You asked - here is the answer from the NYTimes learning site:

Strictly speaking, a calico cat is a tortoiseshell, or orange and black, cat with the addition of white spots, explains "The Cornell Book of Cats" (Villard Books). Several different genes on several different chromosome locations go into creating such a cat, and one of them, for orange color, is sex-linked, carried only on the X chromosome. The combination that produces black and orange coloring is not ordinarily inherited by male cats. (A cat with two X chromosomes is female, a cat with an X and a Y is male.)

The orange color gene has two forms, or alleles, designated by feline geneticists as O (for orange) and o (for nonorange). Each X chromosome that a cat inherits has either an O or an o.

Another gene determines what other color the cat will be besides or instead of orange; in tortoiseshell cats, it is black, designated B. For a male cat to be orange, it needs to inherit only one allele, on its single X chromosome. A female cat must have two O alleles to be orange; if it inherits one O and one o, plus B, it is a tortoiseshell.

Male tortoiseshells also inherit an O and an o, but one comes on an extra X chromosome, which it gets because of an error in chromosome separation. With two X's and a Y, such a cat has chromosomes that are mismatched when it comes to mating, and is almost always sterile.

Another gene has two alleles, designated S for white spotting and s for no spotting. A calico cat would carry O and o alleles, plus B, plus S. Calico males, which are very rare, are also almost invariably sterile.