Calico cats aren't an actual cat breed. Calico is just a color pattern that occurs in numerous breeds of cats, and the genetics behind how this pattern is created are fascinating.
Calico Color and Pattern
Calico cats have a combination of black, orange, and white fur. Each color has its own separate patches, and the colors are never blended or mixed as they are in tortoiseshell cats. Sometimes the colors are diluted to shades of blue, chocolate, cream, or fawn instead of the standard black and orange on white.
Genetics Behind the Coloration
Calico cats are the result of a specific genetic code. Since genetics can be a bit difficult to follow, it's usually easiest to focus on the X chromosome.
The genetic code for color is contained in an X chromosome. Female cats receive an X chromosome from each parent, making them XX, while male cats receive an X chromosome from their mother and a Y chromosome from their father, making them XY. Since two X chromosomes are necessary to create a calico's distinct color patches, you'll find nearly all calico cats are female.
In calicos, one X chromosome carries the orange/yellow color, while the other X carries black/brown. According to the University of Miami College of Arts and Sciences, one of the two X chromosomes in each cell inactivates at a certain point in an embryo's development and then coils into a structure known as a Barr Body. The process is referred to as Lyonization; it's very random, which is why each calico cat looks so unique.
Once inactivated, the same X-chromosome is maintained through repeated cell divisions. So if the X for orange color is inactivated, the fur patch will be black. If the X for black is inactivated, the fur patch will be orange. The areas of white coat color are the result of an autosomal gene that affects the migration of pigment-producing cells known as melanocytes. The amount of white fur a particular calico cat has depends on how slow the migration of melanocytes is, so a very slow migration results in larger areas of white fur. All of these factors ultimately give a calico her patchy coloration.
Since most males only receive a single X chromosome, they do not carry the genetic coding to be calicos. However, there are rare occasions when a male inherits two X chromosomes making him XXY and therefore able to present the calico pattern and coloring. This rarity is basically a genetic anomaly, and for this reason the few male calicos you'll find are nearly always sterile.
Enjoy Your Calico
Since calicos are such a popular color pattern, you'd think cat lovers would specifically breed for these beauties. In reality, this is not the case. Calico cats remain a happy accident of genetics. So appreciate your patchwork kitty not only for her beauty, but for the unusual genetic sequence that was necessary to produce her.