College of Cat Genetics: Part XIII
by Patricia Turner
Study Unit 13
In last issue's Study Unit it was shown that a cat can have a single or double dose (gene) for a characteristic at one locus while at the same time owning a single or double dose for another characteristic at another locus. The fact that the combination of genes at different loci can, under certain circumstances, result in interaction effects was also described. A cat with dissimilar genes at one or more loci is known as hybrid and is said to be heterozygous. The cat with dissimilar genes at one locus is monohybrid; the cat with dissimilar genes at two loci is dihybrid (sometimes described as the double heterozygote); the cat with dissimilar genes at three loci is trihybrid; the cat with dissimilar genes at four loci is tetrahybrid and so on. A general term for a cat known to be heterozygous at a number of loci is polyhybrid.
Thus the F1 cats used to reconstruct Mendel's experiments in earlier Study Units were monohybrid, dihybrid and trihybrid respectively.
All characteristics which may be altered by a single gene are called monogenic. Therefore consideration again of Mendel's experiments shows that the three characteristics concerned in the reconstructions demonstrated blue dilution, Cornish Rex and piebald white spotting (when the extent and allocation of spotting is not considered) are monogenic.
Most characteristics defining breeds of cats are monogenic, other than in respect of build and shape. For example, the Red Persian is genetically similar to the Cream Persian other than in respect of a double dose of dilution at the dilution locus. Considering Persian breeds in general, the differences between most of them can be accounted for by the presence or absence of monogenic characteristics.
Any breeders concerned with monogenic characteristics within a breed category may undertake matings with a view to the production of different colours and patterns.
The Black is the cat with two genes for the non-agouti mutant--place even one of the genes with normal agouti and the cat becomes Brown Tabby. The Blue is the Black cat with two genes for the dilution mutant; the Red is the Black with the addition of sex-linked orange; the Cream as already described is the Red Cat with two genes for dilution and so on.
It must be remembered that a dominant characteristic cannot be carried and that no characteristic known to be fully dominant can be produced by a pair of cats if the dominant characteristic is not shown in the phenotype of at least one of the partners. Thus, no Siamese bred from a chestnut brown can produce more fully coloured cats unless mated to a non-Siamese partner; no seal point Siamese bred from a Tabby Point (Lynx Point) Siamese can produce more Tabby Point Siamese unless bred to a Tabby Point partner; no Black Persian bred from a White Persian can produce more White Persians unless bred to a White Persian partner and so on.
It should also be remembered that whereas two cats owning dominant characteristics may continue to breed those with recessive characteristics if mated like to like, two cats with recessive characteristics will only produce recessive progeny under those circumstances. Thus two Chestnut Browns may breed Siamese, two Tabby Point Siamese may breed Seal Points, and two White Persians may breed Blacks.
It is possible to produce cats with dominant characteristics that are true breeding. This is the case in the Abyssinian breed where most cats are homozygous for the dominant Abyssinian tabby pattern and so, even if bred to the more recessive striped or blotched tabby pattern cats would produce F1 progeny with Abyssinian tabby phenotype. Methods by which the homozygote for dominant characteristics may be produced and proved will be explained in detail in a later Study Unit.
The cat breeds, being generally divided into classifications according to shape and build known as type, are subdivided into classifications of colour and pattern which, in most cases, are inherited as monogenic characteristics. As well as the sub-classifications for colour and pattern, there are also those for coat quality (short, long, upstanding, flat-lying, rexed, etc.). Not all coat colours and patterns are found in conjunction with all coat qualities and not all coat qualities, colours and patterns found in conjunction with all categories of type. Thus, in the cat fancy, although Abyssinian tabby pattern is well known in conjunction with foreign type and short hair, it is not normally seen in conjunction with Persian type and long hair (although this would be an advantage in breeding the red selfs)
Any one breed represents a particular combination of characteristics from each category. If the Blue Persian is analysed genetically it can be said to represent a combination of two genes for recessive dilution with two recessive non-agouti, and two genes for recessive long hair, together with millions of genes with additive effects for producing the required Persian type. It also has one or two normal genes at the locus otherwise giving chocolate brown. The characteristics of type are described generally as polygenic since they are produced by the additive effects of a large number of genes each with small effect. Just as it is said that the last straw broke the camel's back, so it can be said that the last gene produced the Persian type. Although there are certain monogenic characteristics affecting type, seen for example in the absence of tail when the Manx gene is present, the general characteristics of shape and build are inherited in the manner described.
The cat breeder has a number of considerations to bear in mind when arranging programmes of matings.