(You can read more about this condition, called anhydrotic ectodermal dysplasia, by searching a database called OMIM at That means, when the Xs get inactivated, some patches of the woman’s skin will end up with sweat glands and other patches won’t. Human females may not flaunt calico fur, but we’re patchy nonetheless-and sometimes doctors can spot it.įor instance, a woman might have an “abnormal” gene on one of her two Xs-such as one that interferes with the proper formation of sweat glands. After the Xs get randomly inactivated, the kitten’s fur ends up orange some places, black in others. (Because males have only one X to begin with, it stays on, monotonously, in every cell.)Īnd 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.
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. Which X chromosome gets turned off is wholly a matter of chance. This happens early, when those future cats are just balls of cells. (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. 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. It happens to us too, so if we women had colored fur, we might be wandering around with cunning spots and swirls. 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).Ĭalico 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. Treasure it if you do (or give it to me). It’s also dedicated to anyone out there who might own a male calico cat. This column is dedicated to Smuckers and Chainsaw, two neighborhood cats who strut the local streets flaunting their orange, black and white calico pelts.