What is the structure formed by the inactivation of one X chromosome in females called?

The structure formed by the inactivation of one X chromosome in females is called a Barr body. This phenomenon occurs as a means of dosage compensation in females, who have two X chromosomes, to ensure that the gene dosage between males (who typically have one X and one Y chromosome) and females is balanced. During early embryonic development, one of the two X chromosomes in each cell is randomly chosen to be inactivated, condensing into a compact structure that is not expressed and becomes the Barr body.

This inactivation process is crucial for normal development and helps to prevent an overexpression of X-linked genes. The Barr body can be visualized in the nuclei of female cells and is a direct consequence of the X-inactivation process, which is often attributed to the Lyon hypothesis proposed by geneticist Mary Lyon.

While the term "Lyon body" also references this inactivated X chromosome, it is not as widely used as "Barr body." Other terms, such as "chromatin body" or "X inactivated," are less common and do not specifically denote the condensed structure of the inactivated X chromosome. Thus, the use of "Barr body" remains the most accurate designation for the physical manifestation of X chromosome inactivation.