Final answer:
Calico cats are usually female because their coat color is a sex-linked trait on the X chromosome, resulting in a tortoiseshell or calico pattern when a female cat has two different alleles for coat color and undergoes X inactivation during development.
Step-by-step explanation:
The reason calico cats are almost always female is due to the genetics of their coat color, which is linked to the X chromosome. In cats, coat color is a sex-linked trait with genes located on the X chromosome. Female cats have two X chromosomes and can exhibit a tortoiseshell or calico coat pattern if they have two different alleles for coat color. During embryonic development, one of the X chromosomes in each cell undergoes a process called X inactivation. This inactivation is random in each cell, leading to variegated coat colors where different patches express either of the two color alleles present in the heterozygous female. Male cats, however, only possess one X chromosome, and therefore, can only exhibit either one of the coat colors but not a calico pattern, unless they have a rare genetic condition that gives them an extra X chromosome (XXY).