Mother of Harsesis!
EE aa FF pp Stysty CC Dd rr gg chch zz | toto splspl ww lplp vrnvrn PATN: Large 

EE - homozygous dominant for extension= black-based and will always produce black-based offspring
aa - homozygous recessive for agouti= no bay on a black-based horse
FF - homozygous dominant for flaxen= no change to base color due to flaxen
pp - homozygous recessive for pangare= affects the shades of bay- and red- based horses
Stysty - heterozygous for sooty= affects the shades of many coat colors
CC - homozygous for non-cream= no change to base color due to cream 
Dd - heterozgous for dun= will change the coat color to a dun variation
rr - homozygous recessive for roan= no change to base color due to roan
gg - homozygous recessive for grey= horse will not grey out
chch - homozygous recessive for champagne= no change to base color due to champagne
zz - homozygous recessive for champagne= no change to base color due to silver
toto - homozygous recessive for tobiano= no tobiano pattern present
splspl - homozygous recessive for splash= no splash pattern present
ww - homozygous recessive for leopard complex= no appaloosa pattern present
Lplp - homozygous recessive for white= no white pattern present
vrnvrn - homozygous recessive for varnish roan= no varnish roaning if leopard genes present
PATN: Large - large expression of patterns present
Bottomline: For this horse the extension, dun and sooty all contribute to the coat color. Add them all together and you get a slate grullo. Extension makes it black-based, dun adds the grullo, and sooty adds the slate.

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