College of Cat Genetics: Part V
by Patricia Turner
Study Unit 5
Six Possible Matings
In all there are six possible matings that can be made within any one
pair of alleles.
Using the alleles for full intensity of colour (in this case black) and
dilution of colour (in this case blue) yet once again examples are listed
||Genotype of Parents||Phenotypes of Parents||Genotypes of Kittens||Phenotypes of Kittens|
|1||DD x DD||Black x Black||All DD||All Black|
|2||dd x dd||Blue x Blue||All dd||All Blue|
|3||DD x dd||Black x Blue||All Dd||All Black|
|4||Dd x DD||Black x Black||1 DD:1 Dd||All Black|
|5||Dd x dd||Black x Black||1 Dd:1 dd||1 Black:1 Blue|
|6||Dd x Dd||Black x Black||1 DD: 2Dd:1 dd||3 Black: 1 Blue|
Matings 1 and 2 are between cats homozygous for an allele at the same locus. Mating 3 is between homozygotes for different alleles. Matings 4 and 5 are backcrosses of heterozygotes to homozygotes and mating 6 is between two heterozygotes.
Although these matings relate to Blacks and Blues, the results can, in fact, be applied to any matings between homozygotes and heterozygotes for alleles at any one locus. The genotype ratios will always remain the same but, of course, the phenotype ratios will depend on the dominance or recessivity of the character concerned.
A cat homozygous for a characteristic is true breeding for that characteristic and other than by mutation will not produce kittens of any different characteristic. If dominance is incomplete, it is impossible to have a cat that is true breeding for the intermediate characteristic because all matings of the intermediate type cats result in segregation into 1 of the dominant characteristic :2 or the intermediate characteristic :1 of the recessive characteristic.
However, it is possible, by selection of progeny, to build up a pedigree showing four generations, or even more, exclusively of the same characteristic without the cats concerned being true breeding.
In most cases concerning coat colour and pattern, dominance is complete and it is easy to produce cats true breeding for the recessive alternative.
Backcrossing cats that are recessive to those whose phenotype is of the dominant characteristic but whose genotype is uncertain will prove the latter cat to be either one or the other by ratio of the characteristics appearing in the progeny.
If the cat under study is homozygous for the dominant character, all its kittens will show that character (see mating 3 of the table) but the number of kittens from a backcross has to be high enough to ensure that the result is not due to chance deviation in the segregation and recombination. It would ususally be necessary to make at least two backcross test matings and to be able to classify at least eight to ten kittens.
The ratio of any one character to another is taken over a number of kittens and each litter is subject to the same chances. Thus a cat who by genotype may be expected to produce kittens in a ratio of 3:1 may in fact produce 3 identical kittens in her first litter. The second litter will, however, be subject to the same chance so that it cannot be taken for granted that her fourth kitten will be of the alternative characteristic.
Some lethal genes affect visibility and can be detected by the deviation found from the expected ratios. Such lethal genes cause the death of the homozygote embryo and often have a noticeable effect on heterozygotes too.
The results of matings between cats homozygous for a lethal gene that shows its effect in the heterozygote would be the same as those shown in mating 6 of the table. However, the homozygote embryo would not survive so that the final ratio would be 2:1 instead of 3:1.
Breeds of animal produced by the visible effect of a heterozygote lethal gene can be maintained if in each generation the animals showing the effect (the heterozygotes) are mated with animals which are homozygous for the non-lethal alternative. The breed then becomes an obligated heterozygote since the homozygote cannot exist. There are many types of lethal genes and these will be discussed in a later article.
The First Principle
Mendel published his paper in 1866 but it was only in 1900 that it received any real attention. His conclusions were then summarized in the form of laws or principles.
The experiments which the foregoing matings reconstructed led to the first principle known as the law of segregation and which, applied to cats, reads as follows:
Hereditary factors are present in the individual cat in pairs but each gamete carries only one factor of each pair. The two factors of an individual cat are strict alternatives in that one or the other, but not both, can pass into a gamete. They may be alike or different but even when different they segregate unchanged by their association with each other to give pure gametes.