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X-inactivation (lyonization) is the process by which one of the two X chromosomes in 46,XX individuals is transcriptionally silenced to achieve dosage compensation with 46,XY individuals. The process is random, occurs early in embryonic development, and is maintained clonally.
- Random inactivation: In each cell, either the maternal or paternal X is silenced. The choice is random but once made, all daughter cells maintain the same inactive X.
- XIST: The X-inactive specific transcript is a long non-coding RNA expressed from the X-inactivation center (Xic) on the inactive X. XIST RNA coats the chromosome and recruits silencing machinery.
- Barr body: The condensed, inactive X chromosome visible as a dense body at the nuclear periphery. A cell with n X chromosomes has n-1 Barr bodies.
- Skewed X-inactivation: When the ratio deviates significantly from 50:50. Can be random chance, selective advantage, or due to X-autosome translocations.
- Manifesting carriers: Females heterozygous for X-linked recessive conditions may show symptoms due to skewed X-inactivation (e.g., symptomatic carrier of Duchenne muscular dystrophy)
- X-autosome translocations: The normal X is preferentially inactivated to preserve autosomal gene dosage, which can unmask X-linked recessive conditions
- Turner syndrome (45,X): No X-inactivation needed since only one X is present
"XIST X-its the X" - XIST silences (exits) genes on the X it is expressed from. The active X does NOT express XIST.
"Barr = n-1" - Number of Barr bodies = number of X chromosomes minus 1. A 47,XXX individual has 2 Barr bodies.