Native Plants, Habitat Restoration, and Other Science Snippets from Athens, Georgia

Sunday: 19 June 2005

Cat Genetics!  -  @ 07:18:56
Karen at Rurality has new kittens! And one is a tortoiseshell! Since I love calicos, tortoiseshells, and cats in general, and since I love genetics too, the opportunity to hold forth here is irresistable.

First, most calicos and tortoiseshells are female; the rare male ones are (usually? always?) sterile. In the comments at Rurality it was suggested that orange females are rare. This may be true; I can't say, but I can speculate why that might be the case.

To start with, orange and a certain type of black are sex-linked traits. They're carried on the X chromosome. Sex chromosomes work the same way in cats as in humans: XX is female; XY is male. The Y chromosome is an itty-bitty thing and lacks the vast majority of genes, including coat color, that the X chromosome carries.

So the only way to get an orange AND black kitty is to have two X chromosomes; i.e., kitty must be a female. One X chromosome carries the XO (orange) and the other carries the XB (black). On a white background this gives a calico and on a brown background it gives a tortoiseshell. Since males (XY) only have one X chromosome, they can only be orange (XO/Y) or black (XB/Y), but not both.

Several little details:

Tortoiseshells and calicos are clearly sectored (and what a great name for one: "Sector"!); that is, black and orange aren't evenly mixed, some parts are black and some parts are orange. Patches, not mixtures. How does this happen? During development in females, when there just a few cells, one of the X-chromosomes in each cell gets turned off randomly. So some cells will turn off the orange gene and some will turn off the black gene. All cells that descend from those cells will carry either orange, or black, but not both. Presto! You get unmixed patches of each color. (So "Patches" is a good name too, if you don't like "Sector".)

And by the way - there's a human trait that works this way too - a mutation in an X-linked gene that helps to form sweat glands prevents sweat glands from forming. A female who is heterozygous for this mutation will have sweat glands on parts of her body, but not on other parts.

Second thing - we know there are male tortoiseshells and calicos; they're just rare. A male who is a tortie or a calico has TWO X chromosomes, plus the Y; a chromosomal abnormality. He is then XO/XB/Y, a condition called Kleinfelter syndrome, and it occurs in humans too. My impression is that Kleinfelter kitties would be sterile.

Last thing - again I don't know if female oranges are rarer than male oranges, but if they are, here's a speculative reason:

For a female to be orange she must be XO/XO, that is BOTH X chromosomes must carry orange. For a male to be orange he must merely be XO/Y. The probability that female will get both XO genes is the square of the probability that a male will. If 10% (0.1) of males are orange, then 1% (0.1 x 0.1 = 0.01) of females would be. This is the same reason why sex-linked diseases like hemophilia are more common in male humans than in female humans.

So why aren't black females rare? Possibly because there's more than one way to be a black cat - it could be because of the sex chromosome black gene, or because of a black gene carried on a non-sex chromosome.

Now you know all about genetics.
ADDENDUM: After I wrote this I found an excellent article with pictures at a Clermont College Biology website.

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