College of Cat Genetics: Part XVII
by Patricia Turner
Study Unit 17
Recent articles in this series detailed mutant genes currently reported in the domestic cat and it is possible that many readers were puzzled by the apparently small number in relation to the wide amount of variation seen in cat phenotypes.
In fact, the list was of mutants and, other that t+ for wild type tabby pattern, did not include their alternatives. Also, of course, it detailed only monogenes, those which work by the presence or absence of single alleles. Many genes with effects upon type and temperament are too numerous and small in individual effect to isolate, study and detail, while the same situation exists with genes producing minute differences in colour, texture of fur and other characteristics. The listing of effects produced by modifiers or genic interaction has never been attempted.
The list published can be likened to a cook's shopping list; it details the ingredients (mutant genes) not already present in the larder (cat gene pool). The actual dishes (cats) produced depend on the recipe (combination of genes) and the method (environment) as well as the effect of some ingredients upon others (genic interaction).
As has been noted by authors on dog genetics, much confusion in popular literature has arisen through failure to distinguish true dominance and epistasis, as well as to treat all characteristics as simple dominant/recessive pairs. Examples of the ratios expected where the characteristic under study is controlled by autosomal (non sex linked) dominant genes have been given in earlier articles; similarly, examples were given of expectations where characteristics are controlled by autosomal recessives. But while each cat only carries two genes at each locus there may well be many such gene pairs interacting together to produce the final phenotype and by this means the ratio of dominant characteristics to recessives may appear to be upset. As has been explained in earlier study units, genes are not always fully dominant or recessive and the heterozygotes may show either an intermediate or completely different effect. Other characteristics may result from polygenes: genes with small but cumulative effect.
In fact, the straightforward genetic ratios can be upset in various ways and it is usually by comparison of observed results with expected results in the classic mendelian ratios that other factors working on inheritance can be detected. Mutation, incomplete penetrance and a number of other factors have to be considered. Mutation, as has been earlier explained, is a sudden change in an individual gene resulting in its effect becoming different (point mutation). Mutation is also used as a term to describe alteration or damage to chromosomes. However, mutation is a very rare event and back mutation to the original form is even more rare.
The situation where cats possess the genetic factors which could cause them to show a characteristic but where, in fact, they do not do so, is known as incomplete penetrance; one proposed explanation of white toes in colored cats is that of a recessive gene with incomplete penetrance. If this is correct then the kittens from a mating of two white-toed colored cats would include some with white toes and some without, while a similar mating involving recessive inheritance of a gene with complete penetrance would give all white-toed kittens. This example is given as an explanation of the term incomplete penetrance and not as a statement of fact upon the inheritance of white toes. It should also be remembered that white toes are quite a different matter from white feet which are caused by the effect of the dominant gene S for white piebald spotting.
The degree of penetrance for any characteristic can be calculated by use of techniques involving the collection of data upon siblings of as many cats as possible. If for example the degree of penetrance for a hypothetical characteristic called E were being calculated, it would be necessary to collect information on E cats with two E parents. Having found such cats, the details of their siblings would be scored. If out of 19 litters from E parents there were 16 E kittens out of a total of 72 (in each litter the originally selected kitten would be excluded from the count) then the score of 16 in 72 would give a 22.2 penetrance. If the litters were ascertained by means of one of the parent cats, the scoring would be made in a slightly different way and all the kittens would be counted.
Incomplete penetrance is caused by the fact that some genes are quite unable to express themselves even when they should otherwise be able to do so because, even though they are present in the cat's genotype, the total genotype is unfavorable for the characteristic they control. There can also be variability in the expression of a gene so that one formula in the rest of the cat's genotype will result in one effect while a different one will result in another. Also, the penetrance, which is really the degree of expressivity of a gene, can well be influenced by other minor genes known as modifiers, or even by the environment itself.
Modifiers are currently suspected by some geneticists to be responsible for the differences between chinchilla/silver and silver tabby, both of which are otherwise suggested as cchcch in genotype. Even without modifiers (which are sometimes known as suppressors) and the complications of incomplete penetrance, there are many more genes than there are chromosomes and not only do the number of allelic genes carried at loci on the chromosome vary enormously so do the number of loci on the chromosome itself. Some chromosomes are larger than others and carry a correspondingly larger number of genes.