You are mostly correct-- except BOTH her parents would have to be heterozygous for agouti AND at least heterozygous for black as well. Both parents had to give her a non-agouti gene making her homozygous for non-agouti (aa) as well as as at least one parent giving her a black gene (E) making her at least heterozygous for black. Agouti and black are not either-or-- a horse who has black (E-) can also have Agouti (A-) -- meaning it would be a bay horse.
If either of her parents had been homozygous for Agouti, they would have passed one to her and she would be bay rather than black. (Chestnuts also have the pair of genes that are either homozygous Agouti (AA) homozygous for non-Agouti (aa) or heterozygous Agouti (Aa) but you can't tell by looking, because Agouti only acts on black and chestnuts do not have black pigment to be acted on, by virtue of being homozygous recessive for black (ee)).
Black (E) is dominant over non black (e) (AKA chestnut), And Agouti (A) is dominant over non agouti (a). That is why, even thuogh black is dominant, you do not see so many black horses-- the Agouti gene is very common in most breeds, and it makes a foal that is otherwise genetically black into a bay.
Also, grey is not a color in and of itself-- all horses are born with a genetic base color such as black, chestnut, or bay. When other "modifier" genes like the greying gene are inherited, they do not cancel out base color-- they simply act on that base color.
The grey gene causes a horse's colored hairs to gradually be replaced with white hairs as they age. So anywhere you see a grey in your horse's pedigree, realize that horse was genetically a bay, black, or chestnut before it turned grey-- but its registry simply called it grey so you don't know what its base color is unless you can find someone who saw it as a baby, or if the horse was gentically color tested, has enough various colored progeny to prove what it packs genetically, etc.
Last edited by Eastowest; 09-15-2009 at 11:45 AM.