A shelter in Loveland, Colorado, happened upon a bit of good fortune this summer when a rare cat showed up at their door. At first glance, there was nothing unusual about the 7-month-old tortoiseshell kitten. But then the staff noticed that the kitten was a male, which made him a very rare cat, indeed. He was so rare, in fact, that they raised his adoption rate to $300, which a local couple very willingly paid.

The story that all calico cats are female is not completely true, as you can see in this case. But it’s almost true - most calico cats (tortoiseshell included) are female, and the reason behind it is purely genetic.

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What makes male calico cats so rare?

If you can remember all the way back to high school biology, you might remember a thing or two about sex-linked traits. In the case of calico cats, coat color is related to sex. A normal female mammal has two X chromosomes (XX), whereas a normal male mammal has an X and a Y chromosome (XY).

The coding for the colors black and orange are found on the same X chromosome, and the color white is found on another. Since females have two X chromosomes, they can have black and orange on one, and white on the other, leading to the characteristic black, orange, and white calico color.

So, how does a male become calico? Usually, this is due to a genetic abnormality in which a male cat ends up with an extra X chromosome, giving him an XXY, and allowing for additional coat colors. These cats are generally sterile, though they should still be neutered to prevent behavior issues. (A similar abnormality occurs in humans, called Klinefelter’s syndrome.)

Occasionally, male calico cats are the result of chimerism instead of being XXY. A chimera is formed from four parent cells. Two fertilized eggs or early embryos fuse together, and each population of cells keeps its own characteristics, resulting in an organism that is a mixture of tissues.

Whatever the case, male calico cats are rare, and if you get the chance to meet one, consider it your lucky day!

Jan 5, 2012

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