Q.What is Lyonisation?

Step 1. Recall the origin of the term: Barr and Bertram (1949) first observed a condensed body (sex chromatin, later the Barr body) in female but not male cat nerve cells; Mary Lyon then proposed that this body represents an inactivated X chromosome, coiled tightly into visible heterochromatin — this hypothesis is called Lyonisation.

Step 2. State the mechanism and its purpose: in mammals with an XY system, dosage compensation requires that both sexes end up with only one functional X chromosome per cell, since males naturally have only one X to begin with; Lyonisation achieves this in females by inactivating one of their two X chromosomes in every somatic cell. …

Trace Lyonisation from the Barr-body observation to Lyon's inactivation hypothesis …

• Describing Lyonisation as inactivating a specific (e.g. always paternal) X chromosome, when in fact the inactivation is random per cell. …