Biology / Bio 1077 · Atom · ~20 seconds

X-Inactivation and the Barr Body

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In each XY-system female cell, one of two X chromosomes is randomly inactivated early in development, condensing into a visible Barr body.

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A calico cat's patchwork coat shows two coat-color alleles, each active only where that particular X stayed switched on.

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Inactivation happens randomly per cell, not uniformly, so a heterozygous female is a mosaic of both alleles, not a blend.