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§Explained by X chromosome inactivation resulting in dosage compensation

Lyon hypothesis 1: The second X chromosome is inactive and remains condensed and appears in interphase cells as the Barr body. Xist mRNA transcripts are only detected in normal females, not normal males. But, the RNA transcript is not translated into a protein, rather it remains in the nucleus and coats the inactive X chromosome, which affects replication (inactive chromosome is late replicating) and condensation.

Lyon hypothesis 2-

X-Inactivation occurs early in embryonic life:
~2 weeks after fertilization, at several hundred cell stage
(Note: The inactive X must become re-activated in the female's germ line so that each egg can receive an active X chromosome)

X-inactivation is random:
The inactive X may be either the paternal or the maternal X; with a mix of cells, females are mosaics for the X chromosome

X-inactivation is clonal:
After one X chromosome has been inactivated in a cell, all of the cell's descendants have the same inactive X

Properties of the inactive X chromosome: Transcriptionally inactive, Late replicating in the cell cycle, Most genes inactivated, Undergoes inactivation from a specific initiation region, Stable and remains inactive, Inactivation is "reset" in oocytes. Inactive X chromosome forms Barr body, associated with nuclear membrane. If one normal X and one abnormal X chromosome (deleted, duplicated), abnormal X will be inactive (by selection in most cases). In balanced X; autosome translocations, the normal X is inactive. §5-10% of normal females demonstrate extreme skewing of inactivation pattern "skewed X-inactivation" for X-linked disease gene