“… [G Locus] is a recessive gene that causes the red pigments in the fur to drop away leaving the fur a range of blueish greys.”

The G Locus

The blue gene in rats is signified with either a g (grey – usa) or d (dilute – uk), however it is the same mutation regardless of symbol used and is a recessive gene that causes the red pigments in the fur to drop away leaving the fur a range of blueish greys. There are many variances in color, from light pale blues to deep steel colorings, and while these vary and some even have their own standards it is important to understand that they are all just variance in the same locus.

It is also important to understand there is an actual genetic difference between Blue and Russian Blue, as well the more newly discovered Midnight/German Blue. These alleles fall on different loci, and can even interact with each other to create even more interesting variations in coloring.


Blue rats seemingly appeared randomly in pet stock through out the UK and US in the early 1990s. Blue rats also appeared in Australia, though there seems to be less knowledge available on Australian blue rats origins at least in the US. As well in the Western Cape Province of South Africa they seemingly appeared in pet shops in the early 1990 as well. These blue mutations are all the same or allelic mutations as confirmed over the decades by test breeding. The lines when breed together produce all Blue rats and not Black, meaning they are mutations on the same locus. At this time there seems to be at least two alleles on the locus, one being without and one being with a light or white undercoat. 

The origins of the Blue mutation are a bit mysterious honestly, as they were reportedly randomly found in pet stores in the early 1990s in the US, UK and South Africa around the same time. In 1990 Sheryl Leisure found a unique blue rat at a pet store in California (owned by Sylvia Butler?) and named her Tinker. From what I understand there is also records of British Blue imports by the NFRA into America in 1990 that could as well be the beginnings of many American lines. Other imports have occurred as well over the years, leading to the true origin of Blue in America to sometimes be a topic of debate in international discussion. Breedings have proven the two to be the same genetic mutation regardless of true origin. American Blue’s were generally breed for a lighter blue, while British Blue breeders tended to prefer the standard of a darker slate gray, the lighter shade’s eventually being standardized as Sky and Powder Blue. Most Blue standards how ever call for the same deep slate blue whether American or British with the lighter shades being their own standards.

2/21 Update On American Blues; After discussion with several breeders, it seems to be the general consensus that Am Blue is to be considered the variety with a lighter or even sometimes a white undercoat. From my studies this is not mentioned as a uniqueness from American origins but a variation that was present (and not present) in both American and British lines at some point. The UK breed away from it, while the US did not. I could be incorrect here, but this so far is the recorded information on Blue origins and history I have found so I feel it shouldn’t be discredited by what may be current nomenclatures based on selective breeding over time due. With that said, the term American Blue in the US and possibly other countries may be used specifically in reference to the variation with a lighter base coat color. As my line is one of the lighter undercoated American Blue lines I will be working on trying to breed a matching coat color. I highly suspect the allelic mutation theory to be correct however and it could very well be a futile effort, but modifiers could also be the case as well which could be another interesting challenge.

Australian Blues hold uniqueness in that importation of rodents is banned in Australia and so their mutations should be isolated incidents. This means while the Australian Blue mutation could have been the same it could have very well been a different mutation all together. From what I understand the discovery of Aussie Blues was made public around 2001. Eventually importation into America and crossing the Australian lines with American Blues surprisingly determined they were the same gene in 2003 by Camarattery.

Cape Blue is another local variety that comes from South Africa, though at this time I don’t know much about this coloring and how much breeding has been done to test the genetics in comparison to other Blues. From what I understand testing was done with American Blues, and they are the same or mutations on the same locus. According to Imagine Rattery they are in fact a separate allele on the same locus, and have distinct differences. Whether they are the same as British lines has not been tested but as it is at least allelic with Am Blue it could be the same allelic mutation or a different one as well; bringing the possibilities up to 4 separate allelic mutations, several variations based on modifiers or decades of selective breeding.

Blue rats have been very popular since they appeared, but when they were discovered in pet stores unfortunately not all the lines were of healthy genetics. Many Blue rats at first reportedly suffered from hemophilia and even aggression. However the sample size and first hand accounts are very small, and at this point these genetic issues have seemingly been resolved by proper breeding in reputable breeders lines. All though these lines seem to have disappeared you should still be careful with rats from unknown sources as they may still exist in some places, and I have heard of them popping up occasionally.


There are many quite stunning variations that occur with the inclusion of other genes. Platinum is a very popular variety that is a triple recessive consisting of Blue, American Mink, and Pink or Ruby Eye Dilute. Quicksilver is a similar darker shade that sometimes appears in platinum lines. Another variety that is popular is the Russian Silver, which is the Blue and Russian Blue genes together, but many of these variations require specific shades so can be hard to breed. In the future I hope to have more articles for these variations finished.


BLUE – Color is a slate blue, as dark as possible showing no brown patches or silvering. Eye color is dark ruby or black. [Standardized April 12, 1992]

POWEDER BLUE – Color to be a very light powder blue color with no suggestion of rustiness, grayness, or silvering, not to be confused with Sky Blue. Eye color to be dark ruby or black. [Standardized March 7, 2015; amended October 7, 2015]

SKY BLUE – Color to be in between blue and powder blue, a clear sparkling blue. Color showing no brown patches or silvering. Eye color to be dark ruby or black. [Standardized April 4, 1998]”


BLUE -Slate blue, as dark as possible with no brown patches or silvering with dark ruby or black eyes.

POWDER BLUE – Medium blue, lighter than slate blue, with no brown patches or silvering with ruby or dark ruby eyes.”


BRITISH BLUE – To be a deep steel blue without any brown colouration. Colour to be even with the belly colour matching the top. The colour of the fur when parted to be blue grey down to the skin.

POWDER BLUE [provisional] – To be a pale dove blue colour, with pale silver base fur. Pale silver underbelly. Should be distinctively different to that of the slate colour of the blue rat. Colour to be as even as possible, devoid of dinginess, white hairs or patches. Foot colour to match top. Eyes Black or Ruby.”






Elton is an American Blue that fits the Powder Blue standard, with a Bristle coat.

Elton Murrat [Am Blue Bristle]







This is Jo, an American Blue Double Rex. 

Joseph Caratelli [Am Blue Dbl Rex]







Blue and Russian Blue combined with Marten to create a unique Silver Blue Marten.

Sablinova [Silver Blue B.E. Marten]







An Assortment of Blues

Sablinova[Silver Blue B.E. Marten], Ash[RB Silvermane], Romanov[RB], Gwen[RB Marten Silvermane]



  • The G-Locus is recessive.
  • Many shades of blue are produced, though Russian Blue and Midnight Blue are two totally different genes.
  • Blue rats have many different origins which tend towards different shades.
  • While blue rats once where known to have health issues, these traits have since been eradicated in the majority of lines.


A Final Thought

The G Locus produces an array of beautiful blue toned rats that have become extremely popular in the pet fancy. From the deep slate to the vibrant sky blue these rats are sure to catch your eye. The wide range of tones makes them even more interesting, but also often difficult in obtaining the right shades specially in cross-genetics.