X inactivation.

In mammalian females, one of the two X chromosomes in somatic cells is randomly inactivated during development as a compact Barr body.

A "tortoiseshell" ("calico") cat has 2 different alleles for fur color: one for orange fur and one for black fur.

In a heterozygous "mosaic" female, orange patches are formed by cells in which the orange allele is active; black patches have cells in which the black allele is active.