Calico cats are cats with a distinctive tri-colored coat pattern in which white, black, and orange fur are all present in large patches. In addition, calico cats may also have spots of chocolate, cinnamon, ginger, cream, or gray fur. These cats are distinguished from tortoiseshell or tortie cats by the amount of white in their fur: a calico cat has lots of white, especially around the chest and legs, while tortoiseshells have swirls of color in their fur which can look almost muddy from a distance.
The calico cat is not a breed. Rather, it is a coat color, and it is the result of an interesting genetic trick. Coat color in cats is what is known as a sex-linked trait, meaning that the alleles which code for coat color are found on the sex chromosomes. The alleles which code for orange and black fur are both found on the X-chromosome, with orange being dominant, and black being recessive. In calico cats, both alleles are expressed at once, through a process called X-inactivation, in which the alleles are randomly turned off in some areas of the body.
In order to be a calico, a cat must have two X-chromosomes. This means that the vast majority of calico cats are females, with the exception of XXY males, who are quite unusual (and sterile). Female calicos, however, are perfectly fertile, although they will not necessarily pass the calico trait on. Their male kittens may be orange or black, depending on which X-chromosome they pass on, and the female kittens may develop into calicos, or they may not, depending on how the genetic roll of the dice works out.
The next time you look at a calico cat, you can think about this fascinating example of genetic inheritance. If the cat happens to be friendly enough for you to touch it, you can part the fur around the patches of color and take note that the skin changes color as well; skin color changes are often especially evident around the nose and on the ears, where it is easy to see the skin.
The striking appearance of calico cats has intrigued people for centuries. One breed of cat, the Japanese Bobtail, is specifically bred with the goal of bringing out the calico color pattern, and many cultures regard the calico cat as lucky. Calico cats were often used as ship's cats, because people believed that a ship could not sink with a calico on board, and some people also thought that calicos brought in good fortune.