Tortoiseshell cats are cats with coats in which red, black, and cinnamon colorations are all present. They are sometimes referred to as “tri-colors,” referencing the three colors which appear in their coats. When a tortoiseshell also has a great deal of white coloring, it is better known as a “tortoiseshell and white” or “calico” cat. The tortoiseshell coloration can appear in a wide variety of cat breeds, and in an assortment of hair lengths, from short to long.
In addition to the classic red, black, and cinnamon, tortoiseshell cats can also have spots of gray, cream, and other colors in their coats. When the colors are slightly muddy and faded, the cat is known as a “dilute” tortoiseshell. Tortoiseshell can also have coats dominated by gray fur, or they may mingle tabby and tortoiseshell traits, creating a mixture of stripes and bold patches of color. Each tortoiseshell cat is entirely unique, as the coloration pattern is random in nature.
This unique coloration is due to a genetic happenstance. The alleles or pairs of genes which code for orange and black fur are found on the X-chromosome, and if a cat happens to inherit a set of orange and a set of black alleles, sometimes both colors will manifest, due to a condition called “X-inactivation” in which the dominant orange allele is randomly turned off in certain parts of the body. In addition to affecting hair color, this trait also has an impact on skin color, with tortoiseshell cats having patches of darker and lighter skin which correspond with their coats.
Because the tortoiseshell color scheme requires two X chromosomes, the vast majority of tortoiseshell cats are females. The trait also shows up in males with two X chromosomes, classically in cats with Klinefelter's Syndrome, in which the cat has two X chromosomes and one Y chromosome. Male torties, as tortoiseshell cats are affectionately called, are sterile. Females will bear kittens which may be black or orange.
Because tortoiseshell cats are essentially genetic abnormalities, it can be difficult to breed for the tortoiseshell trait. Tortoiseshells have historically been regarded as lucky in some cultures, leading people to attempt to refine the color variation through breeding, but this can be both challenging and frustrating. Some specific cat breeds have “points” or other markings which may have developed through efforts to breed tortoiseshell cats consistently.