Cat coloration trivia

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One of the amazing things about cats is how much coloration variance there is. After all, your average human is pretty much the same color all over, but cats display all different kinds of patterning. I've been reading up on it lately and the science is really cool.
  • The Cat Colors FAQ has a really interesting section on how coat coloration works. Amazingly, all that variety is provided by two pigments eumelanin (blackish) and phaeomelanin (reddish), plus a bunch of genes that control pattern.

  • The first cloned cat, CC, has different coloration from her clone mother, Rainbow. CC is a a tiger tabby and Rainbow is a calico. The reason for this is what's called X-linked inactivation.:
    First of all, calicos are almost always female, which means they have two X-chromosomes (versus the male's XY). One of these X chromosomes contains a gene for orange coat color and the other contains a gene for black coat color (white patches are specified by a different set of genes which are not relevant here).

    For reasons which are not fully understood, as the embryo develops, a phenomenon called "X-linked inactivation" occurs, in which one or the other X-chromosome in every cell in the Calico embryo becomes randomly inactivated. If the specific X-chromosome containing the gene for orange coat color becomes inactivated, that cell will go on to produce black coat color (assuming it becomes a coat follicle cell). The inverse is true if the X-chromosome containing the gene for black coat color becomes inactivated.

    Given that the inactivation is random, one would expect a very fine distribution of orange and black hairs within the coat, but for reasons which are not germane here, the inactivation occurs in larger patches of orange and black.

    "Mosaicism" is the term for distribution of different cell types within a single organism. Mosaicism is three-dimensional, meaning that the inactivation of orange or black-producing genes occurs within cells throughout the calico's body regardless of whether the cells have anything to do with production of the animal's coat. Thus, even the specific cumulus cell used to clone CC would have been inactivated for either orange or black coat color.

    If the nuclear transfer process were to reset the inactivated X-chromosome the way it resets the nuclear differentiation, then one might expect to see a calico clone with a calico coat. On the other hand, if nuclear transfer does not reset X-activation then one would expect to see a clone with a black coat if the donor cell used had an orange coat gene on the inactivated X-chromosome, and conversely one would expect a clone with an orange coat if the donor cell used had an black coat gene on the inactivated X-chromosome.

    The fact that CC has no orange in her coat is consistent both with the theory that nuclear transfer does not reset X-activation, and also that the cumulus cell used had an orange coat gene on the inactivated X-chromosome.

  • Siamese trait (the dark pigment on the nose, ears, tail, and feet ["points"]) is temperature-sensitive. The points tend to be colder and so the hair grows out dark. According to Jennifer Gates, who first told me about this, if you shave the fur of a Siamese, the first resulting coat in the area will grow in dark.

Nan Hampton's slides on feline genetics are also a good resource. Interestingly, the wild type cat color, short hair black mackerel (tiger) striping is actually a pretty uncommon pattern in modern domestic cats.

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