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There have evidently been some updates in what we know about how brown and other variations of bay are transmitted genetically. Ammit has brought the game's genetics more into line with what is now known.

The agouti or A gene affects how black is expressed in horses that carry at least one dominant E (black based) allele.

Dominant A, as it has in the past, pushes the black pigment into the points, that is, the mane and tail and lower legs. Recessive a, as before, has no effect at all.

At is the most recessive of the expressions of A. It moves the black pigment slightly toward the points, but especially in conjunction with dominant sooty (Ssty), leaves enough black in the body pigment to produce brown. A horse with AtA, will be bay because the dominant allele A prevails over the At. A horse that is Ata will be brown, whether or not it has dominant sooty. Ssty makes the brown darker. AtAt produces a light brown, especially combined with Ssty because it doubles the effect of the allele, making the horse a slightly lighter bay.

Ata SstySsty
http://www.huntandjump.com/horse.php?horseid=2848728

Ata SS
http://www.huntandjump.com/horse.php?horseid=2849363

AtAt Ssty
http://www.huntandjump.com/horse.php?horseid=2848754

A+ is the most dominant of the bay variations. It produces Wild Bay, a very light bay with muted dark points. It will be expressed when combined with any other bay allele. When combined with one dominant cream allele it produces Buttermilk Buckskin, where the body color is very yellow.

Wild Bay with a single Ssty.
http://www.huntandjump.com/horse.php?horseid=2851903

AA+ SS
http://www.huntandjump.com/horse.php?horseid=2865354

Buttermilk Buckskin
http://www.huntandjump.com/horse.php?horseid=2853533

Thank you. Sometimes I get things wrong, but Ammit is usually quick to correct me, *G*