Have you ever wondered why tortoiseshell and calico cats are usually female? It all comes down to the unique way in which the basic pigments are controlled in a cat's DNA.

Cats can have two kinds of pigment that color their fur. Eumelanin creates black, gray, and various shades of brown, while phaeomelanin creates orange and yellowish colors. For simplicity's sake, I'll refer to eumelanin as black and phaeomelanin as red for the remainder of the article.

As it turns out, there's one simple gene that determines whether a cat will produce black or red pigment, and it's located on the X chromosome. By convention, we refer to this gene as the O locus. "O" stands for red, and "o" stands for black, when we look at a cat's genotype. If a cat only has one copy of the gene (i.e., it's a male), we put a dash after the single letter.

Since this gene is on the X chromosome, male cats can only receive their pigment type from their mother. An all-red female cat would have the genotype OO, and all her sons would, therefore, be red, with the genotype O-, regardless of what color the father is. The same holds true for an all-black female.

A tortoiseshell cat has the genotype Oo, requiring two X chromosomes. Now, each cell will only make use of one X chromosome (and therefore only one of the Os), leaving the other inactive. Which X chromosome is inactivated is random, which is how torties end up with their distinctive mottled appearance. The cells that produce pigment for each hair will only produce either red or black, depending on which X chromosome was inactivated.

This, then, is why a male cat is not usually going to be a tortoiseshell—because he will only have one X chromosome, and therefore can only produce one type of pigment. There are exceptions, of course, such as in the case of Kleinfelter's syndrome, in which a male cat will be XXY, and is usually sterile. The other case in which you could see a male tortoiseshell would be chimerism, which is when two fertilized eggs fuse, creating a cat that appears to be a mosaic of two different cats. A chimera male cat is usually fertile. Chimerism does not always result in a tortoiseshell appearance, though, and neither does Kleinfelter's syndrome.

Just as a practical example of how one can apply this knowledge, I'll take a look at some of my family's cats. Randa and Minka (shown in the cover photo) are sisters, and their mother was a rescued red tabby. Although we don't know what their father looked like, we can make some guesses, based on the fact that Minka and Randa are tortoiseshells. Their mother has the genotype OO, and can only pass on red, but the girls both have the genotype Oo, meaning their father cannot be red. As their mother could not contribute the black coloring, it has to come from their father—which means that he cannot be red. Exactly what his color and pattern was, we can't really guess, but we know that he had to at least be black or gray (the shades of brown are rare or nonexistent in our local street cats). Figuring out genetics is basically just one big logic puzzle!

*I should mention that everything I know about cat genetics I learned from the book Legacy of the Cat by Gloria Stephens, as well as the website Messybeast.