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Sooty has 3 alleles

S = no sooty, no darkening


Ssty = Normal sooty which darkens coat color.


Ssty+ = Sooty +, super dark, causes dappling


The more Sooty or Sooty+ alleles a horse has the more the coat is darkened. Sooty affects black and red pigmented horses.

Dense Pheomelanin DP is a hidden gene that doesn't show up in a color genetics test. Presumably there is a dominant allele where DP is expressed darkening red pigmented horses and a recessive allele that results in a normal coat.

Example of a normal chestnut (no DP):


And two horses that are tested to be liver chestnuts (having DP) though one could be called brown and the other black if you saw it in real life and didn't know its genetic makeup:

Enjoying learning about the genetics as I find posts like this- thanks for asking it, Cheers :)
I have never seen a dapple like that Ssty+ and its soooooo pretty!