A female calico cat shows random distribution of orange and black patches of fur. The gene for this locus is X-linked. Since the inactivation of X chromosomes occurs randomly early in embryonic development, all progeny cells from the embryonic precursor cell have the same X chromosome inactivated, causing a patchy expression of the alleles.

The white patches are due to a separate epistatic "spotting gene" that prevents pigments from migrating to the skin.