Gerbil Genetics ~Section 5

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When Punnet squares deviate from the known phenotypic ratios

The phenotypic ratios of 3:1 and 9:3:3:1 are theoretical predictions based on Mendel's laws of inheritance, but deviations from these expected ratios occur in certain genetic scenarios, and some of these are applicable to crosses involving gerbil coat colours. Below are some examples that cause deviations from the expected ratios.

  • The alleles in question are physically linked on the same chromosome.
  • The survival rates of different genotypes are not the same. For example if a combination of alleles occurs that are lethal and the affected offspring die in utero.
  • The alleles are incompletely dominant or codominant.
  • There are genetic interactions between alleles of different genes (epistasis).
  • One of the parents lacks a copy of the gene. For example, in humans the male has only one X chromosome which is from his mother, so only the maternal alleles have an effect on the organism These are called sex linked genes.
  • The trait concerned is inherited on genetic material from only one parent. For example mitochondrial DNA is only inherited from the mother.
  • The alleles are imprinted. Normally in diploid organisms somatic cells possess two copies of the genome. Every autosomal or non-sex chromosome is represented by two alleles that are inherited from each parent at fertilization. In the vast majority of these autosomal genes, expression occurs from both of these alleles simultaneously. However a small proportion of genes are imprinted, which means that the expression of the gene occurs from just one allele, and is also dependent on its parental origin.

Of course there are other instances such as polygenic traits, multiple allele series, modifying and regulator genes, incomplete penetrance, sex related genetic effects, pleotropy, stuttering alleles and of course environmental influences which can all alter simple Mendelian inheritance, and as scientists learn more about the inheritance patterns for differing traits, it is becoming clear that genes that follow the simple rules of dominance seem to be becoming increasingly rare! It wouldn't be suprising in the future that scientists will discover many other exceptions to the rules of Mendelian genetics.

In 2005 there was an amazing example of this reported in a type of cress plant. These plants seemed to be able to overwrite the genetic make-up inherited from their parents! They were found to be able to revert back to the DNA sequences of their grandparents including genetic information that was lost in the intervening generations. Researchers on the subject have theorized that since the DNA sequences weren't present in the parents, that there could be some form of "template-directed process that makes use of an ancestral RNA-sequence cache".

Genome-wide Non-Mendelian Inheritance of Extra-genomic Information in Arabidopsis, Nature, Vol. 434, No 7032, March 24, 2005

If we take one of the above examples, that where the combinations of alleles are lethal, and the affected offspring die in utero, we can then show how the ratio changes when this occurs using Punnett squares.

Dominant spotting, like its name suggests is a dominant gene, and is usually best dealt with separately from other genes as it works as a pattern that is overlaid onto any existing gerbil colour, and variations in the patterning, like collared, mottled, variegated, are thought to be achieved by modifying genes that work on the spotting gene to achieve this effect. When using shorthand notation here, the wild- type gene is described as '+' as this gene hasn't been allocated a defining symbol in any published literature. Gerbils that are unspotted have two copies of the wild-type gene so are allocated the symbol '++'. Spotted gerbil are allocated the symbol 'Sp', and gerbils are either '++' or 'Sp+' because the 'SpSp' combination is pre-natal lethal and pups with two copies of the Sp gene are reabsorbed in the womb.

We can show how the spotting gene interacts when bred in the following Punnett square demos below...

This 2:1 ratio in the last example is what you would expect from a lethal gene (in the case of dominant spotting it is deemed pre-natal lethal), The normal 3:1 ratio of a dominant gene isn't seen and is replaced by the 2:1 ratio. Many people have misinterpreted these results and chose not two breed two spotted gerbils together believing that a spotted x spotted cross results in small litters because of the death of the homozygotes. However the truth of the matter is that the homozygotic embryos that are reabsorbed in the womb are simply replaced by viable embryos, and this is why we do not see small litters when mating 'Sp' to 'Sp' in gerbils. Their litter sizes remain normal and comparable to their unspotted counterparts.

Separator Gerbils

If we have a gerbil of unknown ancestry and wish to know its genetic code, there are several ways we could go about this. If for example we take our Golden Agouti gerbil again and cross it to another Golden Agouti, and then find that in the offspring that a quarter of them had the black coat colour, we can quickly conclude that both the parents carried the non-agouti mutation and were therefore 'Aa' at the Agouti locus. However if it were just our Golden Agouti that was 'Aa' and the other Golden Agouti was 'AA', then this type of cross wouldn't help us out at all. The most obvious way to find out if our Golden Agouti was carrying the non-agouti mutation would be to breed it to a Black gerbil. This would ensure us that the Black parent only had 'a' to pass on to its offspring, so half of the pups that may inherit 'a' from the Golden Agouti would then be Black themselves.

So, with careful crosses, we can quickly discover what recessive genes our Golden Agouti with unknown ancestry has. If we logically take this a step further and attempt to cover two gene loci, we could cross our Golden Agouti with a Lilac. We know that our Lilac is 'aapp' and has two copies of the mutations at both the A and the P locus, so we could expect four possible outcomes in our offspring. These outcomes could be either:

  • Only golden Agouti gerbils appeared in the litters: this tells us that our Golden Agouti must be 'AAPP', and wasn't carrying either mutation.
  • A mixture of Golden Agouti and Blacks appeared: This tells us that our Golden Agouti was carrying the non-agouti mutation, but not the pink-eyed dilute mutation. So our Golden Agouti must be 'AaPP'
  • A mixture of Golden Agouti and Argente Goldens appeared: This tells us that although our gerbil wasn't carrying the non-agouti mutation, it was carrying the pink-eyed dilute mutation. So our Golden Agouti is therefore 'AAPp'
  • A mixture of Golden Agouti, Argente Golden, Black and Lilac pups appeared: This then tells us that our Golden Agouti gerbil was carrying both the non-agouti mutation and the pink-eyed dilution mutation. So our Golden Agouti is 'AaPp'.

Over the course of several litters we would expect the ratios of Golden Agouti to the various colour mutants to come out as approximately 1:1. This is because in our previous monohybrid and dihybrid crosses we involved both dominant and recessive genes in both of the parent gerbils, and these dominant genes result in the recessive mutations not being expressed in the offspring. As a result the odds are stacked in favour of the dominant traits being expressed, so our ratios change. Here we observe ratios of 1:1 because we have chosen our Lilac carefully (As we did in our backcross/testcross example) and it only has the recessive genes at both the A and P locus.

In this mating we are only concerned in which genes are being passed on to the pups, and essentially it was only our Golden Agouti of uncertain ancestry that we were testing. If we take for example that our Golden Agouti was indeed 'AaPp', there were four possible gene sets that could have been passed on, and all would arise with equal probability, these sets being 'AP' 'Ap' 'aP' and 'ap'. These genes when combined with the Lilacs genetic contribution of 'ap', must then give us Golden Agoutis being 'AaPp', Blacks being 'aaPp', Argente Goldens being 'Aapp', and of course Lilacs 'aapp'. If however our Golden Agouti was only carrying just one of the recessive genes, and was for example 'AAPp', then in the scenario it could only pass on two gene combinations, those being 'AP' and 'Ap' and would give us approximately a 1:1 ratio of Golden Agouti to Argente Golden pups. Even though the Lilac gerbil was contributing 'a' to the offspring, this would then be trumped by the Golden Agouti's dominant 'A' , so all pups would therefore be 'Aa', which only left us with variables of 'P' and 'p' for the offspring to be either Argente Golden or Golden Agouti.

We can of course take this several steps further and use other gerbils with recessive genetics in which to test our Golden Agouti for various genes at the other loci. For example, with our Golden Agouti, if we wished to test for the presence of 'a' 'p' and also 'g' we would then use a Ruby-Eyed White gerbil, but our Ruby-Eyed White would definitely need to be 'aaggpp', as we know that there are several known genetic combinations that can dilute the coat to an off-white and the eyes to a ruby colour. So to do this seriously our Ruby-Eyed White would need to be bred as a pure strain and test crossed to other gerbil strains of known ancestry, all the time eliminating unwanted genes, and repeatedly back crossing to known strains to ensure that only the recessive genes that are wanted are carried in your 'aaCCDDEEppuw(d)uw(d)' separator gerbil.

A very well known colour strain in gerbils that was first developed by the Gerbil Genetics Group (G.G.G.), was the C-separator or as it later became known by fanciers as the Red-Eyed Silver Nutmeg, which was an odd and confusing description of the phenotype because it looked nothing like a Silver Nutmeg with red eyes, and is in reality a cream self coloured gerbil, and its genetics are 'aaCCDDeeppuw(d)uw(d)'. However the name has unfortunately stuck with it, but thank heavens breeders didn't decide to call their Pink-Eyed White colours by the fanciful names of Red-Eyed Burmese, or colourpoint Argente Goldens! In their course of research on gerbil coat colours, the G.G.G. developed several strains similar to this that enabled them to quickly identify the genes of any new colours of gerbils that appeared, as they effectively separate the genes of a gerbil of unknown ancestry.

The main separator gerbils used in the G.G.G's research were constructed after several years of intentional breeding for these valuable genetic "research tools" and includes the following genotypes and phenotypes as set out below:

  • aaCCDDeeppuw(d)uw(d) - This gerbil is a cream self gerbil, and although you may have expected that ee would of brought another diluting factor into the fur colour, it does in fact add more yellow, which effectively gives it its cream colouring.
  • aac(chm)c(chm)DDEEppuw(d)uw(d) - This gerbil resembles the Ruby-Eyed White in coat colour.
  • aac(chm)c(chm)DDeePPuw(d)uw(d) - This gerbil looks like a washed out Silver Nutmeg, with more pigment on the extremities which you would expect from a colourpoint coat colour. Its eyes are dark and are similar to that of a Grey Agouti.

 

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