Why are most calico cats female?

Most calico cats are female because the genes that determine their distinctive patches of black and orange fur colors are located on the X chromosome, one of the two sex chromosomes (X and Y) in mammals. Because female cats carry two X chromosomes, they may receive two different color-coding genes from each parent—one for black fur and one for orange, resulting in the recognizable calico pattern. The white patches of a calico cat’s coat are produced by a separate gene that is not located on the sex chromosomes.

In contrast, male cats generally carry only one of each chromosome, X and Y. This means that they can inherit either the orange or the black color-coding gene (on the X chromosome) but not both. Male cats can be calico only in rare cases—about 1 in every 3,000 calico cats—when an extra X chromosome (XXY) is present. These males are usually sterile because of this chromosomal anomaly.