Articles Index/Breeding
Posted 02/24/03

FOOD FOR THOUGHT: BREEDERS NEED TO HAVE A PLAN TO DEAL WITH POTENTIAL GENETIC PROBLEMS

In a discussion relating to the elimination of genetic disorders in dogs, including whether known carriers of certain conditions should be utilized in a breeding program, Jim Seltzer of Willowind Dalmatians made the following comments. They are presented here, with his permission, as food for thought.

Let me suggest at the onset that this subject is far too complex and multivariate to allow simple `yes' or `no' prescriptions. I shall try to circumscribe the problem as I see it as succinctly as possible. Some of the factors/considerations that will influence breeding policies are:

1. Genetic defects vary as to the degree of severity of the affliction: some are lethal, others merely cosmetic. These should be treated differently.

2. Genetic defects vary with respect to their heritability, penetrance, and expressivity. There probably are no true simple recessives. Not every DNA-flagged "affected" will express the disorder.

3. The gene frequencies of defective alleles will differ for different breeds and may even vary across breed-lines within the same breed and for various disorders.

4. The mean breed COI (coefficient of inbreeding) is not as important as the specific population structure; i.e. intra-breeding kinship may be much higher than inter-breeding kinship implying a greater intrinsic genetic diversity than the mean breed COI would predict.

5. The registered population size is not always an adequate measure of the effective population number which will always be smaller and will depend on breeders' general preferences for or avoidance of clustering about a few popular sires.

Even for a rare breed, elimination of carriers from the breeding pool is unlikely to have a deleterious effect if the gene frequency for a defect is very low. Even in breeds with small populations, not every dog is bred, and selecting against a carrier and choosing instead a clear with a high kinship should preserve much of the genotype of the carrier without actually breeding to the carrier.

On the other hand, even popular breeds that carry genetic defects with high gene frequencies cannot avoid breeding to carriers and still avoid the loss of genetic diversity.

When formulating a breeding policy that aims to diminish the rate of affecteds while preserving genetic diversity, it is most important to consider mean kinship and/or genome uniqueness when culling carriers from the breeding pool. There is little risk to the future of the breed if some of the produce of a popular sire are eliminated, whereas the loss may be irreparable if carriers with low mean kinship or high genome uniqueness are culled.

Jim Seltzer
ANTIC December 2002

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