Half a cat's chromosomes come from the mother, and half come from the father. The gene that determines a cat's coloring is found on the X chromosome.
Female cats have two X chromosomes, and each one can carry a different color. In calicos, one X has the black gene and the other X has the orange gene. At some point in the female cat's development, one X chromosome becomes inactive. The timing of this determines the amount of calico patches.
So calico coloring isn't that uncommon among female cats. It's just a matter of the right chromosomal combination. In males, things are more complicated because they only have one X chromosome and it's never inactivated.
A male cat can be calico if it's created with two X chromosomes and a Y, allowing one X to be inactivated. This is a genetic defect known as XXY, and it's very rare. Only one out of every 3,000 calicos is male!