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The Expression of Characters

A kitten may receive a gene for one particular character from its sire and a similar one from its dam. Such a kitten, one with two identical genes, is said to be homozygous for the character concerned. On the other hand the sire may pass on one gene and the dam may pass on an alternative at that particular locus. The kitten that has two different genes at one locus is said to be heterozygous for the character concerned. Thus homozygotes have identical genes for a character and heterozygotes will have different ones. If the cat or kit is homozygous for any one character then that character will be expressed irrespective of dominance or recessivity. If the cat or kitten is heterozygous for any one character then expression will depend on dominance and recessivity of the two genes concerned. A good example of this is seen in the Siamese queen who "gets out". She may mate with the next door tom cat who has no Siamese in his ancestry at all. Any kitten from such parents will receive the gene for the Siamese dilution from its mother because she has no genes for full color to pass on. Similarly it will receive genes for full color from its father because he has no genes for siamese to donate. Such a kitten will therefore be heterozygous at the locus concerned--the Albino locus. Since full color is dominant to siamese dilution the kitten will be fully colored and the recessive siamese dilution of color will not be apparent to the eye. Thus the kitten, although it has siamese in its genotype, will not show this in its phenotype.

As has been described, genes governing a particular character and occurring at the same locus are called alleles. Many, but not all, alleles show dominance or recessivity. In Mendel's original experiments the alleles studied were either dominant or recessive, however, there are certain circumstances where the matter is not quite so simple. In writing of dominance and recessivity it is usual practice to describe the dominant character by the use of a letter or letters symbolizing the locus concerned in upper case. Thus, the dominant gene for agouti in the Agouti Locus is symbolized A. Similarly it is usual practice to describe the recessive character by the use of a letter or letters symbolizing the locus concerned in lower case, and the recessive gene for non agouti at the Agouti Locus is therefore symbolized a.

In fact geneticists have evolved a sort of code and to make certain that the symbols used were common to geneticists all over the world a special international committee was set up to review the use of symbols in cat genetics. Members of the committee were drawn from scientists with a special interest and experience in cat genetics and representing a number of different countries. The British members are Dr. A. G. Searle, Mr. Roy Robinson and Mr. C. E. Dyte. Other members are B. W. Moffatt, Dr. N. Todd, C. E. Keeler, and T. Komai. The committee produced its report in 1968 and further details will be given later.

A group of alleles at a locus is known as a Series and a series may include two or more allelic genes. The Agouti Series already mentioned is concerned with the banding of hair and present knowledge classifies either A Agouti or a non agouti. Agouti is the normal or wild type and non agouti is the mutant. Very often the plus sign is used to describe the normal or wild type gene with whatever letter symbolizes the mutant being used in either upper or lower case depending on its dominance or recessivity, as the alternative. For example white is dominant and symbolized W--its alternative (non white) is usually symbolized + while Cornish Rex which is recessive and symbolized r has as its alternative +.

In some cases the homozygote is identical in appearance to the heterozygote and the heterozygote is in its turn different to the alternative homozygote. This is called full dominance and to use the Agouti Series as an example it can be said that AA is equal to Aa but not equal to aa, which means that the homozygous agouti is equal in appearance to the heterozygous agouti but that heterozygous agouti is different in appearance from homozygous non agouti. Therefore Agouti is said to be fully dominant. In other cases the homozygote is not identical to the heterozygote but the heterozygote is identical to the alternative homozygote. This is called recessivity and can be seen in the Cornish Rex--rr is not equal to r+ which is equal to ++ which means that homozygous Cornish Rex is not equal to heterozygous Cornish Rex but that heterozygous Cornish Rex is equal to homozygous normal or wild type.

 

Dominant and Recessive Expression
Dominant Expression as in Agouti Recessive Expression as in Cornish Rex
Gene 1 Gene 2 Expression Gene 1 Gene 2 Expression
Homozygous agouti giving agouti phenotype Homozygous rex giving rex phenotype
Heterozygous agouti giving agouti phenotype Heterozygous rex giving non rex phenotype
Homozygous non agouti giving non agouti phenotype   Homozygous non rex giving non rex phenotype

 

Some genes are only partly dominant, the homozygote being dissimilar both to the heterozygote and to the alternative homozygote. In this case the heterozygous individual shows the intermediate expression. Recessive genes can be partially dominant or fully dominant over their mutant alleles. The effect of a recessive gene being only partially dominant over its allele next down the scale can be seen in the Albino Series where burmese cb is only partially dominant over cs siamese. Both are recessive to full Colour C. Thus the burmese siamese heterozygote, sometimes known as Tonkinese, is the result of partial dominance of cb over cs. Thus cbcb is not equal to cbcs and not equal to cscs meaning that the burmese homozygote is different to the burmese/siamese heterozygote which, in its turn, is different to the siamese homozygote. The Tonkinese is thus an obligated heterozygote inasmuch as it could not occur without heterozygosity.

 

Incomplete Dominance Expression
Gene 1 Gene2 Expression
Homozygous burmese giving burmese phenotype
Heterozygous burmese/siamese giving intermediate or Tonkinese phenotype
Homozygous siamese giving siamese phenotype

 

Genes at different loci can affect each other. e.g. sex linked Orange is epistatic over agouti. Epistasis describes the masking effect of one gene by another gene and such a gene is therefore described as epistatic over the other. Some characters are depemdemt on two or more genes acting together and these characters cannot be formed by either of these genes alone. It is probable that many phenotypic characters are the result of interaction between several or many genes. It is also probable that most, if not all, genes influence several characters although their influence on one character may be most evident. When a gene causes changes in more than one character the gene is described as pleitropic. Most pleitropic effects known are pathological and may involve a variety of different tissues. Most genes probably affect viability and fertility.

Genes which have a marked effect on viability lead to a disturbance of the ratio of progeny expected and in the most extreme examples the genes cause the death of the embryo. In canaries, for example, there is a type of white which causes death in the homozygote embryo while the heterozygotes are not affected; in man there is a disease of the red blood cell which is lethal when homozygous but has very little effect when heterozygous.