X-chromosome inactivation

Females carry two X chromosomes and males only one, yet both need roughly the same dose of the hundreds of proteins those X genes make. Doubling up would flood a female cell with twice the X-gene products. The body's solution is drastic and elegant: in each female cell, one of the two X chromosomes is shut down almost entirely, early in development, and stays shut for the life of that cell and all its descendants.

Early in a female embryo, each cell independently and at random picks one of its two X chromosomes — the one from her mother or the one from her father — and silences it. The chosen chromosome is coated by a long noncoding RNA called Xist, which spreads along it from a single control center and recruits the machinery of silencing: histones lose activating marks and gain repressive ones, DNA methylation locks promoters off, and the whole chromosome condenses into a dense, visible blob called a Barr body. Once a cell makes its choice, it remembers it through every future division — this is a textbook case of mitotically heritable epigenetic memory. Because the choice is random and made cell by cell, a female is a mosaic: in some patches of her body the maternal X is active, in others the paternal X.

X-inactivation matters as the clearest, large-scale demonstration that an entire chromosome can be epigenetically silenced and the state inherited through cell division. It explains the patchy coats of calico and tortoiseshell cats — a female cat heterozygous for a coat-color gene on the X shows orange patches where one X is active and black patches where the other is — each patch a clone descended from one cell's original random choice. It also explains why carrier females of X-linked diseases are often mildly or unevenly affected: their healthy and faulty X chromosomes are each active in different cells. A useful nuance: silencing is not total — a minority of X genes escape inactivation and stay active on both copies.