Mammalian females have two X chromosomes per cell. The gene products of only one X chromosome are needed. One X is inactivated in each cell. Inactivation is random. The other X chromosome is condensed and inactive and is called a Barr body. It is visible in micrographs. It may be either the maternal or the paternal X chromosome.
Because in some cells the paternal X chromosome is inactivated, while in other cells the maternal X chromosome is inactivated, each adult female is a mosaic of X-linked traits, called Lyonization.
This mosaic effect is seen in human females affected by anhidrotic ectodermal dysplasia in which a mutant gene on one X chromosome results in patches of skin with no sweat glands.
This animation (Audio - Important) describes how calico cats result from X Chromosome inactivation.
REVIEW: The mosaic effect in human females can be observed in
REVIEW: X chromosome inactivation is an example of _____ .
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