The term “calico” does not designate a cat breed, but rather refers to a tri-colored coat pattern. Calico cats are among the most beautiful of cats with their distinctive white coats generously spotted with large blocks of orange and black fur. Calico cats with the dilute gene will have a muted version of the calico coat colors, that is, white with pale orange and gray.
A unique characteristic of calico cats is that they are almost always female. This is because coat color is a characteristic related to gender, and is determined by the X chromosome. Just like humans, female cats have two X chromosomes (XX), and each X chromosome can carry a different color. The gene for white coat color is completely separate. This allows for the combination coat color of white, orange and black seen in calico cats.
Male cats have one X and one Y chromosome (XY). The Y chromosome does not carry any color genes. Again, the white coat color gene is separate. Therefore, male cats can be solid orange, solid black, orange with white, or black with white, but not orange, black and white.
There are very rare exceptions to this; a male cat with a calico-patterned coat will have an extra X chromosome (XXY) and as a result will usually be sterile.