Monthly Archives: March 2011

Why Are Calico Cats Always Female?

What’s the scoop?

Calico cats are beautiful. Their fur is mostly white with spots of black and orange. Each cat seems to be a little different than the next, like little furry snowflakes that leave you dead birds in your slippers.

Many know that calicos are (almost) always female. But not many actually can explain why. So let’s get to the bottom of this so you can impress your friends.

Warning! Scientific Content:

You probably learned at some point earlier in your life about human sex chromosomes: normal females have two X chromosomes (XX) and males have one X and one Y (XY). The same system for determining sex holds true in all placental mammals (as far as I know, at least), which of course includes cats.

In case you didn’t know, genes are found in the DNA-histone complexes called chromosomes, and genes code for proteins in our bodies. With females having two X chromosomes, you would think that they have more proteins being made because they have more genes to code for them.

Wrong. In mammals, and this includes us Homo sapiens, an X chromosome is randomly inactivated in each of the female’s somatic (body) cells, which is simply called X-inactivation. It occurs early in cell development and is permanent. This means that females have only one working X chromosome, while the other exists as what is called a Barr body, a heterochromatin mass that does not code for any proteins, named after the cytologist who discovered them.

Accordingly, females are sometimes described as “mosaics.” If we were to randomly pick two skin cells from a woman’s body, say one from her shoulder and one from her big toe, there is a 50-50 chance that they are the same, that they have the same X chromosome active. If we were to do the same with a man, there is a 100% chance that those cells are the same, that the same sex chromosomes are active.

So what does this have to do with calico cats?

Well, cats have it the same way as humans. All females undergo X-inactivation and are mosaics.

In calico cats, the genes that code for the pigment of fur resides on the X chromosome. The genes can express either an orange pigment or a black pigment, with white meaning no pigment expressed. The Y chromosome has no gene for fur pigment.

It follows, then, that a male, with only one option of an X chromosome, will either make orange or black pigment, but not both. Females, on the other hand, can express orange in one cell and black in another, leading to cats that are white, black, and orange. The placement of the black and orange splotches is random with X-inactivation but relates to the developmental history of the individual cat.

This phenomenon can also commonly be observed in mice, as well as other mammals.

Wait a sec! I have a calico cat and he’s male!

Aha, here is the “almost” from the introduction. I had to include it there because not all calico cats are female. Very rarely, we can find a male calico cat, but not is all that it seems.

Klinefelter’s syndrome is the name given to the condition where an individual has two X chromosomes and one Y chromosome (XXY), meaning he has an extra chromosome. The individual has reduced testicles (microorchidism) and is usually sterile. It is the most common sex chromosome disorder in males.

The individual appears male instead of female due to the inactivation of one of the X chromosomes, which in effect means the person is XY in expression, with some differences in hormone levels and development and such.

Essentially, the X-inactivation in the XXY individual acts in the same way to produce orange pigment in some cells and black in others. Interesting stuff, huh?

So next time you talk to the crazy cat lady next door, try impressing her with some knowledge of genetics before she starts hurling cats at you.