X-Chromosome Inactivation: Xist & the Barr Body (2D)

This 2D diagram splits X-chromosome inactivation into its two real scales. The top panel zooms into a single cell: two X chromosomes sit side by side, one is stochastically chosen (Math.random(), exactly once) to express Xist from its inactivation centre, and the animation genuinely simulates that lncRNA spreading in cis along the chosen chromosome — coating density and repressive chromatin marks both accumulate as a wavefront moving outward from the XIC, compacting that chromosome into a Barr body while the other X stays open and active. The bottom panel zooms out to a mitotic division tree: a founder cell's random choice is copied unchanged into every descendant across several generations, and many independent founders together tile a population of cells into the same clonal-patch mosaic responsible for calico-cat coats and the patchy phenotypes of X-linked disease carriers. A running counter accumulates every founder cell ever sampled across reseeds and tracks how the population-level maternal/paternal split converges toward the skew probability as the sample grows.