Calico and tortoiseshell cats are usually female. The reason has to do with genetics. Normal cats have 38 pairs of chromosomes. Half of these pairs of chromosomes are from the father; half are from the mother. Female cats receive an X chromosome from both the mother and father. (Male cats get a Y chromosome from the father and an X chromosome from the mother.)

The gene that determines the color of a cat’s coat is on the X chromosome(s). Calico and tortoiseshell cats receive one X chromosome with the black coat color gene and one X chromosome with the orange coat color gene. The white coat color seen in calicos and tortoiseshells comes from a different gene. Because 2 X chromosomes are required for the calico and tortoiseshell coats, almost all cats showing these coat colorings are female. Thus, calico and tortoise-shell coat colors are considered to be sex-linked traits.

Very rarely, a calico or tortoiseshell cat is male. These cats are born with 2 X chromosomes (only one of which becomes active) and one Y chromosome. In addition to the rare coat color, the XXY gene defect in male cats also causes sterility.