There are two different blue genes in the fancy – British blue and Russian blue. Both are entirely separate genes, with the only similarity between the two being that they give a grey colouring that’s been named blue.
British blue is at the D (dilute) locus. The alleles at this locus are:
- D – Dominant wild type (so “no effect”), the rat has normal dense pigment.
- d – Recessive dilute blue. The black pigment is diluted to an even mid grey-blue. Yellow pigment is diluted to a soft fawn colour.
Russian blue is at the Rb (Russian blue) locus. The alleles at this locus are:
- Rb – Dominant wild type (so “no effect”). The rat has normal dense pigment and a normal coat.
- rb – Recessive Russian blue dilute gene. The black pigment is diluted to a deep steel grey, and the yellow pigment is reduced to fawn. In this case, the pigment doesn’t dilute evenly, resulting in clumps of pigment on the hair shaft – this gives a speckled effect to the colour called heathering. This gene also affects the fur, giving a somewhat shorter and denser coat than average in rats without the Russian gene.
Confusingly, different symbols are used for these genes in different countries, but in the UK the notation used is pretty fixed. In the US, British blue (which is known as slate blue, American blue, or just blue) is given the symbol “g” (for grey), and Russian blue is assigned the “d” notation.
Occasionally breeders across the world talk about other varieties. In Europe there have been claims of a third shade called German Blue, in America a new gene has been called Midnight Blue, and “Leaden” has been talked about in the past. While these varieties may well exist genetically they have not been widely bred or standardised in the fancy.