I feel like I'm on a role here, going through all these stacks of paper about this and that, and getting the ideas in a collection that makes sense. Of course, the piles barely look dented, but I'm trying at least!!

Hooded Rats...
The hooded locus is a piebald locus, producing the white color or absence of pigment in specific areas of the coat. There are two alleles on this locus: H and h. The capital or dominant H produces a solid color, while the lowercase or recessive h produces the lack of color. A rat that is HH will be solid and without white markings on the coat. Hh is where there is the most variation, from the Irish to the Berkshire and everywhere in between. The body of these rats will be colored, but there will be anything from a small spot of white on the belly to an entirely white belly. Finally, the hh rat will be hooded. Again, there is some range here too, based on other genetic modifiers. In general, a hooded rat has a colored head and dorsal stripe, while the rest of the body is white.
But, how does the color appear on the body like that from a genotype? Well, simply, this hooded gene affects how pigment cells are distributed on the body. The mutation causes a delay in the migration of cells, but before I get to far, let me explain this migration. As an embryo develops, there is a clustered area of cells along the neural crest (dorsal area) that migrate to different parts of the body. These cells then differentiate to form everything from organ tissue to pigment cells (melanocytes). Within these cells, there are vesicles called melanosomes that actually contain the particles of pigment. When these vesicles travel to the edge of the cell, they lyse and release the pigment into the growing hair, where we then can see this coloration. Now, the coloration itself is a whole seperate animal (agouti banding vs. black mutation) that will be addressed some other time! At this point, lets just say that there are two types of pigments, the yellow-red ones called phaeomelanins and the brown-black ones called eumelanins. And, to further complicate things, the pigments can switch back and forth in production, creating the banded agouti colors. But, I digress! As stated earlier, the hooded mutation delays the melanocyte migration, so the areas of the body furthest from the dorsal area do not have the pigment cells, so they remain white or unpigmented. Because there is much variation seen with the hooded allele, it is proposed that there are different alleles that help determine how much delay occurs, creating the irish, bareback, berkshire, etc. Interesting stuff for sure!

References:
http://www.spflrc.org/user/rats/CoatColor.htm
http://www.spflrc.org/user/rats/CoatColorMutations.htm
http://www.ratbehavior.org/ExperimentalCoatTempmt.htm#Hooded
http://embryology.med.unsw.edu.au/Notes/ncrest.htm 
http://links.jstor.org/sici?sici=0027-8424(19880715)85%3A14%3C5325%3ACAADPO%3E2.0.CO%3B2-H