Final answer:
A Barr body is an inactive X chromosome within female cells, which is a result of X inactivation process to ensure only one set of the X-linked genes is expressed in each cell.
Step-by-step explanation:
A Barr body is an inactive X chromosome found in the cells of female mammals. Early in female embryonic development, when the embryo consists of only a few thousand cells, one of the two X chromosomes in each cell undergoes X inactivation. This X chromosome condenses into a structure known as a Barr body, which ensures that each cell expresses only one set of the X-linked genes, similar to males who have only one X chromosome.
This process of inactivation is random; whether the maternal or paternal X chromosome gets inactivated is by chance, but once it occurs, all descendant cells have the same inactive X. In this way, females compensate for their double genetic dose of the X chromosome. Females typically have two X chromosomes and to prevent an excessive dosage of genes from the X chromosome, one of them is made inactive during embryonic development, forming the Barr body.