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C2005-30

Dear Christopher Cat

Please settle a bet. I say calico cats are always female, but my friend says that’s not the case. She insists that any cat, male or female, can have any coat color. Who is correct?

Christopher Responds

I never lived with a calico cat, but I did have a sister who was a tortoiseshell.

According to her, almost all calicos and tortis are female.
Before I share her explanation, I should mention that calico cats are tricolor: orange, black and white. Tortoiseshell cats are orange and black.

What is striking about them is that their coats are comprised of both orange and black fur – and most are female.

That’s because the gene for orange fur is located on the X chromosome in the same location as the gene for black fur.
So to have both orange and black fur, the cat must have two X chromosomes.

Two X chromosomes, or XX, make the cat female.
Thus, a calico has one X chromosome with an orange gene, and a second X chromosome with a black gene.

On the other hand, males have only one X chromosome, plus a Y chromosome that makes us male, so we are designated XY.

With a single X chromosome, we can be either orange or black – not both.

But don’t collect on your wager yet. As you recall, I said almost all calicos are female.

Rare genetic defects can produce calico males. The most common is an XXY, or Klinefelter, male.

Good luck settling your bet, which surely will be more difficult than explaining the genetics of coat color in the calico cat.

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