College of Cat Genetics: Part XII
by Patricia Turner
Study Unit 12
Genes in Action
Genes are not simply separate elements controlling individual effects. In fact they work with each other and with the cat's environment in a variety of ways. There are no genes individually responsible for characters such as an undershot jaw, thick tails, etc., although it is true that, in many cases, characters such as coat colour may be affected to a greater degree by one particular gene. In such circumstances, the gene is named for the effect it produces but it should be remembered that it may well have other perhaps less noticeable effects. Any one character is affected by a number of genes and its final phenotype is dictated by the way in which the genes work together and with the environment.
When two gene pairs, each having two alleles, are segregating independently, the consequences are easily predicted. As has been shown, a cross between heterozygotes for both pairs will give kits of nine genotypes, i.e. D/d.r+r x D/d.r+r = 1/16 D/D.r+/r+; 2/16 D/D.r.r+; 2/16 d/D.r+/r+; 4/16 d/D.r+/r+; 1/16 D/D.r/r; 2/16 d/D.r/r; 1/16 d/d.r+/r+; 2/16 d/d.r/r+; 1/16 d/d.r/r. All the while assortment at each gene pair is independent of the other and the existence of the genotypes not impaired by lethals then the above ratios will hold true for all crosses. However, genotype ratios can only be assessed by phenotype ratios unless programmes of test matings are undertaken with all those cats owning genotypes in question. It is only the phenotype that is seen and it is because some of the different genotypes show similar phenotypes that the ratios of the second do not always reflect those of the first. In the case of complete dominance the phenotype ratio of progeny from such a mating as that described above is 9 Black Non Rex; 3 Black Rex; 3 Blue Non Rex; 1 Blue Rex, i.e. 9:3:3:1.
Table showing the correspondence between Increase in Variability of Genotypes and Phenotypes with the Increase of Gene Pairs when Dominance is Complete Number of Gene Pairs Different Types of gamete formed by F1 heterozygote Number of possible combinations of F1 gametes (perfect population size) Different types of genotype in F2 Different types of phenotype in F2 1 2 4 3 2 2 4 16 9 4 3 8 64 27 8 4 16 256 81 16 5 32 1024 243 32
Most of the crosses so far demonstrated have been with cats homozygous and heterozygous for alleles proved to act as simple dominants. In simple complete dominance the heterozygote, although genotypically different, appears identical in phenotype as one of the two homozygotes, i.e.
a/A = A/A
The presence of the recessive gene is hidden. Any lesser effect in the phenotype of the heterozygote towards the recessive alternative can be accounted for by partial or incomplete dominance.
Incomplete dominance seems to be the rule throughout the albino series, the chinchilla or silver/burmese hybrid has been demonstrated as a phenotype showing characteristics intermediate to those alleles. There also seems ground for the opinion that there may be incomplete dominance between chinchilla (or silver) and siamese and this could perhaps account for the cats reported in gene surveys by A.G. Searle and B.J. Marples. These cats are reported as showing faint tabby pattern on the body but a darker one on the extremities and are regarded by Searle to be different from Siamese dilution. As their eye colour is unknown they could, of course, be tabby burmese but incomplete dominance would account for the cat in the possession of the author whose genotype combines chinchilla/silver and siamese. This cat is similar in phenotype to the White or Chinchilla Tiger with muted tabby pattern on an off-white ground and blue eyes.
There are many types of interactions between genes of different loci, one of these can be seen in the way that variations in the blue eye colour of the Siamese are caused by the presence or absence of brown or dilution, etc., in the genotype. Other genes mimic each other, similar phenotypes complementing each other's action, so that crosses between them give normal wild type progeny.
If one gene is epistatic to the other then the resultant ratio is different again. Epistasis is a type of interaction between non-allelic genes where the presence of a gene at one locus prevents the expression of that at another. Examples in cats are seen where the non-agouti mutant such as the blue or black masks expression of, and is thus epistatic to, the tabby alleles, so that except for ghost markings in kittens the actual patterns cannot be seen. Fanciers normally use the word tabby to describe the combined effect of genes for agouti and tabby patterning but, technically, tabby is a word describing the patterns carried in the genotype of all cats, whether agouti or not. Agouti which controls hair banding also allows expression of the tabby alleles but the presence of homozygous recessive non-agouti prevents the expression of the tabby pattern as well as that of hair banding at its own locus.
Orange (or red or yellow) masks the expression of non-agouti. Here then the tables are turned and even the non agouti cat will show tabby pattern if it is also orange. The effect of tabby pattern can be lessened by selection for the abyssinian allele, but nevertheless, it remains present. In this context, abyssinian describes the tabby pattern seen in the Abyssinian cat and not the breed itself. The epistasis of orange over non agouti accounts for difficulties in the breeding of Self Reds as while in the long haired varieties the pattern may be broken to a certain extent there are still noticeable tabby markings seen in the shorter hair of the head.
Sometimes genes are described as hypostatic, this condition being the exact opposite of epistatic much as recessive is the opposite to dominant. Thus tabby pattern is hypostatic to non agouti and non agouti is hypostatic to orange etc. A proposed explanation for the inheritance of blue eyes in white cats involves the hypostasis of a single gene to the normal alternative at the white locus. Simply explained this suggests that the gene concerned can be present in the genotype of both white and coloured cats but that its effect is suppressed in the latter. This hypothesis would seem to explain the inheritance of blue eyes in white kittens from yellow, copper or gold-eyed whites mated to coloured partners and since it suggests that deafness is associated with the same gene but randomly expressed, it also seems to explain the production of deaf white kittens from non deaf white x coloured parents. The type of blue eyes under discussion is not the blue eye of the siamese dilution or that of the blue eye resulting from the interaction between white and siamese, but the inference is that both genes, i.e. siamese and the blue eye gene, can be inherited alongside each other.
The older hypothesis proposed that white interacts with white spotting and that certain white masked spotted phenotypes (where the masked colour does not surround the eyes or ears) produced the effects of blue eyes and deafness. On this hypothesis all blue eyed white other than white masked Siamese or deaf white cats of any eye colour would at least have to be heterozygous for white spotting but data collected from British breeders has since proved this is not always the case. However, the phenomena serve as good examples of the way in which genes can work together.
Some genes are known as polygenes and although further consideration will be given to polygenic or quantitative inheritance later, it can be explained that they are additive in effect building up an expression by minute degrees. Sex linked and sex limited genes have already been simply described and they, too, will be discussed more fully at a later date.