Calico cats are almost always female, although there are rare male calicos. The orange and black genes that create the calico pattern are linked to the X chromosome, so only cats who have two X chromosomes can display both colors.
Normal male cats have two chromosomes, one X and one Y. These cats can have either black or orange fur, but not both. Male calico or tortoiseshell cats have an extra chromosome, giving them two X and one Y. This gives them a male sex while still being able to display both orange and black fur together. Cats with this genetic abnormality are almost always sterile, however.