Why Calico Cats Are Almost Always Female
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. 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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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. (0:50)
The tri-colored calico coloring with white happens due to a gene unrelated to the X and Y chromosomes. (2:09)
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. (2:26)