Genetics of Cat Coats, Siamese Kittens and Calico Females
Genetic Explanations Cat Breed Coats
Cats come in a many colours and patterns, they are elegant and beautiful. But if you look at the molecular level, there are fascinating explanations for cat colours. For example did you know that the ‘point coloration’ of siamese cats, is due to a temperature sensitive mutation? Or that the vast majority of calico (tortoiseshell) cats are female (the few males are usually infertile).
The science of genetics has simple, elegant explanation for the coat patterns of different breeds. There are multiple genes which affect cat coats, these act in something of a hierarchy, with many genes affecting the shading of the coat. For example the recessive d (diluting) allele of the dense pigment gene (encoding melanophilin, a protein involved in the transportation and deposition of pigment into a growing hair), affects the base shade of coat so a cat homozygous for the d allele is grey rather than black, or cream rather than orange. However for simplicity I will write about only two variations, mutations in tyrosinase (the ‘C’ gene) and the red gene (‘O’).
Why do Siamese Cats Have Pointed Colouration? Tyrosinase, Temperature and Albinism
Tyrosinase is a copper-containing enzyme that catalyzes the production of melanin (pigment) from tyrosine. In humans the gene encoding tyrosinase is called TYR and mutations in it lead to albinism. In cats the same gene is referred to as ‘C’ and null mutations, which complete destroy the function of the protein, also result in albino cats with no pigment and pink eyes.
But these are other mutations of the gene found in cats, and some can make the enzyme temperature sensitive. Tyrosinase with this mutation it is inactive at normal body temperature but can function in cooler areas of the skin.
It is the second type of mutation (the cs allele) which results in the point coloration of siamese cats, in which the ears, tails and limbs are pigmented. These extremities are usually cooler than the trunk, at the lower temperatures the tyrosinase enzyme is active, and melanin is produced. The mutation also explains the blue eyes typical of siamese cats, there is some tyrosinase activity in the iris, but less than in wild type cats, resulting in a reduced number of melanocytes (pigment cells).
Because of the mutation all Siamese kittens are born completely white (they are warm and cosy inside their mother). Point coloration starts developing a week after birth, as a result of the skin at extremities becoming exposed to cooler temperatures. Also Siamese cats in hot countries are generally lighter in colour than cats in countries with a colder climate.
The 'White Glove' Coloration of the Birman Cat
Not everything about the genetic of cat coat colour is fully understood at the moment. One still unexplained colouration is that of the Birman breed. Birman cats show the same point colouration as Siamese, but they are characterized by completely white ‘gloves’ on their limbs. It is hard to see how this phenotype fits in with the temperature sensitive mutation of tyrosinase, and is thought that the white limb extremities must be caused by an allele of another gene.
Why are Calico Cats Always Female?
Another gene that plays a role in determining coat colour is the red gene, O. In cats with the red variant (O), phaelomelanin pigment completely replaces eumelanin, resulting in orange coat rather than black.
Calico (tortoiseshell) cats have patches of orange, black and white fur. A puzzling observation was that calico cats are almost always female (1 in 3000 calicos is male, 0.03%). The red gene is sex-linked, it is found on the X-chromosome. In cats (and humans) females are XX, while males are XY. Hence female cats have 2 copies of the O gene, and could be OO homozygous red, Oo heterozygous, or oo homozygous black. Males can only be O, red, or o, black.
The added complication is that one of the X-chromosomes in females is always inactive (it can be seen in cells as the 'Barr body'), otherwise females would have twice the dose of proteins encoded by the genes on the X-chromosome. Having the wrong dose of gene products is usually very bad news. The reason different colours occur in patches, is that cells inactivate one of the X-chromosomes randomlly early during embryonic development.
So a female that is Oo is always a calico. In some cells the O-bearing chromosome is active, giving rise to orange pigment, while in other cells the o-is active making the fur black. Once one of the X chromosomes is inactivated in the early embryo, that skin cell will divide giving rise to daughter cells, which themselves will undergo cell division, creating a patch of cells which are all clones with the same X-chromosome inactivated. This is why the red and orange colour of calico cats occurs in patches, not intermingled.
The few calico cats that are male are usually the result of chromosomal abnormalities. Often they are XXY, and although morphologically male, they are sterile. Occasionally they might have chromosomal mosaicism or be the result of a chimera, two embryos which fused at a very early stage of development. In the last two examples (which are very rare) the male might be fertile.