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.
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
The Exception To The Rule
Why Calico Cats Are Almost Always Female
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Key Facts In This Video
The X chromosome from a male cat's mother determines his fur color, but in females, the X chromosomes from both parents take turns determining fur color. 00:50
The tri-colored calico coloring with white happens due to a gene unrelated to the X and Y chromosomes. 02:09
A male calico cat can have tri-colored fur if he inherits an extra X chromosome, making his genetic makeup XXY. In humans, this condition is known as Klinefelter Syndrome. 02:26