Final answer:
A Barr body is a condensed, inactive X chromosome formed in female mammals through X chromosome inactivation. It is related to the Lyon hypothesis, which explains the early and irreversible nature of X chromosome inactivation.
Step-by-step explanation:
A Barr body is a condensed, inactive X chromosome that forms in somatic cells of female mammals. It is a result of X chromosome inactivation, where one of the X chromosomes is randomly inactivated to equalize gene expression between males and females. The formation of Barr bodies is related to the Lyon hypothesis, which states that X chromosome inactivation occurs early in development and is irreversible.