How is a Barr body formed in the cells of many females?

A Barr body is formed through a process known as X inactivation (or "lyonization"), which occurs during early development in the somatic cells of female mammals.1

To prevent females from expressing twice the amount of gene products encoded on the X chromosome compared to males, one X chromosome in each cell undergoes chemical modification and becomes transcriptionally silent. This inactivated X chromosome physically condenses into a small, densely staining structure located at the rim of the nucleus, which is identified as a Barr body.12

This process is generally random, meaning that in different cells, either the maternal or paternal X chromosome may be inactivated. Consequently, normal females are considered mosaics, as their tissues are composed of a mixture of cells expressing genes from either the maternal or paternal X chromosome. While most genes on the inactivated X are silenced, a small number of genes escape this modification and remain active.1