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Foreword

This article is intended to provide a guide to people who want to know how they can use genetics to help them plan the colours they will get from breeding their gerbils. It is based on two articles that originally appeared in the September and December 1996 issues of the NGS Journal but has been rewritten and updated. It is heavily biased towards the colours available in the UK and the standards of the NGS, but will also be a useful guide to people elsewhere who want to learn more about gerbil coat colour genetics. Please note that references to the Burmese gene (c^b) have bene updated to reflect the recently published literature which described this mutation as Chinchilla medium (c^chm).

If you know nothing about the principles of genetics I recommend you start by reading Lewis Stead's excellent Introduction to Rodent Genetics.

All the genetic information here is covered in more detail on the Genetics page. In addition there is some information on the newly discovered dilute mutation.

Almost all the colours mentioned are illustrated in the The Gerbil Colour Palette.

There is a list of references to the Scientific Literature.

Introduction

The colour of a gerbil's coat depends on the genes it carries. Each characteristic depends on the code carried in the chromosomes in each cell. The point of the chromosome that denotes a characteristic is known as a locus. As chromosomes exist in pairs then each locus will consist of a point on each chromosome of the pair. At these points several different genes can exist that work like switches turning on or off the relevant characteristics. Every gerbil inherits one chromosome from its mother and one from its father, and every parent will pass on one of each pair of genes to each of its offspring. At each locus there may be one possibility, two, or in some cases three or more. The genes available at each locus is set out in one of the tables on my main Genetics Page. The alternative genes at each locus are called alleles.

Loci and genes are usually denoted by a letter to indicate the characteristic that is carried at that point. For example the gene that controls whether the gerbil has hairs that are banded and a white belly is denoted by the letter A. A capital letter is usually used to denote a dominant characteristic and a lower case letter is used for a recessive characteristic. The A gene is carried by the dominant wild type gerbil known as agouti. The recessive gene at this locus is the a gene that produces a single body colour such as in the black gerbil. Geneticists often use the symbol "+" to denote the normal gene found in the wild so a gerbil that was AA would also be called ++, and one that was Aa could be +a.

The Loci

The following loci are known to exist in gerbils:

A - The Agouti Locus which controls the white belly and ticking.

C - The Albino Locus which controls the overall level of colour produced.

D - The Dilute Locus which controls the depth of colour.

E - The Extension Locus which controls the balance between black and yellow pigment in the coat.

G - The Grey Locus which controls the intensity of yellow and black in the coat.

P - The Pink-Eye Dilution Locus which controls eye colour and whether the coat is lightened.

Sp - The Spotting Locus. This controls white spotting and by default is not referred to unless a gerbil is spotted.

Black and Grey

A wild Golden Agouti gerbil burrowing away somewhere in Eastern Central Asia will be carrying the regular dominant genes at all loci and can be described as AACCEEGGPP.

However, a Golden Agouti gerbil kept as a pet in the UK is almost certainly carrying recessive genes introduced somewhere in its ancestry. For example AaCCEEGgPP.

Because this gerbil still has a dominant gene on the A locus and the G locus, the recessive genes have no apparent effect. If the gerbil was aaCCEEGgPP it would appear black because of the aa genes at the A locus. If it were AaCCEEggPP it would appear Grey Agouti because of the gg genes at the G locus. A gerbil with both aa and gg such as aaCCEEggPP would display both the characteristics of the black gerbil and the greying effect of the g gene. As a result it is a Slate, sometimes incorrectly known as Blue, which is a very dark blue/grey colour.

Now we know this, we can start anticipating the colours that are likely to appear when mating gerbils. Clearly we can immediately tell some things about a gerbil if it is displaying recessive characteristics. If it is carrying dominant characteristics we can only tell if the gerbil is also carrying a suppressed recessive gene by breeding from that gerbil or by deducing its genes from its ancestry. So it is usual to only note the genes that are known. A shorthand symbol, often simply a dash or a dot, is used for those genes that are uncertain. For example a Golden Agouti gerbil could be described as A-C-E-G-P- because it will appear the same as one that was AACCEEGGPP or AaCc^hEeGgPp.

The Slate above could just as easily be carrying recessive genes at the P and C loci so would be better described as aaC-E-ggP-.

Using this notation to help us we can work out what offspring we can expect from any mating. For example if we breed a Black with a Golden Agouti we have aaC-E-G-P- and A-C-E-G-P-

Because each gerbil produced by the mating will inherit one gene at each locus from each parent at the A locus there are the following possibilities, Aa and a-. The Aa gerbils will all be Golden Agouti. The a- gerbils will be Golden Agouti if the Golden Agouti parent was AA, but if the parent was carrying the recessive a gene then the a- offspring will be black. In addition because of the uncertainties at the C, E, G and P loci of both gerbils other colours will appear if both of them carry recessive genes in the unknown slots.

This analysis can be carried out for all the relevant loci. For example crossing our Slate aaC-E-ggP- with a Golden Agouti A-C-E-G-P- will produce the following possibilities: (NB, all but the A and G loci are ignored as they do not affect this calculation )

AaGg = Golden Agouti

a-Gg = Golden Agouti or Black

Aag- = Golden Agouti or Grey Agouti

a-g- = Golden Agouti, Grey Agouti, Black or Slate

Again, uncertainty at the C, E and P loci could also give rise to other colours if both gerbils are carrying recessive genes at the same locus.

Importantly, because we know the Slate parent had to be aagg we now know all Golden Agouti offspring of this cross will be AaGg and carry both the a gene and the g gene. We can use that information when planning the next generation because we have eliminated some of the uncertainty.

From this you can see the importance of keeping breeding records. The more generations you breed the more information you will have to suggest whether your gerbils displaying dominant characteristics are carrying useful recessive genes.

The ratio of the colours produced can also be calculated from this information. For example, if you have a Black gerbil aaGg and a Grey Agouti gerbil AAgg, you need to follow these steps:

For each gerbil write down the genes at each locus as follows and then write in each box the combinations you get by reading across and down:

This will give you the following combinations that can be passed on:

Black - aG, ag, aG, ag.

Grey Agouti - Ag, Ag, Ag, Ag.

Note that it has been necessary to write some of this repeatedly. This is to keep the ratios straight. Using the table method will always make sure you get these ratios right.

The next step is to write the combinations you got for one gerbil across the top of a table, and the ones for the other gerbil across the side, enter the data in the boxes as before - then you can enter the colours in the boxes as well.

Counting up the outcomes you have here gives you 8 x AaGg Golden Agouti and 8 x Aagg Grey Agouti. This tells you that with your particular Black and Grey Agouti you can expect a 1-1 ratio of Golden Agoutis and Grey Agoutis. This can be used with any number of loci for which you know the genes, however, the size of the table you need will grow experientially as the number of loci increase.

There is a shorthand method if you get repeating combinations as in the above example. IE, the last table could have been written:

Here is another example, take a mating between a Grey Agouti Aagg and a Golden Agouti AaGg.

In this example you have 6 Golden Agoutis, 6 Grey Agoutis, 2 Blacks and 2 Slates. This means you can expect these colours in a 3-3-1-1 ratio. Of course these ratios are only predictions. The rules of chance mean that you are unlikely to ever exactly match these figures.

One important point to remember is that gerbils of the same colour can have different genotypes. For example, in the last mating we looked at we found that we could expect a 3-3-1-1 ratio of colours, but what about the ratio of genotypes? You can see that whilst 2/3rds of Golden Agoutis will be AaGg, the rest will be AAGg. This means that if you want to pass on the a gene you only have a 2/3rds chance from one of these Golden Agoutis. Have a look and see if the same is true of the Grey Agoutis produced by that mating.

Don't worry if this all looks like a lot of work, there are some programs at the bottom of the page that will help you do these calculations.

The following table shows the genetic make up of all the standard colours in the UK for which the genetics have been established. The spotting gene can be added to any of these to produce a spotted gerbil although this will not be apparent in the white gerbils.

The following table sets out the likely genetics of the colours recognised by the NGS.

- indicates that any gene symbol can be at that location.

* these are only four of the many ways of producing these white gerbils by combining diluting genes.

NB. These are the standard genotypes. It is possible to produce the some of these colours with different genotypes. Not all versions of a genotype given above will necessarily look alike.  For example a Pearl with c^chmc^chm will apear darker than one with c^chmc^h. There are also other colours which are either not standardised by the NGS and/or for which the genotypes are not yet fully understood. There is a much fuller treatment of all the colours at The Gerbil Colour Palette.

As you can see from the complex entries in the C locus column there is something pretty strange going on.  I will explain this by first describing the effect of the P locus.

Pink and White

The P locus has two known genes. P, a dominant gene, which in general causes a dark coloured gerbil with dark eyes and p, a recessive gene, which dilutes the colour of the eyes and the fur to produce a lighter coloured pink-eyed gerbil. The most obvious example of this is the difference between a golden agouti gerbil (A-C-E-G-P-) and an Argente Golden (A-CCE-G-pp). As you can see the only differences in the genes are that the Agouti gerbil can be either PP or Pp at the P locus and CC or Cc^h at the C locus. The Argente gerbil must be CCpp.

Another example of this effect is the difference between a Black gerbil which is aaC-E-G-P- and a Lilac which is aaCCE-G-pp. Once again the only differences are at the P locus and the C locus. The effect is the same, the gerbil is a much lighter colour and the dark eyes of the Black gerbil have been diluted to a deep red colour.

You may be wondering why, if C is a more dominant gene than c^h and c^chm, I have described the Argente Golden and Lilac gerbil as CC while I have not needed to be so specific about the Golden Agouti or Black gerbil. This is because the C gene is only dominant where the gerbil is also carrying the P gene. Any gerbil that is pp will have its colour further diluted if it also carries a c^h or c^chm gene at the C locus. Going back to the examples of Argente Golden and Lilac gerbils above ppCc^h produces a "washed out" Argente called Argente Cream and aappCc^h produces a "washed out" Lilac called Dove. Replacing this single c^h with c^chm produces a sort of "in between" colour called Topaz and Sapphire respectively.

We can then speculate on what happens if a gerbil has c^h as both genes at the C locus. Logically if one gene can dilute the colour, two might dilute it even further?

This is of course what happens. A gerbil that is c^hc^hpp is always white all over with pink eyes regardless of the other genes carried. But what if a gerbil is c^hc^hPP or c^hc^hPp? We know the presence of at least one P gene stops one c^h gene having any effect, maybe something else unusual happens where there are a pair of c^hc^h genes? Well this is exactly the case. The presence of dominant dark eyed genes doesn't produce a dark eyed gerbil as we might expect but then again the pair of c^h genes doesn't produce a completely white gerbil either! What we get is a Dark-Tailed White. A gerbil with pink eyes that is white all over except for its tail that can be almost any shade from slightly off-white, through to dark brown or almost black. However, this tail colour never contains much yellow pigment.

Colourpoint Gerbils

If c^hc^h does strange things what does c^chmc^chm do?

In general this produces an animal where yellow pigment is severely reduced, and black pigment is also diluted, but not from the nose, ears, feet and tail. These are often referred to as the points, hence the name colourpoint you will often hear to describe all the colours that can have these darker "points".

The Burmese gerbil, aac^chmc^chmP-, I have mentioned the genes at the P locus because there is evidence that c^chmc^chmpp like c^hc^hpp produces a white gerbil, is the darkest colourpoint and this is becasue it is basically a black gerbil that has had its body colour reduced by the c^chm genes.

The agouti equivalent of this colour is Pearl, A-c^chmc^chmP-, a gerbil which looks very like a washed out agouti. The coat is very light in colour but there is still dark ticking and the base of the hairs is yellowish in colour. The tail etc are a little darker than the body colour.

We know that other combinations at the C locus area bit special so what can we expect of c^chmc^h do? In practise this seems to simply produce a lighter coloured version of c^chmc^chm. The most obvious example is the Siamese, aac^chmc^hP- which is creamy coloured instead of dark grey and still has dark points.

All colourpoint colours vary a lot, so much so that there is no clear dividing line between Siamese and Burmese. This is probably because the genes at other known loci have an effect but it may be due to other factors also.

Extension of Yellow

In January 1997 a new mutation was imported in to the UK for the first time. The E locus is known in many species and controls the balance between yellow and black pigment, especially in agouti animals. Genes more dominant than the wild-type E tend to make the coat darker, and genes recessive to E tend to make the coat more yellow.

The gene imported, e, makes a golden agouti gerbil a warm orange colour with only a little dark ticking. The eyes stay dark and the tail and ears retain dark skin even though the colour of the fur is lighter than golden agouti gerbils. This colour used to be called Sooty Fawn in the UK,  but the American name of Dark-Eyed Honey is now more commonly used. In Europe this colour is sometimes called Algerian.

The non-agouti gene a seems to have an odd effect ee gerbils producing a dark gerbil but still with a lot of yellow in the coat called Nutmeg. aaee gerbils in most other species would be almost completely yellow.

The effect of other genes is what would be expected. For example, A-eepp is very like A-E-pp because the yellow has already replaced black due to the action of pp, A-eegg produces a Polar Fox, a very much lighter, and slightly yellowish, version of the Grey Agouti. There is very little yellow pigment left in the coat due to the action of gg for ee to have any real effect on, but it can still reduce the amount of black pigment. Mutations at the c locus seem to dilute ee gerbils so a Cc^hee or Cc^chmee will tend to be lighter than CCee animals.

A further mutation at the E locus was imported in March 1998. e^f produces similar effect to e but the colouring is a little lighter. However, as the gerbil ages it loses pigment so that when fully adult it is an off-white colour. This is known as Schimmel which is the name Germans use to describe the fading of Lippizaner horses. Gerbils carrying e^f are not yet generaly available in the UK.

Marked Gerbils

Another strange gene is the one that causes white patches or spots. All the other mutant genes that cause non wild colours are recessive but with white spotting we have a gene that is dominant. Therefore a gerbil that carries the Sp gene will always have white spots or patches on its head, neck and possibly tail. If it doesn't have a white belly then there will also be large white patches on the chest and ventral area. Interestingly it doesn't seem possible for a gerbil to carry a double dose of the Sp gene. Those gerbils that should be SpSp never grow in the womb and are never born. Because of this all white spotted gerbils must carry a single Sp gene and all unspotted gerbils have no Sp gene at all. This means that a white spotted gerbil bred with a non spotted gerbil will always produce 50% spotted pups. Breeding two spotted gerbils together will produce smaller litters because the gerbils that would be SpSp never come to exist in the womb and of the remainder, two-thirds will be spotted.

Although the gene causing white spotting has been designated Sp by scientists, they have not named the normal wild-type gene that non-spotted gerbils have. So it is therefore technically incorrect to refer to gerbils as being Spsp or spsp. Instead, it is more proper to use the symbol +. The normal wild gerbil is therefore ++ at the spotting locus and spotted gerbils are Sp+. In practise it is easier and makes as much sense to refer to spotted animals as Sp and leave the locus blank for non-spotted gerbils.

Using these symbols we can demonstrate why spotted gerbils, in general, have smaller litters when mated to other spotted gerbils and why mating a spot to a non-spotted gerbil will usually produce a mix of spotted and non-spotted pups:

Spots and non-spots are produced in a 1-1 ratio.

Spots and non-spots are produced in a 2-1 ratio but litters are smaller because 25% of embryos are unviable and are never born.

In recent years pied gerbils have appeared in the UK that have similar white markings as spotted or patched gerbils but these markings are more extensive, sometimes forming a complete white collar and a white line down the middle of the face as in Dutch mice and Rabbits. Sometimes Pied gerbils also have a white dappling effect across the rump. More recently completely mottled gerbils have been bred from these. Exactly what causes this is not yet known. The amount of spotting is probably controlled partly by several modifying genes. In addition, non-genetic factors almost certainly affect the amount of white spotting. It is fair to say that the genetic cause of the pied markings is the subject of much debate! It is also possible that a form of recessive spotting has appeared but this has not been confirmed. There are pictures showing the range of white spotting. The Sp gene also produces some dilution of the basic colour of the gerbil, so a spotted Golden Agouti will normally be lighter in colour than a non-spotted Golden Agouti. The more white spotting there is the greater the dilution appears to be. This can cause confusion, for example, a marked Lilac may appear as light as a Dove.

Other Genes

In addition to the above genes there is a Dilute Gene (symbol "d") which affects the intensity of pigment by changing the distribution of pigment within the hair cells. The gene is very rare and only a handful of people currently have access to gerbils carrying "d". There is a page with some pictures of dilute gerbils.

With the information in this article, along with that in the Genetics Page and the pictures on the The Gerbil Colour Palette you should be able predict with some certainty the range of colours you can expect when breeding your gerbils.

For more information on gerbil genetics, including the latest developments, subscribe to the Gerbil Mailing List. Full instructions are on the NGS Homepage

Aids

There is a CSH script that can be run under UNIX to predict the outcome of certain matings.

This has now been converted to C,

And compiled to run in Windows 95.

Return to the NGS Homepage?

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