I thought this was interesting:

"The gene which makes a cat ginger (orange) is located on the X chromosome. The gene for ginger will override all other colours. Since males have only one X, they either are or aren’t ginger - no halfway about it. Girl cats have two X’s in each cell. As far as the cells are concerned two X’s is one too many, so each cell deactivates one of the X chromosomes in a fairly random fashion. Sometimes the ginger X will be left on producing a bit of orange fur and in some cells the ginger X will be turned off and the genes for black, brown or other colour fur will be produced. And there you have a Calico or Tortoiseshell Cat.

Since males only need to have the orange gene on one chromosome to become ginger, and females have to have it on two, ginger males outnumber females 3 to 1. To show both orange and another colour such as black or brown a cat has to have two different X chromosomes per cell. Therefore calico (or tortoiseshell) female cats outnumber males by at least 200 to 1, some stats go as high as 3,000 to 1. When a male tortie does appear, (a result of a mutation producing two X’s and one Y) he is invariable sterile and exhibits feminine behaviour such as nurturing nearby young.