X-Chromosome Inactivation: Xist & the Barr Body

This simulator models the two scales at which X-chromosome inactivation happens. On the left of the 3D scene, a field of cells shows the population-level outcome: each cell independently flips a biased coin at the moment of inactivation to decide whether its maternal or paternal X stays active, and that choice is inherited by every descendant, producing the clonal patchwork mosaic that gives calico cats their coat pattern and carrier females of X-linked diseases their partial phenotypes. On the right, a close-up pair of chromosomes shows the molecular mechanism driving that choice: Xist long non-coding RNA spreading in cis from the inactivation centre, coating the chromosome and pulling it from open euchromatin into a compact, silenced Barr body. Adjust the founder-cell count to change patch coarseness, the maternal skew to bias which X wins, and the Xist spreading speed to slow down or speed up the condensation wave.