Agouti! The party responsible for bay and brown.

Agouti! The party responsible for bay and brown.

by Dainelle Kinsel

As we all well know, a bay horse has a red body and black points; the points being mane, tail, muzzle and at least the lower legs (in the case of wild bay.) The ear tips and legs up the cannon bones may also be black.

-But-

The red seen on the body has nothing to do with chestnut. 

Yep, you heard right. The reddish color actually belongs to a black (EE or Ee) horse that has been influenced by the agouti gene (abbreviated as A or a.) What agouti does is restrict the black on the horse's body to the points.

That said, a chestnut horse (ee) can actually carry the agouti gene but is completely unaffected by it. That's because a chestnut horse (ee) has no black pigment on its body to restrict. 

(Kind of like color blindness is carried by human females but expresses itself in human males.)

(Too far off subject? Ooops!)

OK, I think we all understand what agouti does. Now it's important to understand that there are more than one type of agouti...actually, there's three.

A= the "normal" agouti that produces those beautiful bays

At (superscript "t')= the agouti that produces BROWN

A+ = the agouti that produces WILD BAY (*a working theory-see below)

Ah ha! Yes, browns really are genetically different. For the most part, brown (also called seal brown) can be expressed as:

At- E-.

A is dominant to At and a.

At is dominant to a.

A+- we aren't really sure yet, since it seems to crop up sporadically. *So far it seems its not true breeding, so there's still quite a bit of research to be done.

Now we have gotten through that (and I am so dying for one of those chocolate popsicles...) let's talk about agouti. It's named after rodents of a similar color known as Agoutis. If you have ever seen a Chinchilla, you know what agouti looks like. Tabby cats and many other mammals express the gene, as well.

If a chestnut horse (ee) is bred to a black horse (Ee or EE) and they produce a bay foal, you have your proof that the chestnut is carrying one of the agouti genes (either A or A+) but chances are, not At. If the foal is brown, then the chestnut is carrying At. 

The reason for this is that if a black horse carries agouti, then that horse is either bay or brown.

Just like on chestnut horses, modifiers such as sooty and panagre can affect the shade of the coat: i.e. dark bay, red bay, black bay, cherry bay, sandy bay, mahogany bay....I think we get the idea :D Brown works the same way. Often what's known as "seal bay" or "seal brown" is a brown horse (At- E-) that has the panagre modifier, his muzzle will have a lighter ring around it and often his eyelids will be a lighter color and have a lighter ring around the eyes (also called "toad eyes" and "mealy muzzle." 

Pretty neat, huh?

Bay foals often have mealy or almost-white legs when they are born. These lighter hairs shed out with the foal coat and the mature coat will have black points. This is why colors of foals are not accurate until they are yearlings and in their natural length summer coat. A horse's coat can go through some pretty wild changes as it ages--many domestic horse foals are born with a dorsal stripe that has nothing to do with the dun gene.

In the next installment, we will be discussing the cream gene, or why buckskins and palominos cannot be breeds.