At first, genetics may seem very daunting, however, it is very useful to know.
On this page, I will attempt to explain the genetics of gerbil coat colours in clear and simple terms.
I will be starting from the very beginning, assuming you have no prior knowledge of genetics.
All living organisms have a genetic code made of DNA (deoxyribonucleic acid) that determines their physical attributes. DNA is organized into units called chromosomes which reside in the nuclei of all cells.
In most of the body's cells, these chromosomes come in pairs.
In the reproductive cells (sperm and eggs) however, there is only one copy of each chromosome.
This is so that when the sperm and egg join, the resulting embryo winds up with two copies of each chromosome.
Each chromosome is made up of smaller units called genes. These genes carry the information to produce all the proteins in the body, including the ones thar produce coat colour. Since every organism has two copies of each chromosome, they also have two copies of each gene. These genes, however, can come in different forms, which have slightly different sequences of DNA. These forms, which are called "alleles" result from genetic mutation. You can either have the same allele on both chromosomes, or different ones.
Alleles can interact with each other in different ways. The natural or "wild-type" allele is usually dominant. This means that if a different allele is on the other chromosome, it will be masked by the dominant allele. You will just see the effect of the dominant allele, even though a different allele is also present. Dominant alleles are represented by capital letters.
The allele that is being masked by the dominant allele is known as the recessive allele. It is represented by lowercase letters. Since a dominant allele with always mask the recessive allele, you will only see the recessive trait if there are two of the alleles present (one on each chromosome).
For example, imagine a gerbil that has two different alleles on each chromosome, 'A' and 'a'. If the 'A' allele is dominant, you will only see the 'A' colour, not the 'a' colour. This is because 'A' is dominant over 'a'. The 'a' colour will only show up if there are two of them. The 'a' allele would therefore be called "recessive".
In most cases, the wild-type allele will be the dominant one. The mutation from the natural state, therefore, is usually the recessive allele. In gerbils, most of the genes that create the agouti coat colour are dominant, because these are the normal alleles found in the wild. This is true for all of the genes except for the spotting gene. Even though spotting is a mutation, it is still a dominant allele.
In addition to the normal interaction of dominant and recessive that I described above, there is also an interaction known as "codominance". This is where one allele doesn't mask the other allele, but rather they work together to create a new colour. For instance, if the 'C' allele is codominant with the 'Ch' allele, then you can wind up with 3 different possible colours. You will get one colour when you have 'CC', a slightly different colour when you have 'CCh', and a completely different colour with 'ChCh'.
The Seven Coat Colour Genes
So now that you know some of the genetics basics, I will discuss how it pertains to gerbil coat colour.
The wild-type gerbil coat, agouti, consists of three bands on each hair. The top band is black, the middle band is yellow and the lower band is black. Several of the coat colour genes affect this banding pattern.
In gerbils, almost all of the coat colour genes come in only two alleles, dominant and recessive. The exceptions are the 'C' and 'E' genes, which have three alleles. The seven known coat colour genes in mongolian gerbils are:
This gene controls the colour of the belly. The 'a' allele is recessive.
- A = the belly will be white
- a = the belly will be the same colour as the back (often called a "self" gerbil)
AA = white belly
Aa = white belly, because 'a' is recessive
aa = the gerbil will have a belly the same colour as it's back
This is the colourpoint gene. It controls the distribution of pigment in the extremities. The 'C' gene is an example of codominance and has three alleles (C, Cchm, and Ch). If the gerbil has one copy of either the 'Cchm' or 'Ch' alleles, in combination with either 'ee' or 'pp', the coat will be lightened slightly. In gerbils that do not carry either 'ee' or 'pp', you will not see this effect. If you have two copies of either 'Cchm' or 'Ch', you wind up with a colourpoint gerbil.
- C = gerbil will have normal pigmentation
- Cchm = with the presence of one allele, it causes the coat to be lighter when the gerbil is 'ee' or 'pp'. When two alleles are present, pigment accumulates in the extremeties.
- Ch = with the presence of one allele, it causes the coat to be lighter when the gerbil is 'ee' or 'pp', more than 'Cchm'. With two alleles present, you get a dark-tailed, pink-eyed white gerbil.
CC = normal distribution of pigment
CCchm = coat lighter than normal if the gerbil is 'ee' or 'pp'. Otherwise, the gerbil appears the same.
CCh = coat lighter than normal if the gerbil is 'ee' or 'pp', more than 'Cchm'. Otherwise, the gerbil appears the same.
CchmCchm = colour accumulates in the extremeties
CchmCh = colour accumulates in the extremeties, but is lighter than 'CchmCchm'
ChCh = eliminates all colour except the tail producing a dark-tailed white gerbil
D gene:This gene "dilutes" the colour of the coat and is a recessive gene.
- D = gerbil will be the normal, dark colour
- d = the gerbil will be lighter coloured, or "diluted"
DD = normal dark colour
Dd = normal dark colour
dd = diluted colouring
This gene controls the presence of the lower black band on the hair shaft. When the band is absent, the gerbil is golden-coloured. One of the alleles is also a colourpoint gene. This is an example of both dominance and co-dominance. The 'E' allele is dominant over both 'e' and 'ef'. When 'e' and 'ef' are present, however, the 'ef' allele causes coat lightening.
- E = black lower band is present
- e = black lower band is not present so the gerbil will have gold-coloured fur
- ef = one copy of the gene will cause lightening of the fur when the other allele is 'e'. Two copies will result in golden pigment concentrated in the extremities.
EE = black lower band present
Ee = black lower band present
Eef = black lower band present
ee = black lower band absent, resulting in a golden gerbil
eef = black lower band absent with gold colour slightly lighter than ee
efef = black lower band absent with gold colour concentrated in the extremities
This is a recessive gene that controls whether or not the yellow band on the hair shaft is present.
- G = the yellow band is present
- g = the yellow band is absent
GG = yellow band is present
Gg = yellow band is present
gg = yellow band is absent, resulting in a grey gerbil
This recessive gene controls the colour of the gerbil's eyes as well as the black bands on the hair shaft.
- P = eyes are black and upper and lower bands are black
- p = eyes are red and the upper and lower bands are grey
PP = black eyes and black banding
Pp = black eyes and black banding
pp = red eyes and grey banding
This gene causes spotting and is a dominant gene. A spotted gerbil will also have a slightly lighter coat and a white belly.
- Sp = gerbil will have spotting, a lighter coat and a white belly
- sp = gerbil will not have spotting
SpSp = lethal combination, this fetus will be resorbed
Spsp = coat will have spotting, be slightly lighter and the gerbil will have a white belly
spsp = no spotting
Genetic List of All Known Coat Colours
Now that you know what genes the gerbil coat colours are governed by, here's a list of all the possible colour combinations. Beside them are the recessive genes that produce the colours. This list is a good reference for determining the colours of the potential offspring.
White bellied colours
|Argente Cream||Ch pp|
|Ivory (Cream)||gg pp|
|Dark-eyed Honey (DEH)||ee|
|Yellow Fox (red-eyed)||ee pp|
|Polar Fox||ee gg|
|Cream Fox (red-eyed)||ee gg pp|
|Dove||aa Ch pp|
|Sapphire||aa Cchm pp|
|Red-eyed White (REW)||aa gg pp|
|Red Fox||aa ee pp|
|Pale Red Fox||aa Ch ee pp|
|Silver Nutmeg||aa ee gg|
|Red-eyed Schimmel||efef pp|
|Colourpoint Grey Agouti||CchmCchm gg|
|Colourpoint Nutmeg||aa CchmCchm ee|
|Colourpoint Silver Nutmeg||aa CchmCchm ee gg|
|Dark-tailed White (DTW)||ChCh|
|Very Dark-tailed White||aa ChCh|
|Pink-eyed White (PEW)||ChCh pp - no matter what other recessives are present|
Determining the Colours of Potential Offspring
Now that you know the basic ideas behind genetics, how each gene works and what possible coat colours can result, you are ready to figure out what your pair can produce.
In this section I will explain how how to do this the 'long' way. Many people just input their gerbils' genetics into a 'gerbil genetics calculator', but it is always good to have the background knowledge first.
Since I like genetics, I always do it the hard way *smile*.
The next two paragraphs are for general interest only because I'm going to explain why the gerbil's genetic makeup is so fascinating. We are actually very lucky, because compared to other organisms, the genetics behind a gerbil's appearance is relatively simple. If you don't really want to read about why I think this is so cool, you can just skip the information in the box.
Why gerbil genetics is so fascinating
What makes gerbil genetics so interesting, is that the genes for coat colour are not "genetically-linked".
This essentially means that each coat colour gene exists on a seperate chromosome.
When reproductive cells are produced, only one copy of the chromosome winds up in each sperm or egg.
This means that either copy is randomly picked to be placed in the reproductive cell.
Since the genes for coat colour exist on seperate chromosomes, each coat colour gene is selected randomly during the creation of these cells.
The selection of one gene will not influence the selection of another gene.
First you start by figuring out the possible gene combinations each parent can contribute to their offspring.
When the reproductive cells are being produced, only one copy of each chromosome goes to the egg or sperm.
Since all organisms have two copies of each chromosome, the reproductive cell will either get chromosome 'A' or chromosome 'B'.
Since this process is random, you will get 'A' 50% of the time, and 'B' the other 50% of the time.
Lets take a female gerbil that is 'Aa' for example. 50% of the time the egg will get allele 'A' and 50% of the time the egg will get allele 'a'.
If you mate her with an 'Aa' male, then 50% of the time the sperm will get allele 'A' and 50% of the time the sperm will get allele 'a'.
|Then you just drop the possibilities into the Punnett Square.|
|Once you do that, you can put the possibilities from each parent together as you can see in the illustration to the left.
So this cross will give you 3/4 agouti and 1/4 black.
When you start to work with more than one gene, it gets more confusing.
A good rule is to count the number of genes that are showing two different alleles.
Then the number of different possible sperm/eggs that the parent will produce is 2n, n being the number of genes showing two different alleles.
For instance, if one of the parents is 'Aa Pp', you have two genes that are showing two different alleles. This means the number of possibilities is:
'AP', 'Ap', 'aP', & 'ap'
If the other parent is 'aa Pp', you only have one gene that is showing two different alleles. In the case of the 'A' gene, this parent will always be contributing an 'a', so there isn't actually a choice happening between two alleles. There is a selection happening for the 'P' gene, however, so the number of possibilities is:
'aP' & 'ap'