Rare or Half Baked?

I recently had an all white Mustang foal born with an item in it's geno that I haven't seen before. Even after reading the amazing Genetics topic by the knowledgeable & helpful vos - I have no clue about genetics. I'm wondering if my horse is rare for EV or it just has a genetic marker that I personally have never come across. (The chances of this being a case of that second option are VERY HIGH.) I'm hoping someone can tell me what the item I'm confused about is/does. I'm also wondering if he's a rare color/geno horse. Any help will be great appreciated! Thank you!

Here's my boy's geno:
ee AAt gg crcr dd chch Zz pp stysty rbrb WT oo spsp lplp patn1patn1 patn2patn2

So I know "ee" usually makes the horse chestnut. (I have an obsession with chestnut horses.) I'm confused about the AAt part.

E or e is for extension, which is what decides if a horse will have black hair or not. EE and Ee means it will have black hair. When a horse is ee, that means it can't have black hair, so it will be chestnut-based.

The A's are for agouti, which determines where the black hair will be. If a horse is ee, that means that, whatever is in agouti won't show up, since there will be no black hair. However, even a chestnut horse will pass on its agouti genes to its foals, and if that foal has an E in extension, agouti becomes important again.

There are several possibilities for agouti. Most dominant to least dominant, they are A+ (wild bay), A (bay), At (seal brown), and a (black.) So, any horse that has an E and at least one A+ will be wild bay. If it has an E and an A (but no A+) it will be bay. AtAt or Ata are seal brown, and only E with an aa will be black.

Your horse is chestnut based. The AAt means that, if your horse did have an E, it would have been bay, but it carries a gene for seal brown that it can possibly pass on to a foal. Of course, because your horse has a W, that means it will appear white rather than chestnut. It also carries Z, which is silver, but silver only expresses on black hair. Your horse has no black hair, so the silver won't show up, but can be passed on to a foal.

Seal brown tends to be a fairly rare and desirable color in this game. If you breed your boy to a mare who is EE, you will have a 50% chance of getting a seal brown foal, and 50% chance of bay. If you breed him to a mare that is Ee, you will have a 25% chance of seal brown, 25% chance of bay, and 50% chestnut. Breeding him to ee will always produce chestnut. (Of course, if the foal inherits the W, it will appear white, no matter the extension or agouti.) If the foal ends up seal brown or bay, it then has a 50% chance of being the silver version of that color.

At is brown :)

like A (agouti) it only phenotypically appears on black based horses.

Thank you both for your help! Is the combination of the AAt & W rare here on EV?

@Confessor

Thank you for the amazing explanation. I just learned so much about genetics and it's not even my horse *_*

you can find out the number by using the genotype function in the horse search.

there are currently only 7 mustangs with AAt and the white gene, and all but one are WSb, WRn, or WT. so yours is pretty rare :)

*palm smacks forehead* I totally forgot about the genotype function! Thank you for reminding me, Kahzie. I normally only focus on stats when I'm searching.

Wow! 1 out of only 7 with AAt & white?! Thank you so much for looking this up for me, Kahzie!