Biology / Unit 5 Bio 1077 short lesson, about 20 seconds
X-Inactivation and the Barr Body
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The idea
In each XY-system female cell, one of two X chromosomes is randomly inactivated early in development, condensing into a visible Barr body.
In practice
A calico cat's patchwork coat shows two coat-color alleles, each active only where that particular X stayed switched on.
Where people slip
Inactivation happens randomly per cell, not uniformly, so a heterozygous female is a mosaic of both alleles, not a blend.
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