Beginning
with the basics is probably best, so this first section features Agouti
and Black and how color is
produced in a rat's fur, which is critical to the understanding of other
color mutations.
A
rat's hair is made up of several layers, but it is the cortex of the hair
that contains the pigments that make up the overall color of the rat. As one
would assume, mutations to genes that affect pigment cells or their
development and distribution can create a rainbow of colors and markings.
Pigment cells are called melanocytes, and within them are vesicles or little
sacs (melanosomes) with particles of pigment. These vesicles will rupture as
they go to the surface of the cell, leaving behind the visible color on the
rat’s fur. There are two main types of pigments that are important to
understand rat colors, as well. The first is phaeomelanin, a yellow-red
pigment. The second is eumelanin, a brown-black pigment. To further
complicate matters, these
 two
pigments can switch back and forth in production, creating a banded look to
the hair. In rats with NO mutations or dilutions to color, this banded
pattern will occur. This is the agouti rat, or wildtype. It is referred to
as the wild-type because it is the original version with no modifications
that in turn create differing colors and coat types. Rats that are
non-agouti in color (those that have no bandin g
of color on the hairs) have a mutation that makes the body stop producing
the phaeomelanin pigments. They only produce eumelanin. In these rats, the
mutation causes them to have none of the agouti protein, the protein
responsible for the body’s switching back and forth of pigment production.
Genetically, agouti is a dominant gene. This means you only need one copy of
the gene to have the rat be agouti or with banded hairs. The gene letter
used to represent agouti is ‘A’, and in it’s capital form, it indicates a
dominant allele (one copy of the gene). So an agouti rat is ‘AA’ or ‘Aa’. If
a rat has no copies of the dominant gene, it is ‘aa’ and will appear black
in color because there is no banding on the hair. All rats have a base color
of either agouti or black, and any mutations to coat color are in additional
to this 'A' gene.
For
RSA standards, an Agouti rat is “Golden chestnut brown with dark gray to
black base fur and black guard hair. The line between the top color and
belly color is to be sharp, devoid of irregularities and brindling. The foot
color should match the top, and the belly color should be a silver gray. Eye
color is black.” The RSA standard for a Black rat is "Deep solid black,
devoid of browning or rusting. The base fur is to be black to the skin, with
foot and belly color to match the top. Eye color is black." Similar
standards are found in NFRS, AFRMA, and RatsPacNW. |
One
of the most common mutation discussed, either on it's own or in combination
with other mutations, is the red-eye dilute or RED mutation. This mutation
interferes with melanosome development. When their development is
compromised, as in this case, those pigments that occur in the melanosome
are not distributed properly to the hair and eyes, resulting in a diluted
color from the standard black (aa) or agouti (A-). Of course, the final
color produced by this mutation also depends on this base, as one has the
banding of color and pigment types and one does not. An Agouti rat with a
RED mutation is called Fawn. A
Black rat with the same mutation is called Beige.
Genetically, the RED mutation is recessive. This means that you need two
copies of the mutated genet to have the rat's color be diluted. The gene
letter used to represent RED is 'R', and in it's capital form, it indicates
a dominant allele (one copy of a the gene). So, a Fawn rat is ‘AArr’ or ‘Aarr’,
and a Beige rat is 'aarr'.
For RSA standards, a Fawn rat is "To be a rich, golden fawn evenly ticked
with silver guard hairs. Undercolor to be pale blue/gray carried down to the
skin. Belly fur to be silver. Top color to be even and carried well down the
sides with a clear differentiation between belly and top color. Any tendency
to sootiness or grayness of the top color to be penalized. Eye color is dark
ruby." The RSA standard for a Beige rat is "A warm tan. Eye color is dark
ruby." Similar standards are found in NFRS, AFRMA, and RatsPacNW.
Another thing that seems important to mention here is that anytime you have
a mutation in a cell, there are often multiple things that are affected.
Obviously, color is one in the RED mutation, but their are some less visible
things that are also affected by it. For instance, Fawn and Beige rats may
have abnormal platelet function, which reduces their ability to clot
properly. The disorder is called Platelet Storage Pool Deficiency. Serotonin
uptake disorder can also be found in RED rats. These can lead to any mood
alterations ranging from depression and anxiety to bipolar or
obsessive-compulsive behaviors. How does something like this result from a
mutation in a pigment vesicle's development? Certain platelet dense granules
have a common component in their development, so it is thought that the
mutation affects this particular area. Platelets are also partially
responsible for serotonin uptake. Since this is not the true purpose of the
article, but still needs mention, here are a couple links to read more about
it if you are interested. Visit
http://physrev.physiology.org/cgi/content/full/84/4/11 and
http://bloodjournal.hematologylibrary.org/cgi/reprint/66/5/1196.pdf.
Next is the pink-eye dilute or PED
mutation. This mutation alters a protein that is in the membrane of
eumelanosomes, the brown/black pigment producing cells, which changes the pH
of these cells. Normally there is an acidic pH, and the eumelanin pigments
are produced as usual. But the PED mutation alters the pH to a more neutral
environment, and less of the eumelanin is produced. The other type of
pigment found in a rat's hair, which you will recall is phaeomelanin
(yellow/red pigment), is not affected by this mutation though. The result is
a pale coat and pink eyes. In an agouti (AA or Aa) rat, this is Amber, and
in black (aa) rats, this is Champagne. No significant health issues have
been noted that coincide with this mutation, but melanosomes, in general,
are usually fewer in number and can also be misshapen. Genetically, the PED
mutation is a simple recessive. This means you need two copies of the
mutated gene to have the rat's color be diluted. The gene letter used to
represent PED is ‘P’, and in its capital form, it indicates a dominant
allele (one copy of the gene). So an Amber rat is ‘AApp’ or ‘Aapp’, and a
Champagne rat is 'aapp'.
For RSA standards, a Champagne rat is
"to be a very light beige color with no sign of grayness. Belly color to
match top. Eye color is pink." At this time, the RSA does not have Amber
standardized, as it is quite uncommon in the US. However, AFRMA and
RatsPacNW do have a standard for it, which is "a light golden fawn, evenly
interspersed with light hairs. Belly color to be light, but not white. Eye
color is pink." |