or of consanguinity), "The coancestry of any two individuals is identical with the inbreeding coefficient of their progeny if they were mated. Thus the coancestry of two individuals is the probability that two gametes taken at random, one from each, carry alleles that are identical by descent." (p85, "Intro. to Quantitative Genetics," Falconer & Mackay, 4th Ed., 1996)

The important difference between the COI and the coancestry (one-half the additive relationship) is that one, the COI, refers to the individual animal. The coancestry refers to the genetic similarity between two animals.

A bitch "Jacki" might have a high COI. A dog "Jessie" might have a high COI. But the coancestry of Jacki and Jessie might still be low or even zero. If this is the case, then both Jacki and Jessie are inbred (in fact, the entire breed line might be inbred), but there is still significant genetic diversity in the breed line if Jacki and Jessie have a low coancestry.

Continuing - If Jacki and Jessie have high COIs, they are prone to all the problems associated with inbreeding. If the average COI for the breed line is high, then the whole breed line is inbred and is likely to suffer the consequences as reflected in rates of hereditary defectives, increased puppy mortality, and reduced longevity. Yet, a low average coancestry for the breed line implies that there is a lot of latent genetic diversity in that same breed line. If so, the breed line can be salvaged by the appropriate choice of mates.

So when you discuss the genetic health of a single animal, consider its COI. When you discuss the latent genetic health of a breed (or a breed line), consider the average coancestry. To compare coancestry figures be certain that they are computed to the same depths of pedigrees (same as with COI comparisons).

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