Gerbil Genetics ~ Section 2
Gerbil Mutations
If we look at gerbil genetics today they have at the time of writing 9 known mutations that affect the coat colour, and those that have appeared in the gerbil since its introduction into captivity in 1935 have all been recessive in nature, bar one, which is white spotting. A successful coat colour mutation is seen as an exceptionally rare event in many species and occurs when their DNA gets modified in some way. Alterations in cells happen all the time, however these abnormal cells don't usually prosper. DNA repair mends most changes before they can become permanent mutations, and many organisms have specific mechanisms for eliminating these abnormal cells. Sometimes though, if this alteration occurs very early on in an animal's development, such as in the sperm, ova or zygote, the abnormal cell may succeed and have a chance of survival.
When considering a mutation that affects just the hair colouring in an animal, and not one that can also cause a problem elsewhere in the animal's development, the odds will increase as to whether the chances of this altered gene will survive. In certain instances a mutation that affects the hair or coat colour, may also go on to cause detrimental effects to the normal functioning of the organism. If we also consider that most mutated genes are also recessive in nature, and then the individual with this mutation still has to go on to reproduce successfully, and then if all this has been accomplished, two of its descendents will have to reproduce again before you see the mutation surface once more. Over a relatively long time period these mutated genes will then have to be built up in the population, and that's all providing whether the mutation is actually beneficial for the species.
You can now begin to see how such a rare occurrence a successful mutation really is, and appreciate how high the odds are at a new mutation eventually succeeding!
So what exactly is a mutation?
Now that we've been on a cellular journey into a gerbil's inner ear, we now know that every single cell in a gerbil's body contains a nucleus. The nucleus itself contains a number of chromosomes containing DNA. If we look closely along these chromosomes, we see that coded within their DNA are a series of genes. Each of these genes is there to create specific proteins. The proteins that are created by genes have very specific functions and are very wide ranging. There are many examples of this and we have seen that sometimes they act as a catalyst to help another process to take place within the body, or they can just as easily be used to create something, such as the protein collagen, which helps make body tissues both strong and flexible, or keratin which is responsible for growing hair and nails, and let's not forget about pigment enzymes which give rise to our gerbil's eye and fur colours. In essence this means that these genes that lie within the chromosomes are responsible for a whole range of differing processes that take place within an organism's body, and these processes rely on groups of genes to produce specific proteins to enable them to take place, either producing hormones, body tissues or even a body characteristic. In the case of the gerbil, the characteristic we are interested in is their hair, or more specifically, their coat colours.
The proteins that are produced by the coding of DNA are very specific in nature, so if the sequence of their DNA is altered, even slightly, this can affect the proteins function by either partially stopping, stopping altogether or even completely altering the protein that is produced. This change in the DNA is referred to as a "mutation".
For an organism to function properly, each individual cell depends on differing proteins to function in the right place and at the right time. If a mutation occurs in a protein that plays a critical role in the body, a medical condition will result, which is known as a genetic disorder. However most mutations have little impact on health, for example they may only alter a genes DNA base sequence (see the flash movie above to learn about bases) but will not alter the protein function that is made by a gene. In most cases, as mentioned earlier, mutations that could represent a genetic disorder are repaired by the DNA repair system of a cell. Individual cells have several pathways in which enzymes recognise and repair DNA, and because DNA itself is subject to damage and mutation in many ways, the whole process of DNA repair is an important factor in protecting the body from disease.
In hereditary diseases we are dealing with a mutation that is present in a germ cell, which in turn will give rise to offspring carrying the mutation in all of its cells. However a mutation occurring in a somatic cell of an organism will cause this mutation to be present in all the descendents of this cell, and certain mutations can cause the cell to become malignant, which then will gives rise to cancers.
It has also been shown that mutation rates will vary from species to species, and evolutionary biologists theorise that higher mutation rates are beneficial to a species in certain situations, because they allow the organism to adapt and evolve much more quickly to their changing environments. We can see this with bacteria that are repeatedly exposed to antibiotics, and the selection of resistant mutants can result in the strains of bacteria that have a much higher mutation rate than the original population. In science these are referred to as mutator strains.
In all wild populations of animals there will always be some forms of slight variations in their DNA, and animals that are radically different from the norm may not fair so well for obvious reasons. However some mutations or variations in the DNA can have advantages that benefit the species long term. Mutations create variations in the gene pool, and natural selection eradicates the less favourable mutations from the gene pool. The more favourable types of mutations accumulate in the gene pool, which over time results in evolutionary change within the species. Imagine a butterfly species that through ultraviolet radiation from the sun, produced a change of wing colour in its offspring. Now in most instances this wouldn't do the species any good, and there was certainly no purpose for it at the molecular level, but if this change of colour made it better for the butterfly to evade predators and was effective camouflage, then this mutation increases the butterfly's survival rate and through successful reproduction, this mutation will then be passed on to its offspring. Over time the number of butterflies with this mutation will form a large percentage of the original species population.
Diseases or parasites which take advantage of an animal's specific protein may not fair so well if that protein had been altered due to a mutation, and due to this occurring they maybe resistant to the disease or the parasite. These variations in the DNA can benefit the species over time. An extreme example of this in humans would be the recessive mutation that causes sickle cell disease. This disease is much more common in sub-Saharan Africa, where malaria occurs very frequently, however it can still occur in other ethnic populations. As a result of this, those with one allele of the sickle cell disease are resistant to malaria since their red blood cells are not affected by the parasites. Those with two alleles, although resistant to malaria have the accompanying sickle cell disease which is extremely debilitatating and reduces their lifespan considerably. This mutated allele has incomplete dominance, which means that even individuals who have just one mutated allele will still retain immunity to the disease. Although the positive side effect of this mutation is beneficial and Mendelian laws makes it possible for some people to carry the advantages without the disadvantages of full blown sickle cell anaemia, there would need to be further mutations to solve the problem of the full blown disease in the population.
A very small percentage of all mutations though do have a very positive effect and these mutations leads to new versions of proteins that help the organism to adapt and survive in a changing environment. In humans we have an example of this with a specific 32 base pair deletion CCR5 (CCR5-Δ 32) which results in HIV resistance to homozygotes and delays the onset of AIDS in heterozygotes. The CCR5 mutation is much more common to people of European descent. There exists a theory that maybe a reason for the relatively high frequency of CCR5-Δ 32 in the European population is that it gave resistance to the bubonic plague in mid-14th century Europe. People who had this mutation survived the plague, so obviously the frequency of the mutation increased in the population. This could also explain why this mutation isn't present in African populations as bubonic plague never reached there. Other theories say that it was selective pressure placed on the mutation by smallpox and not bubonic plague. Either way it shows how such a specific mutation becomes beneficial to a population over time.
Understanding The Primary Coat Colours
If we now take the known mutations that have occurred in gerbils, how would these individual mutations look on an Agouti coat colour? We know that the Agouti coat is the true or original coat colour of the Mongolian gerbil; it aids in camouflage and helps it to blend in with its surroundings. The white belly serves a purpose too and aids in thermo-regulation in the extremes of its natural climate. However in captivity there is neither the need for camouflage or thermo-regulation, so a new mutation that affects the coat colour can flourish through selection.
So let's take a look how these coat colour mutations affect the colour to form the initial primary coat colours in the gerbil. Of course if we start combining several of these recessive genes, many other coat colours appear, but firstly we need to understand how just the initial mutations change the coat colour in our Agouti gerbil.
N.B. In all the genecode examples below, they have been written with the intent of showing all the Loci as wild genes except for those at the loci that are affected by the relevant mutations being discussed.
So here's our Agouti gerbil. Now before any mutations occurred it had dominant or wild genes at each of the known loci, these being AACCDDEEGGPP
non-agouti (a)
The Black or non-agouti mutation causes a dramatic change to the Agouti coat. It completely removes the white belly and takes away the yellow band from the hair in the Agouti coat colour. The end result is a black coated gerbil.
With the occurrence of the non-agouti mutation we now essentially have two distinct coat colour types. The White Bellied type and the Self type. These two distinct coat colours form our base colours from which we can apply further mutations to, which will in turn further change the colour. From here we can see how the rest of the mutations affect these two main coat colour types.
Chinchilla Medium (cchm)
With the occurrence of two recessive mutations at the C locus, they enable the Agouti and the Self Black to change coat colour once more. The C locus itself controls the amount of colour in the coat, and the Chinchilla Medium mutation reduces the colour intensity by removing most of the yellow from the coat, but leaves pigment relatively unaffected at the gerbil's extremities. These areas are its feet, nose, ears & tail. On an Agouti coat colour this gives rise to the Colourpoint Golden Agouti.
On a Self Black coat the same mutation gives us the Colourpoint Black, also known as the Burmese Coat colour variety.
Himalayan (ch)
The second mutation at the C locus is known as the Himalayan mutation. It essentially acts in a very similar manner to the Chinchilla Medium mutation, but is more extreme in it's actions, and leaves pigment only at the tail. It also has the ability to remove the pigments from the black eye colouring. On an Agouti coat it produces a gerbil known as the Dark-Tailed White. The tail is a light sepia colour and often it takes many months for any colouring to appear on the tail.
On a Self Black coat, it again produces a Dark-Tail White, but this time the tail is much darker and is a very deep rich brown.
Dilute (d)
The next Locus is the D locus. This controls the depth, or intensity in a gerbil's coat. A recessive mutation here dilutes the coat. In gerbils, this mutation acts mainly on diluting the black pigments in the coat, and only slightly dilutes the yellow pigment. On an Agouti coat it will dilute the colour giving it a slightly washed out appearance.
On a Self Black coat the change is more dramatic, producing the well known Blue coat colour variety.
Extension of Yellow (e)
The following Locus is the E locus, or the Extension Locus. This locus is responsible for controlling the balance between the black and yellow pigments in the coat. To date, two mutations have occurred at the Extension Locus. The first mutation is known as the extension of Yellow. On an Agouti coat this mutation produces the well known Dark-Eyed Honey coat colour.
On the Self Black coat it has a very unusual effect as the gerbil matures into adulthood. As a pup the gerbil is a yellow colouring, but as it moults into its adult coat it develops exaggerated dark ticking to the hair which results in the Nutmeg coat colour.
Fading Yellow (ef)
The second mutation that occurs at the E locus is Fading Yellow. The gerbil as a pup is very similar to the Dark-Eyed honey pup and is a rich yellow colour, however as it ages and moults the yellow coat fades to an off-white, leaving pigment only at the nose and tail. This is known as the Schimmel coat colour. On an Agouti coat the mutation turns the coat into off-white and the nose and tail are a light yellow colour.
On a Self Black the effect is very similar on the coat, however the nose and the tail are a richer yellow colouring.
Grey (g)
The G locus controls the intensity of the yellow and black pigments in the coat. The recessive mutation at this locus reduces the intensity of these pigments. On an Agouti gerbil it will remove most of the yellow in the coat but will also slightly dilute the black pigment as well. The result is the Grey Agouti coat colour.
On a Self Black coat colour the pigments are diluted slightly to produce the Slate coat colour variety. The gene also has the ability to slightly reduce the pigment in the eye as well.
Pink-Eyed Dilution (p)
The Last of the recessive mutations occurs at the P locus. The mutation here is known as the Pink-Eyed Dilution mutation. On an Agouti coat it removes virtually all the black pigment and turns the eye colour to ruby. This coat colour is known as Argente Golden.
On a Self Black this mutation results in the Lilac coat colour variety.
Dominant Spotting (Sp)
As its name suggests, this mutation is dominant in nature. Rather than being a colour, it is seen as a pattern that overlays any existing coat colour. So spotting itself can occur on any coat colour variety. Along with the spotting mutation there exists modifying genes which act on spotting to extend its markings.
These modifying genes enable the gerbil to have an extensive range of white markings from the simple spot to the variegated coat colour variety, as seen below in our colour strip.
Below are two useful flash demos that show you at a glance how the various mutations affect both the Agouti and the Self Black coat colours. Just click the buttons to see how each individual mutation alters the coat colours.
Golden Agouti
Black