The C Locus: The Department of Pigment Taxation

This post is Lesson 5 in our Rabbit Genetics Illustrated series. 

The C Locus is home of the Pigment Taxation Department. The letters you see in this portion of our recipe will tell you if your rabbit gets to keep all of their pigments, if your rabbit has to give up some of their pigments and which ones, or if your rabbit does not get to show off any pigment at all and will be a red eyed albino. There are 5 genes that you can encounter here. Let's start with the two extreme opposites as those are the easiest.

First up is the big letter "C". This is the most dominant gene on this Locus and the kindest member of this department. The big "C" stands for full colour. Nothing is taxed, no pigments are reduced or taken away, the rabbit gets to keep all of their colour. All of our example rabbits from previous lessons were "C" or full colour rabbits. Because "C" is the most dominant gene here, a single copy is all it takes for the rabbit to be able to show full colour. A rabbit displaying full colour can be either "CC" or they can be "C_". Because "C" is most dominant, it can carry and hide all of the other C Locus genes. 

On the opposite end of the spectrum is the small "c". This gene stops the production of all pigment on the entire rabbit (all skin, hair, nails, and eyes) resulting in a rabbit that is pure white with ruby red eyes. We call this colour variety the Ruby Eyed White, often abbreviated to REW. This is the least dominant gene and two copies are needed for it to show. A REW rabbit must be "cc", it can not hide or carry any of the other C Locus genes. 

While visually, all of the colour is erased from a REW rabbit, the genes needed to create colours do remain. It can be helpful to think of REW rabbits as simply being covered with a white blanket. They still have the colour genetics they received from their parents and are able to produce coloured rabbits. They simply can't visibly express those colours as long as two copies of the "c" gene are present. 

Like all recessives, REW can hide for generations and not show itself until the rabbit carrying it is paired with another rabbit who can provide the needed 2nd copy. All rabbits who have one REW parent are automatically REW carriers. When you breed two REW carriers, each kit born will have a 25% chance of being a REW. When you breed a REW carrier to a REW, each kit born will have a 50% chance of being either a REW or a REW carrier. 

Before we go any further, we first need to better understand the different types of pigment that rabbits produce as well as a concept from our previous lesson, the banded coat. 

The pigments produced by rabbits can be categorized into two groups: dark pigments (eumelanin) such as black or brown, and light pigments (pheomelanin) such as red, orange, and yellow. When all of these pigments combine at different rates or in different places, we get all of our various colours and patterns. Any time we see white on a rabbit, that is the result of the absence of pigments. 

The banded coat of the Agouti rabbit gets its unique appearance by splitting the two different types of pigments into separate bands of colour on a single hair. On a Self rabbit, each hair from root to tip is one colour. The most pigment is always deposited at the tip of the newly grown hair and as the hair grows longer, the amount of pigment deposited is gradually decreased. This is very evident on all Angora breeds but is also true for short haired breeds. On an Agouti rabbit, the tip of the hair begins growing with deposits of dark pigment, followed by a period of growth where only light pigment is deposited, and finally ending with a period of growth where the dark pigment resumes, which by this point isn't as densely deposited and gives a diluted appearance. Each of these sections is referred to as a band.  The band at the top of the hair is referred to as the terminal band or simply the hair tip, while the middle band with light pigment is referred to as the sub terminal or intermediate band. When the coat is blown into, the different colours appear as separate bands or circles of alternating colour.

Now we can look at and better understand the effects of the other three C Locus genes. They are in order of dominance from most to least: Chinchilla Dark noted as c(chd), Chinchilla Light noted as c(chl), and Himalayan noted as c(ch). 

The Chinchilla Dark gene taxes or stops the production of practically all of the light pigment. This gene only affects the production of light pigment which is responsible for creating red, orange, and yellow. That means all of these colours are visually removed from the coat, nails, and eyes. What remains in places where the light pigment would have been is either dark pigment or no pigment. On hair, that means portions of the coat that would have been red, orange, or yellow, will now be white. Because technically this gene stops the production of almost all light pigment but not absolutely all light pigment, on some rabbits, such as ones with very high rufus (high amounts of deep red coloration) the parts of the hair turned white can have a more off white, pearly, or even creamy appearance. By removing the light pigment from the eyes, this gene can make the eyes appear a blueish grey.  This gene is recessive only to the most dominant gene of full colour "C" but it is dominant over all the other C Locus genes. This gene can carry and hide the Chinchilla Light (mostly, more on that in a minute), Himalayan, and REW genes. The Chinchilla Dark gene is what turns a Black Otter into a Silver Marten, a Japanese Harlequin into a Magpie, or an Agouti into a Chinchilla.  

The Chinchilla Light gene works very similarly, however it stops the production of some of the light pigments and some of the dark pigments. It changes black into a dark Sepia or Sable colour and because of this, it is some times referred to as the Sable gene. When two copies of the Chinchilla Light gene (or double Sable) are present, the resulting colour variety is called Seal. Seal results in a rabbit that is much darker than Sable, but not quite Black. It can however be so dark that it can be visually indistinguishable from Black. At that point, the only way to know the rabbit's genetic colour is through test breeding. 

While Chinchilla Dark is dominant over Chinchilla Light, it is not a full dominance. When breeding the Chinchilla variety, it is advised to avoid the use of rabbits who are Chinchilla Dark and carry Chinchilla Light. These rabbits would be c(chd)c(chl). That's because it is possible for both genes to visually manifest and cause a muddy appearance to the pattern. The same applies when breeding Magpies which should ideally be crisp alterations between black and white. The Chinchilla Light gene will cause the white to appear more creamy while also lightening the black into sepia, creating incorrectly marked Magpies. 

The last gene in this series is the Himalayan gene. This gene is dominant only over the albino REW gene. The Himalayan gene removes all pigment from the eyes making them ruby red. It also removes all pigment from almost the entire body, while sparing all pigments on the rabbit's extremities: the ears, nose, tail, and feet (including nails). What makes this gene extra peculiar is that the remaining pigmented areas are temperature sensitive! In cold temperatures, the coloured points on the rabbit intensify in colour, while in warm temperatures the colour fades to either faint or completely non existent. This can result in a rabbit that looks just like a REW while it's points are faded. 

Here is a summary of the C Locus genes in order from most to least dominant.

Continue forward! Only one more lesson left. The E Locus!