Why are all calico & tortoiseshell cats female?

Have you ever noticed that all tortoiseshell and calico cats are female? There is a reason!
The trait for hair coat color is linked to the trait that determines gender in cats. Gender is determined by certain chromosome combinations. The XX chromosome combination results in a female. The XY chromosome combination results in a male. The X chromosome can be linked to either the color black or orange in cats. Therefore female cats can potentially be both black and orange (because they have two X’s) whereas males can only be black or orange (because they have only one X chromosome). The one rare exception is if a cat is born with the genetic mutation of being XXY. This rare condition, called Klinefelter Syndrome, is the only possible way to get a male calico cat! Dr. Sarah Myers, Willow Animal Hospital

(photo courtesy of: ionetics.blogspot.com)