College of Cat Genetics: Part IX
by Patricia Turner
Study Unit 9
The 1:1:1:1 & 1:2:2:4 Ratios
Since black is dominant to blue, i.e. full intensity of colour is dominant to dilution and non-rex is dominant to rex the F2 kittens produced from the union of blue and rex gametes must be homozygous for both recessives. Such a genotype is known as a double recessive and some other examples in the cat fancy are:The genotype of the double recessive for blue dilution and rex coat is, of course, ddrr and the phenotype is Blue Rex. Kittens of this genotype, if mated, when mature, to others of the same genotype would be expected to produce only more Blue Rex like themselves. This was exactly what Mendel found with his experiments with green wrinkled pea pods. Black non-rex, on the other hand, could be of a number of different genotypes. They could be DD++, Dd++, DD+r, or Dd+r. For example, the F1 kitten from a mating between a black non-rex and a blue rex would be expected to have the genotype Dd+r. To test this hypothesis, the Dd+r cats can be test mated to the parental types as follows:Blue Point Siamese (ddcscs) and Blue Longhairs (ddll)
Although the F1 is expected to produce four types of gametes in equal rations (D+, Dr, d+, and dr) the double recessive partner in Mating 1 will produce only dr gametes. The resultant phenotype ratio is 1:1:1:1. The mating can be diagrammed as follows:1) Black non-rex Dd+r x blue rex ddrr.
2) Black non-rex Dd+r x black non-rex DD++
D+ Dr d+ dr dr Black Shorthair
Ratio: 1 Black SH: 1 Black Rex: 1 Blue SH: 1 Blue Rex
(Punnett Square showing mating between Dd+r Black non-rex and ddrr Blue Rex)
The second mating between the F1 cat and the homozygous Black non-rex produces kittens of only one phenotype--Black non-rex. As all the gametes of the homozygous Black non-rex are D, the kittens produced should consist of four genotypes in a ration of 1:1:1:1 as shown in the next diagram. But, as pointed out, they will all be identical in phenotype.
D+ Dr d+ dr D+
Punnett Square showing mating between Dd+r Black non-rex and DD++ Black non-rex
In his experiments with garden peas, Mendel carried out three tests with a number of plants resulting from a cross such as that detailed in Mating 2 between black non-rex. He found that the seeds assorted into four different types of equal proportions (1:1:1:1). He later selfed most of these and the results were in accordance with an approximate overall ratio of 1:2:2:4. To continue the experiment with cats, this would involve the mating together of the black non-rex cats of the F1 generation in order to classify their progeny and thus prove their genotypes. If this were done, they would be found to have assorted into a ratio of 1:2:2:4; ie 1 DD++ Black Non Rex: 2 DD+r Black Non Rex: 2 Dd++ Black Non Rex: 4 Dd+r Black Non Rex.
It can therefore be seen that F1 cats produced from parents DD++ Black Non Rex and ddrr Blue Rex carried four elements for the two pairs of characters: so at segregation there were four kinds of spermatozoa and four kinds of ova with 16 possible combinations at fertilisation. These F1 combinations can be worked out in two ways: (a) by algebra; or (b) by the Punnetts Square.
By algebra the answer is arrived at as follows: (D+ plus d+ plus Dr plus dr) (D+ plus d+ plus Dr plus dr) = 1DD++ plus 2DD++ plus 2DD+r plus 4Dd+r plus 2dd+r plus 2Ddrr plus 1dd++ plus 1DDrr plus 1ddrr. Rewritten the answer reads: 1 Black non-rex plus 2 Black non-rex carrying blue plus 2 Black non-rex carrying Rex plus 4 Black non-rex carrying Blue and Rex. (The 1:2:2:4 ratio of genotypes in the 9 Black Non-Rex plus 2 Blue non-rex carrying Rex plus 2 Black Rex carrying blue plus 1 Blue Rex.)
By the Punnetts Square, the answer is arrived at as in the diagram of the September/October 1974 issue of Cat World.
The Second Principle
Mendel's second group of experiments reconstructed in this article with Black non-rex and Blue Cornish Rex led to the formation of the second principle which, when applied to cat breeding, can read: When two or more gene pairs segregate simultaneously, the distribution of any one of them to the kittens is independent of the distribution of the others.
These experiments have been reconstructed with Black non Rex shorthairs and Cornish Rex but the results will apply to any two sets of gene pairs. Independent segregation only holds between genes on different chromosomes--genes which are situated on the same chromosome are said to be linked. Linkage has been mentioned briefly in a previous Study Unit and will be discussed more fully later.