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Calico cats

NZ Appaloosas February 17, 2011 February 22, 2011

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Was doing some totally non-horse related research (or well, that's how it started, looking into the cause of the Elephant Man's tumours), went into mosaicism, and came across this statement: "True mosaicism should not be mistaken for the phenomenon of X-inactivation, where all cells in an organism have the same genotype, but a different copy of the X chromosome is expressed in different cells, such as in calico cats." Anyone got further insight? Diane

Maigray Thu, 02/17/2011 - 08:30

You know how all females carry 2 copies of the X-chromosome, right? Well, we don't use both copies. One is redundant (in the somatic cells, anyway). It is inactivated during early embryonic development, creating Barr bodies. The inactivation is more or less random. The gene controlling red or black color in cats is like extension in horses; they are alleles of each other. So a cat can be either red or black on one chromosome, but not both. If they are heterozygous for red and black, then x-inactivation of cells means one chromosome will be expressed and the other will not. Voila - patchwork of red and black over the coat.

rodeoratdogs Thu, 02/17/2011 - 09:40

8-} Ummmm, I really wish I understood what you just said Maigray, because it sounds really interesting. I get the coolest colors and patterns in my Rat Terrier dogs and I understand very little of what makes them and what I do understand I actually learned from Heidi of this forum.

Dogrose Thu, 02/17/2011 - 12:25

I've been trying to find the mechanism behind ej in rabbits, which give the harlequin type pattern of yellow/red and black. Its not sex linked or chimera though it tends to makes a striped pattern like chimeras.

Maigray Mon, 02/21/2011 - 07:45

Oh thank goodness someone understood!

CK - let's think of this in terms of horses. The base coat can be either red or black, right? They can be heterozygous or homozygous for either color. One gene can be black, one red, or both can be black or both can be red.

What [i]cannot[/i] happen is for [b]one[/b] chromosome to carry BOTH a black and a red gene. They are located on the [i]same[/i] gene - you cannot have a black AND a red allele at the same position on the chromosome. You can have black OR red and something else - like cream, you have red and cream on the same chromosome. Two different mutations at different points. But not 2 mutations at one point, at the extension locus.

Okay, so...Cats have a red/black locus that works the same way. They can be heterozygous for red and black or homozygous for either color. IF they are heterozygous for black and red, and they are female, they have 2 copies of the X chromosome. One x = red and one x= black.

Due to X-inactivation in the somatic cells only (the reproductive cells due do something different), one of those X chromosomes is "turned off." Which one is "turned off" is random. It could be the one which carries the red allele or the one which carries the black allele. The result is random patches of black or red throughout the coat.