Practice question

Dosage compensation in female mammals equalizes X-linked gene expression relative to XY males. One X chromosome randomly inactivates early embryogenesis through coating by XIST long noncoding RNA expressed from inactive X, recruitment of Polycomb repressive complex 2 depositing H3K27 trimethylation, H2A K119 ubiquitination, promoter DNA methylation and late replication shift. Condensed inactive X persists as heterochromatic Barr body at nuclear periphery in interphase somatic nuclei, detectable in buccal smears or neutrophil drumsticks. It remains transcriptionally silent except pseudoautosomal regions escaping inactivation, reflecting epigenetic silencing not Y-related.