Coat color of cats is linked to gender. Female cats have 2 X chromosomes while the male has 1 X chromosome and 1 Y chromosome. White is a separate gene. The colors black and orange are found on the X chromosomes. Because female cats have 2 X chromosomes, they can be bi- or tri-colored (with the addition of white). Since males have only 1 Y chromosome, they can be either orange or black. There are other factors involved but that's why calico cats are female. A calico cat may be male but it is an extremely rare occurence and it is usually sterile.
