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Genetics of calico and tortoiseshell cats

Genetics of calico and tortoiseshell cats

Q: My veterinarian was excited to meet Thomasina (Tommi), a calico kitten I adopted from a friend. It turns out s/he is really Thomas (Tommy), a male calico, a rarity according to my veterinarian. Why are calico cats almost always female?

A: My cat Carlie is typical of calico cats with her orange, black and white fur. Calico and tortoiseshell cats, those with marbled orange and black fur, are almost always female, thanks to the genetics of coat color.

A cat with two X chromosomes (called XX) is a female, while a cat with one X and one Y chromosome (XY) is a male.

The gene for orange fur is on an X chromosome, and the gene for black fur also sits on an X chromosome, but not the same one. So, a cat with orange and black fur must have two X chromosomes.

The most common reason for a calico cat to be a male is if he has two X chromosomes, which give rise to the orange and black fur, and a Y chromosome that produces male genitalia. This XXY condition is rare in cats.

XXY cats are sometimes used as research models to study the XXY condition in humans, called Klinefelter Syndrome.

Thomas should have no difficulties with his unusual status. Like any pet cat, he should be neutered to prevent spraying, roaming and fighting with other cats. Enjoy your unique boy!

Lee Pickett, V.M.D. practices companion animal medicine. Contact her at

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