Originally Posted by Celeste
What is the genetics of brown?
Brown is a mutation of the agouti locus, related to but different from bay. It restricts black differently, in that it actively tries to keep black off the "soft" points of the horse first and foremost, namely the muzzle, eye, elbow, flank, and under the tail. A brown horse can be very light, enough to mimic bay, or dark enough to be close to black.
Genetics wise, brown is notated as an "At" on the agouti locus. It is thought to be recessive to classic bay, but the order of dominance is not really well understood as some bay horses have produced brown foals, while some brown horses have produced bays. As it is a form of agouti, the horse must be black based, as it will not express on a red based horse.