"Calico cats are almost always female because the X chromosome determines the color of the cat, and female cats—much like all female mammals—have two X chromosomes, whereas male mammals, including common male cats, have one X and one Y chromosome. Since the Y chromosome does not have any color genes, there is no chance he could have both orange and non-orange together. One main exception to this is when, in rare cases, a male has XXY chromosomes." - Wikipedia

(Did you understand that? Me neither.)