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3562825 brindle

Why is it called non dun 1, when it is clearly dun 1.

Recessive Dun (d) has no effect on horse color or markings. nd1 is dominant over recessive dun (d); it gives partial dun markings principally the dark shoulder bar and slight dilution of the coat. Dominant dun (D) is dominant over both of the other alleles and is thought to be the most ancient of the 3 versions of the gene.

In this way, it is analogous to the 4 alleles of the agouti gene. Most recessive (a) has no effect on horse color. At (brown) is next on the scale. It is dominant over recessive a but recessive to the other two alleles. This is why a horse must be Ata or AtAt to be called Brown (although AtAt has to be reinforced by Sooty, I believe, for the name to be changed from bay to brown. Dominant agouti (A) is dominant over both At and a, so a horse with AtA is bay, not brown. Wild Bay (A+) is the most dominant of the four alleles and overrides all the others, giving patchy gray areas on the legs and sometimes some lightening of the mane and tail.

This page in the Genetics Guide gives a clear statement of the agouti varieties (clearer than mine, I think).
http://www.huntandjump.com/geneticsguide/?page_id=48 . The information there on the Dun gene has not yet been updated.