Why are Calico Cats Almost Always Female?

Only one in about 3,000 calico cats are male. You knew that, but the explanation of how sex and cat fur color are so intertwined is a new one for me. The explanation starts with a refresher on X and Y chromosomes. Females have two X, and males have an X and a Y. In cats, the X chromosome determines fur color. Unless that color is white. A cat with two X chromosomes (female) only uses one of them, while the other will go dormant.

The important thing here is that the same X-chromosome does not inactivate for each cell. One cell may shut off the X-chromosome from the mother while leaving the chromosome from the father. That cell then creates more cells, each of which will use the father’s X-chromosome to determine the fur color. Likewise, another cell may silence the X-chromosome from the father and instead use the chromosome from the mother.

So, for instance, if the female offspring receives the chromosome for black fur from both of its parents, she will have black fur. In the case of calico cats, the same process occurs. However, the offspring receives the chromosome for, for instance, black fur from one parent and orange fur from the other. One cell inactivates the chromosome for black fur resulting in orange fur. Other cells use the chromosome for black fur instead. In both cases, these cells are replicated and the inactivated chromosome will always stay inactive. Those two colors then combine on the cat’s fur to create the orange and black patches of fur. If the cat only has these two colors, it is known as a tortoiseshell cat.

But what about white? That’s a different story, because white cat fur isn’t dependent on sex chromosomes. Any cat can have white fur, or patches of white fur. And then there are the rare male calico cats. Read how that happens in the full post at Today I Found Out.

(Image credit: Flickr user Erica Zabowski)