What are Barr bodies?

Barr bodies are indeed the inactive X chromosomes found in females. In organisms with two X chromosomes, such as human females, one of the X chromosomes in each cell undergoes a process called X-inactivation in order to balance the gene dosage between males (who have one X and one Y chromosome) and females. This inactivation typically occurs early in development and results in one of the X chromosomes being condensed, making it transcriptionally inactive; this condensed structure is what is referred to as a Barr body.

The presence of Barr bodies is significant in genetics and cytogenetics as it serves as an indicator of the number of X chromosomes present. Typically, if a cell has more than one X chromosome, there will be one Barr body for each additional X chromosome present beyond the first. For example, in females with Turner syndrome, where there is only one X chromosome, no Barr bodies are formed, as there is no second X chromosome to inactivate.

Understanding Barr bodies provides insight into genetic dosage compensation mechanisms, particularly in the context of sex-linked genetic conditions and disorders.