College of Cat Genetics: Part VIII
by Patricia Turner
Study Unit 8
The 9:3:3:1 Ratio
The study of single gene differences in what are known as monohybrid crosses led to Mendel's discovery of the principle of segregation and through the use of this principle it became possible to predict the segregation between two different alleles in any single gene pair as well as to predict their subsequent behaviour in succeeding cat generations. However Mendel did not stop his experiments at that stage but went on to consider the progeny of individuals differing in two pairs of genes. Such crosses are known as dihybrid crosses. He chose plants which were true breeding for two dominant characters (yellow and round seeds) and two recessive characters (green and wrinkled seeds). In the F1 generation he obtained seeds which were all yellow and round and even by varying the parents as to which one carried the dominant and recessive characters he never obtained any F1 generation of any different character.
When the F1 plants were allowed to self-pollinate, Mendel obtained four kinds of F2individuals in what appeared to be a new ratio of numbers. The actual results were:
Number of Seeds Seed Color Seed Shape 315 yellow round 101 yellow wrinkled 108 green round 32 green wrinkled
In terms of proportion, these numbers are very close to the 9:3:3:1 Ratio.
Mendel's experiment can be reconstructed with cats as follows:
A cat homozygous for both full intensity of color (black) and straight (or non-rex) coat is mated to a cat that is homozygous for dilution of color (blue) and Cornish Rex coat. All the kittens will be black and straight coated (or non-rex) and when mature and mated either together or to others of the same genotype they will produce an F2 generation in the ratio of:
Number of Cats Coat Color Coat Shape 9 black straight 3 black rex 3 blue straight 1 blue rex
Looking carefully at the F2 kittens, it can be seen that each pair of characters (ie black & blue and straight & rex) shows a ratio of three black to one blue and three straight to one rex . The two pairs of characters are segregating independently and combining independently as well. So the chances for a cat to be black or blue are independent of its chances to be straight or rex coated. This applies to many sets of gene pairs: black & brown and full color & siamese which segregated and recombined can give blacks, seal point Siamese; chestnut brown Foreign and chocolate point Siamese is but one other example.
Exercise:Using the gene pairs black & brown and full color & Siamese, recreate the chart above. Here's the Answer.
The experiment reconstructed with black & blue and straight & rex is shown diagrammatically as follows:
Segregates into gametes:
Segregates into gametes:
The mating of two cats, one homozygous for black color and straight coat and the other homozygous for blue color and Cornish Rex coat. The F1generation are all black and straightcoated. All have the genotype Dd+r
The kittens produced by the mating of the F1 heterozygotes are as follows:
Number of Kittens Genotype Description Phenotype 1 DD++ homozygous black, homozygous non-rex
2 DD+r homozygous black, heterozygous for rex
2 Dd++ heterozygous black, homozygous non-rex
4 Dd+r heterozygous black, heterozygous for rex
1 DDrr homozygous black, homozygous rex
2 Ddrr heterozygous black, homozygous rex
1 dd++ homozygous blue, homozygous non-rex
2 dd+r homozygous blue, heterozygous for rex
1 ddrr homozygous blue, homozygous for rex
Click here to see the Punnetts Square.
Although the kittens carrying genes Dd+r, DD+r, and Dd++ are heterozygous for one or more characteristics they are, of course, identical in appearance or phenotype to DD++ kittens as a result of the dominance of genes for full intensity (black) and non-rex or straight coat.
These kittens therefore have the same phenotype but not the same genotype and until they have matured and produced sufficient kittens for classification it is not possible to state their genotype with any degree of certainty. Two new combinations have arisen in the course of this experiment. The experiment started with two types of cats:
black non-rex and blue Cornish Rex ,
and finished with four types of cats:
black non-rex, black rex, blue non-rex, and blue rex.
The black rex and the blue non-rex are the new combinations and are described as recombinants.
The expected ratio of combinations from a large enough number of matings would, therefore, be 9 black non-rex:3 black rex:3 blue non-rex:1 blue rex. As dilution of colour and Cornish rex coat are said to be on different chromosomes (in point of fact it would be truer to say that they have not been shown to be linked), the ratios given pre-suppose no linkage.
Of course the 9:3:3:1 ratio refers only to the phenotypes produced and by denoting the gene for full intensity of colour as D, the gene for dilution as d, the gene for non-rex coat as + and the gene for Cornish rex coat as r the original cat parents were denoted as DD++ and ddrr. The gametes formed by each of the parents either had the formula D+ or dr which combined to form the genotypes of the F1 kittens Dd+r. Obviously the dominance relationship between D and d was not affected by that between + and r since all the kittens of the F1 generations were black non-rex.. According to the principles of segregation, the gametes of the F1 generation contained D and d in equal proportions. Similarly for +r gene pair, half the gametes contained + and the other half r. There were then equal proportions of D and Dr gametes and also equal proportions of d and dr gametes. To sum up: the four types of gametes produced by the F1 were D+, d+, Dr and dr in a ratio of 1:1:1:1 or 1/4 to 1/4 to 1/4 to 1/4. The probability that each of the four types of gamete from one parent would combine with any one of the four types of gamete from the other is 1/4 x 1/4 = 1/16.