Because this inactivation occurs randomly, all normal females have roughly equal populations of two genetically different cell types and are therefore a type of mosaic. In roughly half of their cells, the paternal X chromosome has been inactivated, and in the other half the maternal X chromosome is inactive. This has a number of important biological and medical implications, particularly with regard to X-linked genetic diseases. Cats provide a unique opportunity to observe X chromosome inactivation and help visualize how it affects all females. Tortiseshell cats, as seen below, have a coat that is a mixture of black and orange hair.
Calico cats are similar, but also have patches of white, which is encoded by another gene.
Black color is encoded by either a co-dominant allele on the X chromosome or, more likely, an autosomal gene that is masked by the orange gene. Normal male cats have a single X chromosome and can carry either the O or o gene, leading them to have an orange or black coat, respectively. The tortiseshell pattern of fine patches of black and orange reflects the pattern of X chromosome inactivation in the hair follicles. In black patches, the X chromosome bearing the orange allele has been inactivated and the X chromosome bearing the non-orange allele is active. Precisely the converse is present in patches of orange fur.