College of Cat Genetics: Part VII
by Patricia Turner
Study Unit 7
Single Dominant Gene Difference
Inherited characteristics other than those passed down as a single recessive can be traced in a similar manner and an example of a pedigree involving the inheritance of a dominant characteristic--in this example, agouti--is given. The red tabby cat symbolizes the heterozygous agouti cats, the blue tabby symbolizes the homozygous agouti cats, and the gold cat symbolizes the homozygous non-agouti cats.
C D E F GH I
J K L M
R S T U V
N O P Q
Part of a pedigree showing the inheritance of agouti as a dominant characteristic. AA is homozygous agouti, aa is homozygous non-agouti, and Aa is heterozygous.
The genotype of the agouti cats D, F, H, I, J, K, R, S, and U is bound to be heterozygous agouti since one of the parent cats in each mating is non-agouti. Cat B cannot be homozygous agouti as if he were all his progeny would be agouti even when he was mated to a non-agouti mate. The fact that he produced one non-agouti cat (G) proves him to be heterozygous. Since the cats N, O, P, and Q are the result of a mataing between a known heterozygote (H) and an agouti female of unknown genotype (I) their genotypes are uncertain without classification of their progeny from test matings. Sufficient records on their progeny would not only give the answer to the question of their genotype but also to that of their dam (I)
The Lack of Blending
During this century there have been thousands of experiments carried out in hundreds of species and showing that genes segregate in a precise and constant manner in each generation. Of course, genes may become changed by mutation. On the whole, genes are not changed by the presence of other genes. For example, the gene for dilution of colour does not change because it finds itself in the presence of the gene for full intensity of colour in the heterozygote Dd or in the presence of genes for other characters such as rex coat or long hair.
Recessive genes normally show no modification even when kept in the heterozygous state for generations. Some stocks of laboratory animals have been kept as heterozygotes for many generations--as an example, a colony of mice inbred with forced heterozygosis for two separate characters has been maintained for over 30 generations. There is no evidence that either the recessive or dominant genes are changed because of their association. No blending has taken place. Certain stocks of fruit fly have been kept as heterozygotes for both recessive and dominant genes since 1919 and the same fact has emerged.
It should not be thought that the presence of a particular genotype excludes any variation in development of the kittens. In fact, as everyone knows, many variations do occur and these can be the result of other factors. It is most unlikely that littermates of the same genotype will be exactly alike in phenotype even though there is usually a family resemblance.
Definite differences can be caused by disease, diet, as well as by other factors and the differences so caused are the subject of a later article. The genotype of the cat provides it with the basic coding necessary and places limits on its development. Within these particular limits, there are factors that may not remain constant between generations.