Xs Mark the Calico's Spots

This column is dedicated to Smuckers and Chainsaw, two neighborhood cats who strut the local streets flaunting their orange, black and white calico pelts. It's also dedicated to anyone out there who might own a male calico cat. Treasure it if you do (or give it to me). Sure, it may seem to do nothing but eat, shed and cough up hairballs, but just by existing it's actually rarer than jewels (though of course every cat seems to deem its existence that way).

Calico and tortoiseshell cats are almost always female because their cool, splotchy and stripy fur patterns are caused by a weirdness that happens to the female sex chromosomes early in the development of a cat fetus. It happens to us too, so if we women had colored fur, we might be wandering around with cunning spots and swirls. (It's kind of a pity, really.)

The reason for calico-ness begins with the fact that female cats--like female humans or any female mammal--have two X chromosomes in each cell. (Males have one X, along with a Y.) To keep things balanced, to make sure that the products of X chromosome genes are equally abundant in females and males, one of the Xs is turned off--inactivated--in every cell. This happens early, when those future cats are just balls of cells.

Which X chromosome gets turned off is wholly a matter of chance. But once it's off, it stays off. That means every female is actually made up of two types of cells--ones with an active X from their dad, and ones with an active X from their mom. (Because males have only one X to begin with, it stays on, monotonously, in every cell.)

And so, if a tomcat's X carries a gene for orange fur and a female cat's X carries a gene for black fur, their female kittens will carry black and orange Xs in each cell. After the Xs get randomly inactivated, the kitten's fur ends up orange some places, black in others.

The only way you can get a male calico cat is a freak event--get a cat that looks male but actually has abnormal numbers of sex chromosomes. A male calico cat has not only a normal X and the Y--but also another X, like no cat usually should.

In theory, you could check to see if this is true for your male calico cat, because if it has more than one X, each cell will contain a little dark spot (the bundled up, inactivated chromosome). You could see this by scraping a few cells from the inner cheek to look at under the microscope.