Pigments & Dilutes
First, recall that the pigment cells produce two types of pigment: eumelanins (brown/black) and phaeomelanins (yellow/red). And also, note that the switching back and forth of pigment production is what produces the Agouti color-banded hairs, and this is the wild-type or dominant allele.
In non-agouti rats, the only pigment produced is eumelanin. There is no agouti proteins in these rats, and it is that lack of protein that keeps the switching of pigments from occurring. And for my purposes here, it is just that simple!
And on to the pigment mutations.
A mutation in the brown locus on a rat also affects just the eumelanin by altering an enzyme that normally converts brown colored pigment to black. The result is a rat with brown coloration. In the agouti, the produced color is not standardized; and in the black, this is chocolate. The brown mutation is recessive and occurs like this:
BB/Bb - no dilution
bb - diluted to chocolate
And, on to another mutation. Sooner or later, I swear I'll get through all my little notes here and there..some day!
This time, we'll look at mutations in the chinchilla gene. This gene codes for an enzyme that is responsible for pigment production. If a rat has this mutation, the enzyme cannot function, and there is no pigment at all, producing what is known as an albino. The albino is white with pink eyes and is commonly referred to as Pink Eye White or PEW. The chinchilla gene is interesting though, because there are mulitple types of mutation that can occur, giving way to a variety of genotypes AND phenotypes. The mutations are recessive and occur in this way:
CC/Cc - normal pigmentation
cc - no pigmentation/albino
The other mutations are a bit different though. They do affect the enzyme production, but do not halt it altogether. A diluted color results and the resulting enzyme is fragile and temperature dependent. This type of mutation occurs in lots of animals from cats to rabbits to rats and results in the siamese and himalayan color patterns. The pigment forms only on the cooler areas of the body, like the nose, ears, feet, and tail. Pink eyes are also typical of these animals. The genotype for these rats are as follows:
c(h)c(h) - diluted color/siamese
c(h)c - more diluted color/himalayan
These rats tend to have poorer vision than their normally pigmented counterparts, as well.
References:
http://www.spflrc.org/user/rats/CoatColorMutations.htm
http://spoiledratten.com/articlebasicgeneticscolorcont.html
http://www.afrma.org
http://www.sgul.ac.uk/depts/anatomy/pages/MRholdings.htm
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