Contrary to popular belief, a calico cat is not a specific breed of cat. It is instead a pattern of coloring that can occur in many breeds, including manx, Persian and domestic short-hair cats. To be considered a calico cat, a cat must be tri-colored, with patches of white, black and red or orange fur. Some breeds have rules as to what percentage of the cat's fur must be white for it to be called a calico cat.
Determined by Chromosomes
Calico cats are nearly always female because the genes that determine the color of the cats' fur are carried on X chromosomes. A calico cat has one X chromosome that has the gene for orange fur and an X chromosome that has the gene for black fur. Females have two X chromosomes, so it is possible for them to carry one X chromosome that has the orange color gene and one that has the black color gene. Male cats, however, normally have one X chromosome and one Y chromosome, so it is possible for them to have only one color gene or the other. Some male cats that have a genetic anomaly that causes them to have XXY chromosomes can be calico cats, but they are extremely rare and are almost always sterile.
Calico cats have sections of different colors in their fur. Usually, the calicoes that have more white fur also have larger and more distinct patches of color. If the orange and black fur is mixed, the cat is called a tortoiseshell. Tortoiseshell cats, or "torties," as they are commonly known, have little or no white fur.
A variation on the usual calico color pattern is seen in dilute or pastel calico cats. These variations get their name from the fact that the color pattern is a diluted or pastel version of the typical colors. A dilute or pastel calico cat has a pattern of buff or cream instead of orange as well as blue — a slate gray color — instead of black.
Traditions and More
Throughout history, many cultures have considered calico cats to be lucky. At one point in Japan, calico cats were thought to bring good luck in houses and to protect sailors on their ships. The calico cat was adopted as the Maryland state cat on 1 October 2001 because its colors are the same as the Baltimore Oriole and the Baltimore Checkerspot butterfly — the state bird and insect, respectively.