Few questions fascinate the pet-loving public as much as why there are (almost) no male calico cats. Here’s what a recent reader asked, and my answer:

Q: Why is it so unusual for male cats to be calicos?

A: I’m glad you asked. Feline color genetics is always a fascinating topic. To get started, let’s define our terms. A tortoiseshell cat has patches of orange or red and patches of black, chocolate or cinnamon. When those patches are combined with a white background, the cat is called a calico, after a type of colorful patterned fabric.

A study done by researchers at the College of Veterinary Medicine at the University of Missouri found that only 1 of every 3,000 calico cats is male. That’s because the gene that determines how the orange color in cats displays is on the X chromosome, one of the two chromosomes that determines gender. Females have two X chromosomes, while males have an X and a Y chromosome.

While any cat, male or female, can be orange, in males the orange almost always occurs in the tabby pattern. Females can be orange tabby, calico or tortoiseshell. In rare instances, though, a male cat turns up with not only his allotted X and Y chromosomes, but also an additional X chromosome. If both of those X chromosomes happen to carry the gene for orange coloration, bingo: You have a calico male.

This genetic anomaly is called Klinefelter syndrome, after the doctor who identified it in the 1940s. In human and feline males, it typically causes sterility, which is one reason you don’t see people getting rich off breeding their rare male calico cats.

Interestingly, the source for calico coloration was traced in the 1970s by Neil Todd, who was studying the migration routes of domestic cats. The orange mutant gene that causes the patched appearance originated in Egypt and then spread to Mediterranean port cities in Greece, Italy, France and Spain.

Read more, including how to care for your long-haired pets, in this week’s Pet Connection!