Author(s): TrommershausenSmith A, Suzuki Y, Stormont C
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Abstract The chestnut rule in equine coat-color genetics asserts that the inter se mating of chestnut horses never produces bay, black, brown or gray offspring. The gray rule asserts that a gray offspring must have at least one gray parent. Nine alleged exceptions to the chestnut rule, all involving bay offspring, and eight alleged exceptions to the gray rule, including four offspring that were also exceptions to the chestnut rule, were examined for parent-offspring genetic incompatibilities in as many as 17 genetic systems of blood-group markers. In all except one of the 17 cases it was possible to show that parentage had been incorrectly assigned. In 9 of the 16 exclusions it was possible to exclude the stallion irrespective of the mare and in one of the 16 exclusions it was possible to exclude the mare irrespective of the stallion. The percentage of exclusions, i.e., 94, was closely in line with expectation based on the established efficacy of these tests, about 90 percent, in excluding the incorrect stallion or stallions in paternity cases. Although the results strongly uphold the validity of the chestnut and gray rules in equine coat-color inheritance, they do not completely exclude the possibility that there could be rare exceptions to one or the other or both color rules. Insofar as equine registries may be concerned, the results clearly indicate that no alleged exception to the color rules should be considered eligible for registry in the absence of these tests.
This article was published in J Hered
and referenced in Journal of Veterinary Science & Technology