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.[2][8][10] 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 (see Klinefelter's syndrome), in which case the male could have tortoiseshell or calico markings. Male calico or tortoiseshell cats are sterile due to the abnormality of carrying two X chromosomes. Few of these males can breed (1 in 3,000) and are rejected by breeders for studding purposes and would, in any event, pass on only one of those X chromosomes to any male offspring, producing normal (non-calico) male kittens. "In the case of a calico cat, the feline’s parents passed on different versions of X chromosomes genes related to coat color."[8] The color of calico or tortoiseshell cats is determined by the X chromosome(s). Tortoiseshell and calico cats are almost always female because of X chromosome inactivation.[11][12]
