This article was first written by Eddie Crutchley in 1998 as a supplement to the NCS gazette.
INTRODUCTION
Whilst there are a number of books which explain colour inheritance, I thought that it would be useful to write a document for this website.
The idea is to introduce the common mutations and how they breed paired to Standards and also how they breed when crossed together. The colours dealt with will be those which are commonly available. Fur colour falls basically into two categories which are either dominant or recessive to the Standard agouti type. Whilst this type of inheritance is relatively easy to understand, most features such as Veiling coverage, Density, Length and strength of fur are cumulatively dominant or recessive. This means that a number of genes are involved which can, by careful selection, be increased to give the required result. Often the seeking to enhance one feature will produce an adverse effect upon another. With apologies to those who already understand the basic concepts of genetic inheritance I feel that a short explanation may be necessary before continuing with the colours.
GENETIC INHERITANCE
A knowledge of genetic inheritance will help the breeder to improve the quality of the stock by providing the vehicle by which the required characteristics can be improved. It is assumed that at the paramout of importance are health traits amongst your chinchillas, but aside from these, the characteristics may be colour, size, disposition, or any of the many fur qualities required. The essential point is to remember that any offspring are a result of the union of two individuals and that each contributes a half to their genetic make-up. The results which follow from this union may be influenced by the previous breeding of one of the partners, the offspring being more like one than the other. It is rare for the young to be intermediate between the parents and therefore you are presented with the problem of selection. In any species there are very few outstanding individuals and it is necessary that when such an animal becomes available that it is used to move the whole stock toward the goal of soundness and high quality. If this high quality individual is a male then his influence can be spread quickly through the stock since he can be mated to several females.
The speed with which a female of high quality may be spread will of course be slower. Only when one of the partners has been inbred whilst the other has not, or chancing upon a prepotent animal of indiscriminate breeding (which rarely happens), will these influences be shown.Subsequent generations will tend toward the mean condition unless selection is practised. The means of genetic inheritance is via a coded chemical message contained in millions of genes arranged along the length of a number of chromosomes which occur in pairs.
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A Pair of genes, forming a Chromosome.
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Each type of animal or plant has a particular number of chromosome pairs, the Chinchilla has 16 pairs, the horse 30, monkeys, rabbits and humans each having 24. The same number of chromosomes are situated in each cell of the body. Obviously some bodies have far more cells than others, the amoeba has a single cell whereas we have many millions. However, the message contained within the cell in that particular part of the body has its own coded message which enables it to act in a certain manner. Cells in the liver, kidneys, muscles, eyes etc, each doing their own thing in order to promote correct functioning of the whole body. As stated earlier there are thousands of genes to each chromosome and these are also arranged in pairs. One of each pair of genes to a corresponding chromosome
There is a particular position or locus for each gene on the chromosome and the genes in this position have a certain function. For instance the genes controlling white coat colour are opposite to each other on the same locus on each of the corresponding chromosome pair.
During normal growth or replenishment the cells go through a process known as mitosis,
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Comparison of Mitosis to Meiosis
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when each cell divides into two absolute replicas of the original. There is another form of cell division which is known as meiosis which is where the cells of each parent divide before conception. Remember each cell of each species has to have the same number of chromosomes, half from each parent. Thus the cells of each parent divide and it is pure chance as to which of the gametes (separated pairs of chromosomes ---- sperm or ovum) from one parent will combine with the gametes of the other. After combination they are said to be a zygote, a fertilised ovum having a complete set of chromosome and gene pairs. The genes at the same locus on corresponding pairs of chromosomes are described as alleles.
When the two allelic genes both have the same characteristic message the cell is homozygous ("Homo" = the same), but when the allelic gene pair have differing messages the cell is heterozygous ("hetero" = different). We can now look at a pair of allelic genes and see how they pass on their inherent characteristic. The male has the two genes P and P', whilst the female has the genes Q and Q'
The offspring shown are those which it is possible to produce, and over a number of matings these theoretical results should prove to be correct. It is however perfectly possible to have all of one litter to be of one genetic possibilities. How the offspring look will depend on the relative 'strength' of each of the genes in the possible pairs. It is usual that some genes are dominant whilst others are recessive to the standard for that species. For the moment we need only consider these two conditions.
As an example we should look at a characteristic with which we are reasonably familiar. We know that cattle may be horned or polled. The gene for polled is dominant to that for horns therefore, when an animal which is a homozygous poll is mated to one which is homozygous horned then the offspring will all be polls, receiving a poll gene from one parent and a horned gene from the other, the single poll gene taking precedence over the gene for horned in the phenotype of the offspring. When these heterozygous polls are mated together then the offspring will in the main be polled with some, about one in four, being horned. In this case the offspring may receive either type of gene from each parent and only those which receive a horned gene from each parent will have horns, this is typical for a recessive characteristic. Those which have either one or two poll genes will not have horns, showing the dominance of the poll characteristic.
Mating a horned animal to one which is a heterozygous poll will give half polled and half horned offspring, the offspring taking their phenotype in proportion to the two different alleles of the hetero-poll animal. The presence of a gene for poll being sufficient to produce this phenotype. (I leave you to work out why most wild cattle were horned?)
When we observe an animal we see its outward appearance - its phenotype. We may have no idea from observation what the animal might breed. This is somewhat like switching on an electric toaster, you cannot see the electric current but you can observe the results. It is only when you 'switch on' the animal and breed from it that you can see how it is really made - its genotype.
A dominant gene will cause the characteristic it represents to be exhibited in its phenotype, even when in the heterozygous state.
A single dominant gene will show itself in the phenotype.
Recessive genes will only exhibit their characteristic as the phenotype when in the homozygous state.
A single recessive gene will be masked by the phenotype of its allele.
Another concept of dominance is to consider the mutation genes for Wilson White, Tower Beige and Gunning Black as the top of a three tier system. The gene for Standard fur colour has the mid-position with the mutation genes for Larsen Sapphire, Sullivan Violet and Brouke Charcoal as the lower tier. When the lower tier genes are in the homozygous state then the recessive colour they represent is expressed in the phenotype. If a lower and mid-tier gene are together in the heterozygous state then the Standard colour is seen as the phenotype, the Standard being more dominant. The phenotype of the homozygous Standard is the same as that of the heterozygous recessive. The combination of a mid and upper tier gene in heterozygous form will result in the phenotype of the higher tier gene, the most dominant. Only one of the dominant colours mentioned above exists in the homozygous dominant state, the effect of the double gene enhancing the action of the heterozygous condition. The Tower Beige is the dominant mutation where the colour in the heterozygous state is darker than that of the homozygous condition. The other two dominant colour genes cause a lethal effect and so far as is known there have been no homozygous Wilson White or Gunning Black mutations produced.