Why Calico Cats Are Almost Always Female

Ever talk about your friend's cat and find yourself saying "It's so cute! I mean...he's so cute. Or she? I don't know, but it's cute." Well, we've got a little tip for you: If the cat has two colors, go with "she."

In The Genes

Chances are that any cat you've ever seen with more than one color (not counting white—we'll explain later) was female. Female cats, like female humans, are born with two X chromosomes (one apiece from the mother and father) whereas males are born with one X chromosome from the mother and one Y chromosome from the father. A cat's fur color is decided by the X chromosome.

That makes things simple in a male cat: whatever fur color the mother's X chromosome determines is the color the male cat will be. But this is a bit more complex when it comes to female cats. If both X chromosomes call for one color of fur, the cat will be all one color. But if the X chromosomes call for different colors, they'll take turns: one clump of cells will use information from the mother's X chromosome and another clump will use information from the father's, resulting in tortoiseshell or calico markings.

The Exception to the Rule

White fur, however, happens through a process unrelated to X and Y chromosomes, which is why male cats can be white and black but hardly ever orange and black. Because a cat needs two X chromosomes to be calico, there's only one way a male cat can have those markings: by inheriting an extra X chromosome, making his genetic makeup XXY instead of XY. This occurs in only 1 of every 3,000 male cats.

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Written by Ashley Hamer June 19, 2016

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