Chromosomes and Genes

The genetic material in all cells is made up of chromosomes, each chromosome comprising many different points (loci) which we know as genes. Each gene or in some cases group of genes is responsible for all the functions that occur within the body and the outward appearance of the body.

Each chromosome, except during division of the cell and in sex cells and red blood cells occurs as a pair of strands. The genes on one strand of the chromosome match up with the genes on the partnering strand with genes at the same location on each strand being responsible for the same characteristic. So when we say that all genes occur in pairs we do not mean on the same chromosome strand but on the actual chromosome.

Each chromosome location is specific for a particular gene(s) and will not allow any other gene to occupy that location. Some locations may allow several different possible genes to occupy the space for example the genes that determine coat colour. The gene that produces the black colour is located at a locus (point) where the dilution gene (turning the black to blue) may also occur. Other locations only allow one type of gene to occupy that particular space as to allow any other gene to do so could result in a lethal trait being produced. This does occur but generally the foetus dies before birth and the results of the ‘mistake’ are never seen. An example of this are the genes that govern the shape of teeth. If the incorrect gene occupies the locus the teeth will not grow and function in the correct manner thus causing the animal to be unable to eat.

Whilst all genes (except those previously mentioned) occur in pairs on the same chromosome pair of strands they do not have to match each other exactly. For example the gene for the black colour (strength of pigment gene) denoted as B may occur at one locus it’s partner on the other locus does not necessarily have to be another B. Two Bs would give a homozygous pairing (homo meaning same) but you could also get a b as the corresponding gene to the B thus giving Bb which is a heterozygous pairing (hetero meaning different).

Types of Genes – Dominant & Recessive

If one of the genes from the pair expresses itself more dominantly than the other it is said to be dominant (and is represented by a capital letter), the other gene being recessive (and is represented by a lower case letter). If the dominant gene expresses itself completely over the recessive gene then this is known as complete dominance. If the recessive gene exerts some effect then the dominant gene is said to be incompletely dominant resulting in incomplete dominance.

Passing on the Genes

When the sex cells (egg and sperm) are being formed the chromosome pair splits and each cell only receives one half of the chromosome pair e.g. one half of each pair of genes. This is so that when the egg and sperm fuse each chromosome strand in the egg will fuse with its partnering strand from the sperm cell and you once again end up with a full compliment of chromosomes. Hence half of the buck’s genes and half the doe’s genes make up the offspring.

 Determining What Genes are Inherited

This is purely random and is based on the mathematics of probability e.g if a rabbit carries the genes Bb then the probability that the B gene will be passed on in each of it’s sex cells (egg or sperm) is 50%. This is because either of the two genes B or b could be passed and so as all probabilities are out of 100 it must be half of 100. If there were three genes in question then the probability would be 100/3 which gives a 33% chance of the gene being passed on.

E.g. Looking at a simple pair of genes from buck and doe.

Doe is Bb. Chances of either gene being passed on are 50%

Buck is Bb Chances of either gene being passed on are 50%

So the chances of the offspring being BB are 50% (from the doe) X 50% (from the buck) = 25% i.e. 1 in 4 of the babies will be BB.

Chances of bb are also 25% as this is the same as BB above.

The Bb combination is a little different as there are two possibilities: The doe could pass on B and the buck b or the doe could pass on b and the buck B. Both possibilities give the same end result Bb but there are two ways to achieve this. So the chances of this combination occurring are 50% (from the doe) X 50% (from the doe) + 50% (from the buck)X 50% from the doe) which basically is 25% + 25% which give 50%.

Another way of working this out is in the grid below. The doe’s genes are along the top of the grid and the buck’s genes are along the side. The possibilities in the grid itself reflect the genes that will be present in the resulting offspring (in theory).          

So you can see that getting BB occurs 1 in 4 times (25%), getting bb occurs 1 in 4 (25%) and getting Bb or bB (same thing) occurs 1 in 2 or 2 in 4 times (50%).

This pattern of inheritance is the same for all simple pairs of genes. It also works the same way if you are comparing two pairs of genes at the same time. These two pairs of genes may or may not act together e.g. the B gene (strength of pigment) and the D gene (dilution of pigment) – the presence of two recessive d genes dilutes the black rabbit to a blue rabbit. The grid for the inheritance of these genes is slightly more complicated as each rabbit may pass either copy of the B gene on together with either copy of the D gene. 

As you can see the following offspring are possible from the above mating

Generally speaking most rabbit breeders are only interested in the genes that govern coat type and colour except when certain undesirable genetic traits ‘pop’ out and then we want to know here they came from and are both parents at fault (carriers). At the minute we shall keep things simple and only look at coat colour.

Colour Genes

Hair colour in the rabbit is controlled by different genes at different locations Some of these genes act in conjunction with each other and others act independently.

There are five major genes that govern coat colour along with several minor genes and other genes known as modifiers such as those which intensify the red in the coat, produce spots and control the intensity of the colour. Modifiers are more complicated than the major genes as they are groups of genes which have a cumulative effect.

The five major coat colour genes are inherited very simply and in general the presence of a dominant gene will overshadow the other gene present in the pairing. The five major groups are:

A – Pattern

B – Strength of colour pigment

C – Colour saturation

D – Colour strength (dilution)

E – Extension of colour

A Genes

These are known as the agouti series and determine the coat pattern of the rabbit. There are three types and in order of dominance these are: Agouti gene (A), tan pattern gene (at) and the self gene (a). The most dominant, the agouti A gene is the wild type pattern as seen in wild rabbits. This produces animals with bands of colour throughout the coat (slate base colour, orange band, black tips), white belly, nostrils and eye circles. As the agouti gene is the most dominant gene a rabbit cannot carry agouti (as this would make it an agouti) the only exception being a white rabbit as the gene causing the white ‘masks’ all other genes present. (see the C gene series).

The tan gene at is the next most dominant and produces a rabbit as above but with no banding throughout the fur and also tanning is seen around the areas which are white (eye circles, belly etc). Tanning is also seen on the nape of the neck. This gene is recessive to the agouti but dominant to the self gene and so an agouti can carry the tan pattern gene but a self rabbit cannot (as it would exhibit the tan pattern).

The self gene a is the most recessive of the A series and as such cannot carry either of the other two genes. This gene produces a rabbit that is one colour throughout (other genes may act to create different shades within the coat).

B Genes

These are known as the colour pigment genes and determine the strength of the dark pigment seen in the hair. There are two types of genes the black gene B and the chocolate gene b. Black is dominant to chocolate and is the ‘full strength’ colour pigment. Chocolate is the ‘watered down’ version of the pigment. Being dominant a black rabbit can carry the chocolate gene but not the other way around. In an agouti animal B produces the black band and in a self coloured animal this produces a solid black colour. The recessive b gene produces a chocolate band in the agouti coloured animal (cinammon) and in the self coloured animal it produces solid chocolate.

C Genes

These genes are known as the colour saturation genes (amongst other things). There are five genes in this group and in order of dominance they are:

C – Full colour saturation as seen in any coloured rabbit e.g. black. Allows all colour pigments to be expressed and is completely dominant over all other C genes.

cchd – dark chinchillation gene as seen in chinchillas. This prevents three quarters of the orange pigment (as seen in agoutis) from being expressed and produces a white/pearl colour. Thus a chinchilla coloured rabbit is just a chinchillated agouti i.e. the orange has been stripped away. This gene is completely dominant over all the C genes listed below.

cchl – light chinchillation as seen in sable and smoke coloured animals. This acts in a similar way to the cchd gene but allows half of the orange pigments to be expressed. This gives rise to animals with shaded coats and lightens the overall coat colour to varying shades of sepia. This gene is only partially dominant over the C genes listed below and it is this series of genes that gives rise to the most colour variations.

ch – Himalayan gene as seen in Himalayans and sealpoints. This is a strange gene as it is temperature sensitive. It causes dark points on the rabbits coldest areas (its extremities) e.g. the nose, ears, feet and tail. It is partially recessive to the cchl gene and partially dominant to the c gene.

c – albino gene as seen in red eyed whites (REW). This gene, when present as a homozygous (same) pair completely blocks the expression of all the other colour genes. Any rabbit, regardless of colour of parents, if carrying two c genes will be white. The colour of the eye is seen to be red as all pigment is removed and the blood vessels can be clearly seen. The actual coat is not white but albino (no colour), it appears white because of the way that light reflects off it.

D Genes

There are two genes in this series and these represent the strength or dilution of the colour. Full strength colour is represented by D and this is seen in a black rabbit. the recessive d gene produces fewer pigment granules in each hair shaft and so the colour looks diluted as seen when two d genes are present as in the blue rabbit (dilute black). When this gene is present in an agouti coloured animal it produces a blue band instead of the black and you get a blue agouti (opal). This gene also dilutes an orange to a blue orange (fawn), a chocolate to a lilac and a chocolate agouti (cinammon) to a lilac agouti (lynx).

E Genes

There are four genes in this series and they control the extension of black pigment seen in the animals coat. In order of dominance they are:

E – normal extension of black as seen in the black band of an agouti.

Es – super extension of black as seen in a black agouti known as a steel. This produces white guard hairs so gives the appearance of ticking.

ej – bridling of the black pigment (also called the Japanese gene) as seen in harlequin and magpies.

e – non extension of black pigment as seen in sooty fawns and oranges. This gene removes all or most of the black pigment from the coat leaving orange. Hence an agouti with ee will have the black removed from its coat and become an orange.

Other Genes

There are a whole host of other genes that control the overall appearance of the rabbits coat. Some of these are listed below with a brief description. Many are expressed in the same basic way as the B gene series.

En – causes spotting. Enen causes normal spotting as seen in the English rabbit, EnEn causes sparse spotting known as charlies and enen causes no spotting.

Du – causes the dutch pattern as seen in dutch rabbits. Dudu causes partial dutch markings, DuDu causes no dutch markings and dudu causes the normal dutch markings.

V – Vienna gene – produces blue eyes in a white rabbit as seen in the Vienna rabbit.Vv causes dutch markings in coloured rabbits and coloured spots in white rabbits. VV produces a normal coloured rabbit and vv acts similar to the c gene and causes no colour to be expressed (i.e. produces white) and gives blue eyes hence BEW (blue eyed white).

Si – Silvering gene as seen in the Silver rabbits. Si produces normal coloured rabbits. si present in its homozygous pairing produces white tipped hairs throughout the coat giving a ‘silvering’ effect.

W – band width gene. W produces normal band width as seen in the agouti. ww produces double band width and causes the colour of the orange to appear red as seen in the New Zealand Red, Belgian Hare, caster rex etc.

R – rex gene as seen in the rex rabbit. R produces a normal coated rabbit, rr produces a rex rabbit. This gene shortens the length of the guard hairs so that they are the same length as or shorter than the undercoat. this gives the coat a thick, plush, velvety feel.

L – length of fur. L produces normal coat length and ll produces long hair as seen in the angora.

Sa – satinisation of the fur. Sa produces normal fur and sasa produces satinised fur as seen in the satin rabbit.

Wa – waved coat. This is a very rare gene and is hardly ever seen at all today. Wa produces a normal coated rabbit and wawa produces a rabbit with a wavy coat as seen in the astrex.

Whilst it may seem that we have gone into pages of detail on the genetics of colour this is really only the basics. Obviously these genes have only been discussed independently of any other genes. Put them all together and things become a little more complicated. Below is a table of some of the most common rabbit coat colours and the genes that make them up. Hopefully with the explanations above you should be able to work out where the colour came from and why it is what it is.

If you have any slightly more technical genetics queries please feel free to email us and we will do our best to help. Also photos may be emailed to us if you are unsure of colours that have appeared in your litters.

The symbol - represents an unknown gene in the pairing.