PART 2 – SOME COLOURS
by Sheila Sowter
What the rat looks like (its phenotype) will depend on what
instructions its genes give (its genotype). Rats with different genotypes may
have the same phenotype (remember that a rat that is cc will be albino, no
matter what colour instructions are being given by other genes) and rats with
the same genotype for the genes at the major loci (the only ones I shall deal
with) may look slightly different due to genes at other loci, which have only a
slight effect (making the coat a little paler or darker for instance). It may
help to know that there are two colour pigments in the rat’s fur, one is black
or brown (eumelanin) the other is yellow or red (phaeomelanin). [This is true
of most mammals, which is why furry animals are not green or purple]. The genes
act on one or both pigments to give the range of coat colours we see. Let’s
look at the loci to see the possibilities.
A locus, the
agouti locus. An agouti rat has hairs with two bands of colour, black with
orange outer tips. The agouti allele A, which is dominant will ensure the rat
has two colour hairs, the other allele, a non-agouti, is recessive and aa rats
have hairs that are the same colour all the way along; these rats are the self
coloured rats.
B locus, the
brown locus. The dominant allele B is a ‘do nothing’ gene. There’s nothing to
notice, but the recessive turns black pigments brown, an aabb rat will be
chocolate and an A*bb rat chocolate agouti.
C locus, the
albino locus. Here we meet the problem of more than two alleles needing more
symbols. The most recessive is c, the albino gene and a cc rat is albino. A
bit more dominant is the ch the Himalayan gene, chc rats
are Himalayan, chch rats are Siamese, completely dominant
is C full colour, so CC, Cch, and Cc rats are all full colour. There
may be other alleles on this locus [In other species chinchilla is due to a gene
on this locus, but not chinchilla in the rat, which is due to the interaction of
several different genes].
D locus, a blue
locus. It is probably the Russian blue that is on this locus. The recessive
allele fades the black pigment to a bluish colour and the yellow to something
very pale.
G locus, a blue
locus. Here the recessive allele produces the ordinary blue, also called
English blue or American blue. It seems fairly certain that the blues we have
had for some while are gg blues, and dd blues are Russian blues.
M locus, the
mink locus. Like B, the recessive allele turns the black pigment to a brown
colour (not the same one) so aamm rats can be mink, A*mm can give cinnamon.
Pe locus, the
pearl locus. The allele Pe that produces a noticeable effect is dominant but
only shows itself in an mm rat (mink or cinnamon) where its effect is to bleach
the base of the hair leaving the mink colour only at the tip. A ‘mink’ rat with
a pearl allele, is a pearl, a ‘cinnamon’ rat with a pearl allele is a cinnamon
pearl. This allele Pe is ‘homozygous lethal’, that is, rats that get Pe from
both parents and so would be PePe are not born.
P locus, the
pink eyed locus. The recessive gene p turns eyes pink, black pigment to pale
brown and may make the yellow pigment slightly paler, so a black rat becomes
champagne and an agouti becomes silver fawn. Pink eyed minks are also
champagne, pink eyed cinnamons are silver fawns.
R locus, the
red eyed locus. The recessive gene r turns eyes ruby (though they may look
black), and has an effect similar to p, giving topaz from agouti and buff from
black. It is said that r is not completely recessive so an Rr rat may be
slightly different from an RR rat, but I have no first hand experience of this.
These are the genes known to
affect coat colour in the rat. There may be others we haven’t identified yet.
Since they all affect coat colour, there are combinations which give slightly
different colours. A rat that is blue but also mink mm (a mink-blue or
blue-mink) is a paler blue than the one that is not mink, MM or Mm, and as
mentioned earlier there are many genes that have a very slight effect.
So much for the colours, next
time we can tackle the patterns.
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