Bay horse question

There is probably no easy answer to this, but I am curious, what makes one bay horse be a deep almost black looking bay, and what makes another horse be a firey red bay? And when breeding bays together, if you breed two dark bays are you going to get a dark bay? Or could they be any shade of bay?

If you breed two bays, you'll get either bay, chestnut, or black. My friend's thoroughbred is a flaxen chestnut and both of her parents are bay.

You're right. There is no easy answer.

Sooty is one factor that can make horses different shades. Countershading is another. Both of these put dark hairs into the coat. Given that both of these are thought at this time to be independent of any specific colours, they may or may not be passed down to offspring.

Even without sooty at all. I always wondered the same. Like a dark/black bay and a "blood" bay. Whats genetically the difference? Is this what you mean? I would like to know myself as well!

As of right now (I am prefacing with this as you never know what they will discover about color genetics in the future), there is no genetic difference between the shades of bay (or shade of chestnut/sorrel or shade of black).

I was under the impression that there are genes which can alter the amount of dark/light "shading" on any horse? (like Sooty, or Pangere (sp?))

Sooty and pangere can do that. BUT those are not the genes that affect what shade a color is expressed as. They modify it.

Basically black and red (chestnut/sorrel) are the only two base colors. Everything else is a modification of those color, I.e. A bay is a black that has had the color modified by agouti into restricting black to the "points," and so on and so forth with the rest of the genes.

Ah...yes, that's right. I think I was not quite "getting" the original question at first so I reread it, I understand, now, what's being asked.

A "black bay" is caused by the At allele, while a bright bay is caused by the A allele. A is dominant over At. Pet DNA of Az is the only lab who tests for At, with all other labs, they only test for a (no agouti, so black stays black vs being bay) and so anything not a is reported as A, even if it's At.

At is brown.

Black bay is a misnomer. There are three variations to agouti. Bay (A), Brown (At), and Wild Bay (A+)