Why Calico Cats are Female - It's Not Just Genetics

Photo by Andras Ostrom

Calico and Tortoiseshell cats are generally always female. Why? It actually isn't so much an issue of genes as much as development. Cats, just like humans get one set of chromosomes from their mother and one set from their father. Of the chromosomes that determine sex, females have two X chromosomes, one from each parent, and males have one X (from mom) and one Y (from dad) chromosome. In females, only one of the X chromosomes is actually used, and the other is deactivated at some early stage of development. However, some cell division had already occurred when this deactivation happens, which X chromosome will be deactivated is random. Some cells may shut off one X and other cells may shut off the other, and any cells that are descended from those cells would share the same active X.

Coat color is linked to the sex chromosome, and so if each X has a different color gene on it, the result is patches of different colors. Since males have only one X chromosome, there is no deactivation, and no calico or tortoiseshell patterns develop. Now there is always an exception, right? A genetic abnormality in males can result in a calico pattern. If a male were born with an extra X chromosome, so instead of having only an X and Y, they have XXY, there is the potential for the same coat patterning to occur. An extra chromosome can result in other problems, however, and these rare cats are usually sterile as well. A similar condition occurs in humans, sometimes referred to as Klinefelter syndrome.

Apparently the presence of the 'white' coat gene increases the chance of larger patches of color, explaining why calicos (with white) tend to have large patches, and tortoiseshell's (no white) tend to have small patches. However, how this occurs is not well understood at this time. The exact timing of when the extra X chromosome is deactivated varies somewhat, and since which X gets deactivated is random, trying to breed specifically for a calico is difficult. Breeding an orange cat to a black one can increase the odds, but there are still many uncontrollable variables that come into play.



The very first cat to be successfully cloned was a calico cat, named Rainbow. A surrogate cat, a tabby, carried the cloned kitten. The cloning actually provides a great example. The kitten, named CC for 'carbon copy' does not share her mother's coat patterns. She does have all the same colors, but his patterning is very different. This makes sense because we know that the patterning is developmental, not just genetic. As a side note, the later reports I found about CC as she became an adult say that her personality and behavior is also different than Rainbow's. Behavior is also strongly influenced by environment and development. So if you fantasize about cloning a beloved pet when they pass, keep in mind, you'd still end up with a different animal.