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NZ Rat Genetics

In New Zealand there are not many different genes for rat colour and markings, making it reasonably easy to identify what genes each rat has, and what colours and markings are expected in each litter. Obviously it is essential to correctly identify the colour of each parent you in order to have a correct view of what offspring they could have.

Colours

There are only 11 acknowledged colours in NZ, resulting from 5 different genes working in combination with each other. These genes are:

• The ticking gene (A) - this is the base gene that determines whether the rat is ticked (each strand of fur is multicoloured) or solid (each strand is one single colour). It is also known as the agouti gene or black gene. The dominant form of this gene is given the symbol 'A', and results in a ticked (agouti-based) rat. The recessive form 'a' results in a solid (black based) rat if there are two recessive alleles present.
• The mink gene (m) - this is a recessive gene that lightens the fur colour of a rat. Two copies of the recessive gene 'm' are required to see any effect in the phenotype of the rat. If a rat does not have this recessive gene, we write the allele as 'M' (dominant, no effect).
• The ruby-eye gene (r) - often called the RED gene (red eye dilute), this is another recessive gene whereby if the rat has two copies the gene changes the eyes from black to ruby, and also lightens the coat. Again, the absence of this gene is denoted by the dominant symbol 'R'.
• The pink-eye gene (p) - often called the PED gene (pink eye dilute). Changes the eye colour from black to pink, and drastically lightens the fur colouring
• The albino gene (c) - a rat with two copies of this gene is born albino, which overrides and colour/marking information contained in the rest of the rats genes and simply results in a white rat (Pink Eyed White, PEW).

These genes are independant of each other, meaning the appearance of a single rat is a result of the combined effect of each of these seperate genes it carries. To characterise the colours, we add more recessive genes.

Ticked colours:

• Agouti - the wild colour. These rats do not display any recessive genes. They can be heterozygous or homozygous 'A', and must have a copy of the dominant (non effective) genes for the other variations, M, R, P and C.
  • An example of an agouti rat might be Aa MMRRPPCC, which has the phenotype of agouti, and is carrying a single copy of the solid (black) gene, 'a'. An agouti rat carrying a copy of the ruby-eye gene would have the genotype AAMMRrPPCC.
  • Generally, writing the full genotype is tedious, so the gene for an agouti rat is simply written as A-. The dash means an agouti rat could be AA or Aa, the second allele being specific to each individual rat based on its parentage.
• Cinnamon - a chestnut brown colour with black eyes. This colour is present in rats that have both the ticked gene 'A' and the mink gene 'm'. Two copies of the other recessive genes (p and c) must not be present.
  • An example of a cinnamon rat might be AammRRPPCC.
  • More generally it must be A-mmR-P-C-, and since the R, P and C genes code for 'no effect', we simply write the genetic combination as A-mm.
• Fawn - a bright gingery orange, with dark ruby eyes. Caused by having the combination of the ticked gene 'A' and the ruby-eye gene 'r'.
  • Written A-rr, or in full, A-M-rrP-C- (where the dashes represent either the dominant or recessive form of the allele)
• Argente - similar to fawn, but darker and more brown in colour. Caused by the ticked gene, the ruby-eye gene and the mink gene all being present.
  • A-mmrr (or in full, A-mmrrP-C-)
• Silverfawn - honey-coloured rats, with pink/red eyes. Again, the fur is ticked, with the pink-eye gene responsible for the light coat colour and pink eyes.
  • A-pp (or in full, A-M-R-ppC-)

Solid colours

• Black - black rats should be 'true' black colours, but are sometimes more chocolate in appearance, often leading to rats being called the wrong colour. Caused by two copies of the solid (black) gene, 'a'.
  • Genetically aa (in full, aaM-R-P-C-)
• Mink - a dark grey coat, with black eyes. Combination of the solid (black) gene and the mink gene.
  • aamm (or in full, aammR-P-C-)
• Buff - light grey coat, with a tan look to the grey, and dark ruby eyes. Combination of the solid (black) gene and the ruby-eye gene.
  • aarr (or in full, aaM-rrP-C-)
• Dove - very similar to buff, with a slightly darker grey coat - darkness is around half way between buff and mink. Combination of the solid, ruby-eye and mink genes.
  • aammrr (or in full, aammrrP-C-)
• Champagne - very light cream colour, with pink/red eyes. Caused by having both the solid and the pink-eye genes.
  • aapp (or in full, aaM-R-ppC-)
• PEW - pink eyed whites, or albino rats. These rats are any of the above coloured rats, with the addition of two copies of the colourless 'c' albino gene.
  • cc (or in full, --------cc, the blanks indicate that the rest of the genotype has no effect whatsoever on the phenotype).
  • For example, the following genotypes are all PEWs, Aammcc, aarrcc, aaPpcc, etc.

Determining the colour outcome of litters