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Mosaic Expression

A good example of an animal that exhibits mosaicism is a tortoiseshell cat, which has patches of black and orange fur. There is a dominant gene on the X chromosome that makes the cat's fur orange. If a female cat has this geneon only one of its two chromosomes, then the pigment-producing cells in which this chromosome is active will generate orange fur, while those that have the gene on the inactive X chromosome will make black fur.

The choice of which X chromosome to inactivate occurs very early in development, when an embryo has less than one hundred cells. While this initial choice is generally random, the same inactivation pattern is then passed on to descendant cells through subsequent cell divisions, resulting in a patch of cells with one or the other X chromosome active, and therefore producing orange or black fur in the tortoiseshell cat.

Because the single X chromosomes in males is never inactivated, male cats do not have tortoiseshell coats. XXY male cats, however, which have an extra X chromosome, can have such coats.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 3Mosaicism - The Sex Chromosomes, Mosaic Expression, X Chromosome Inactivation, Effect Of X Inactivation On Human Disease