Q.What is Lyonisation ?

Lyonisation is the process, described by Mary Lyon, by which one of the two X chromosomes in every somatic cell of a female mammal is randomly and permanently inactivated, forming a Barr body.

Because female mammals (XX) have two X chromosomes while males (XY) have only one, females would otherwise produce twice the dose of X-linked gene products compared to males. To correct this imbalance, dosage compensation occurs: early in embryonic development, one of the two X chromosomes in each somatic cell of the female is randomly chosen and inactivated (condensed into transcriptionally inert heterochromatin), a phenomenon called Lyonisation after its discoverer, Mary Lyon (the Lyon hypothesis). The inactivated X chromosome appears as a small, darkly-staining mass called a Barr body attached to the inner nuclear membrane, and can be used clinically as a marker of the number of X chromosomes present. Because the choice of which X (maternal or paternal) is inactivated is random and is then faithfully maintained in all descendant cells of that particular cell lineag …