In human cells, the inactive X chromosome visible as a dense nuclear structure is called a Barr body.
Most cells with more than one X chromosome silence all but one X chromosome early in development. This process, called X-chromosome inactivation, helps balance the activity of X-linked genes between cells with different numbers of X chromosomes. The silenced chromosome becomes highly condensed and can appear as a Barr body under a microscope.
The inactive X is not genetically empty. Some genes escape inactivation, and the pattern can differ among tissues. In individuals with two X chromosomes, which parental X becomes inactive is often random in each early embryonic cell, creating a mosaic of cell populations.
The number of Barr bodies is generally one fewer than the number of X chromosomes in a cell. Thus, a typical XX cell has one, while a typical XY cell has none. This rule is a useful guide, but it does not replace chromosome analysis.