Final answer:
Calico and tortoiseshell cats are almost exclusively female because the coat pattern is due to X chromosome inactivation, which only occurs in females who have two X chromosomes. Males, having only one X chromosome, typically do not exhibit these varied coat patterns, making a calico coat a strong indicator of a female cat.
Step-by-step explanation:
To distinguish a female calico cat from a male calico cat by the coat appearance, one must understand that the tortoiseshell or calico coat pattern is a phenotypic expression related to the X chromosome. The coloration in tortoiseshell and calico cats is due to X inactivation in cells during the embryonic development of females.
Given that female mammals have two X chromosomes, females heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their bodies.
This occurs because one X chromosome in each cell is randomly inactivated, resulting in a coat with a patchwork of colors, called a tortoiseshell or calico pattern.
Since male mammals, including cats, have only one X chromosome, they do not have the capability for such varied coat patterns in most cases. Therefore, the appearance of a calico or tortoiseshell pattern is an indicator that the cat is likely female.