Rabicano-The Rabicano gene(RB) is one of the color or pattern genes that is not completely understood as of yet, and because of that I cannot go into too much detail about it. However, after looking at many lines, especially in Quarter Horses, that appear to produce Rabicano, it appears to be simple dominant gene, which means, of course, that it only takes one gene for Rabicano to express.
Rabicano expresses as roaning, usually concentrated on the flanks, between the front legs in the arm pits, occasionally under the throat latch, and rarely very scattered roaning throughout the coat, but usually only if the horse has a very loud expression of rabicano. The roaning on the flanks can also form into what are called "rib stripes," and that is when the roaning appears to "stripe" over the horses barrel. Rabicano also causes what is known as a "skunk tail" and that is a tail head, and rarely the whole tail, that is striped with both white from the rabicano, and the color of the horse.
Rabicano can express on any color or pattern including gray. It is fairly common in both Arabians and Quarter Horses, though I have seen it in most breeds, and chances are it is in all breeds except where white is prohibited, such as Frisians.
Here are a few Rabicano, also with their theorized genetic combination (based on pedigree, phenotype, and if possible, offspring) on a few different colors.
This is a Black Rabicano Arabian by the name of Aswad Shahwan from Laurence of Arabians. He is tested EE aa, and Arabians have no dilutions so this is a Black Rabicano.
This is Dancing Colors. A loud Chestnut Rabicano Arabian from Amentaah Egyptian Arabians. She is ee ??. You can see the loud roaning through her flanks, and under her throat latch. Though she has flaxen as well, you can slightly see the white in her tail head from the Rabicano gene.
This is KRH Matar owned by Holly Arabians. He is a bay Arabian Rabicano stallion, and he is E? A?. You can see the skunk tail on this horse, and the very faint rib striping and roaning on his flanks.