College of Cat Genetics: Part VI
by Patricia Turner
Study Unit 6
Pedigrees of Single Gene Differences
It has been explained that the characteristics of kittens can be predicted from the genotypes of their parents providing that the breeder had sufficient knowledge of the genes he is manipulating in arranging the mating. It has also been explained that the genotype of a cat can be determined sometimes by its phenotype and sometimes by the ratio of characteristics appearing in its progeny.
Segregation for single character differences has been reported in practically every species it is possible to study. One character widely observed in cats to be inherited as a single gene difference if that of dilution of colour. Dilution converts black to blue, brown or chocolate to lilac, and red or orange to cream. Dilution of black to blue was used as the example in the reconstruction of Mendel's experiments. Normal pigmentation is designated D for full intensity of colour and the dilution gene is designated d for dilution. As dilutiuon is recessive to full intensity of colour, it is designated by a lower case letter and the dilution cat must be homozygous for the gene and is designated dd. The appearance of a recessive dilution therefore requires the inheritance of one dilution gene from each parent cat. In some cases one or both of the parent cats can show dilution, in other cases both parents may appear normally pigmented but carry the dilution gene in heterozygous condition.
When sufficient information is available, the exact method of inheritance for any single gene can be traced through pedigrees. In this context, however, the word pedigree does not describe the printed form probably issued by a cat club and detailing the registered names of the kittens ancestors but, instead, describes a sort of chart where complete details of the litters from all the kitten's ancestors and details of the kitten's littermates are given. Males are signified by squares, females by circles, and cats whose sex is unknown (or has not been recorded) by triangles.
Single Recessive Gene Difference
An example is given of a hypothetical pedigree for recessive dilution of colour on black cats. The black cats are symbolized by the white symbols and the blue cats (the mutants) by the shaded symbols.
The male parent cats are represented as squares and the female parent cats (represented as circles) are connected by a mating line and their offspring are connected by a horizontal litter of sibship line below the parents. Each litter is connected to an individual litter line. When, as very infrequently, two kittens are reported as sharing only one placenta at birth (identical twins) the method used to designate them is to draw a line to each cat coming from the same base point.. Some litters are quite large and include a number of kittens similar both for sex and the gene understudy. When this is the case, it is not necessary to symbolize them separately and one symbol can be given accompanies by the code numbers or letters used to denote the kittens concerned. This is shown in the pedigree example at W to Z. However, this method cannot be used if it is intended to record progeny from any of the kittens so grouped. In that case they have to be drawn separately. (In the table below we have note differentiated the sexes or the twins. Blue cats are used to represent blue cats, a black cat is used to represent a homozygous black and a dark grey/black cat is used to represent a heterozygous black. Gold cats are used to represent cats with a black phenotype but unkown genotype.)
E F G HI
K L M
N O P Q R S
T U V W-Z
Pedigree showing the inheritance of dilution of colour as a recessive characteristic. DD is homozygous full intensity (black); dd is homozygous dilution (blue); and Dd is heterozygous (black carrying blue).
Study of the example pedigree will show that dilution is inherited in a single recessive gene. Cat A is blue, Cat B is black. Both C and D are black but because they produced 2 blue cats in their progeny K and L they are clearly heterozygous (carrying blue). Cats E, F, G, H and I are all black but because one of their parents is known to be blue (A) they too must all be heterozygous Dd. The mating of I and J (both black but heterozygous) results in two litters totalling 13, 7 of which are blue. Until cats P, S, W, X, Y and Z are mated, their genotypes cannot be stated with any certainty: they could be homozygous black or they could be carrying blue dilution. The ratio of kittens appearing in their progreny will give the answer.
Had dilution of colour been a dominant characteristic, it would appear in every generation in order still to be present in the third. It would not have been able apparently to disappear in the second generation (as in litter E to I) and then subsequently reappear in the third generation litter.
It is by tracing pedigrees that the mode of inheritance of characteristics can be established, and this explanation must make it apparent that full details of all kittens in every cross must be available before this can be done. It was by this method that the mode of inheritance for the gene producing White or Chinchilla Tigers was established.
Clearly, details only of a single kitten and its parents could be misleading and, unless there was supporting evidence, no firm answer relating to the inheritance of any particular characteristic could be given. It is for this reason that requests from cat fanciers for information on any particular gene difference are invariably countered by requests from the advisor for full pedigree details of the cats concerned.