The answer turns out to be fairly simple. One gene for fur color lives on the X chromosome. Female cat cells have two X chromosomes — one is paternal, one is maternal. However, because of something called random X inactivation, a random X chromosome gets randomly inactivated in each cell. Thus, if the paternal gene for fur color is different than the maternal gene, this random inactivation can lead to the non-uniform color patterns so distinctive of calico and tortoiseshell cats.
There’s More To It
That’s the very short and simple explanation. Here are some answers to questions you might now be wondering.
Q: Are all calico cats female?
A: It turns out that male cats can also be calico (due to mutations, for example), although more than 99% of calico cats are female.
Q: Why are there black and orange patches? Why is the pattern not more random?
A: The inactivation process happens early on in the cell division process (i.e. when there are only around 8 cells in the embryo). That means the randomness only applies to a handful of cells, which then divide over and over again (with the same X inactivated) to create the rest of the cells.
Q: Why does this random X inactivation thing even exist?
A: It’s because otherwise, there would be a “double dose” of genes on the X chromosome, i.e. they would be expressed twice. This could lead to bad things. Note that males have one X chromosome and one Y chromosome, so X inactivation doesn’t apply to them.
Q: Is X inactivation always random?
A: Not always — see the nature link for more details.
Also, note that if us humans carried the skin color gene on our X chromosomes, we would look a lot different 😛
- The Gene: An Intimate History, pg. 399