Barr body

When one X chromosome is switched off, it does not vanish; it scrunches up into a small, dense lump that can be seen under a microscope at the edge of the cell's nucleus. That visible lump is the Barr body, named after the researcher who first described it.

Physically, the Barr body is the inactivated X chromosome packed into tightly condensed, transcriptionally silent chromatin. Its dense state is the structural face of X-chromosome inactivation: the same processes that silence the chromosome's genes also compact it enough to stand out under a stain.

Counting Barr bodies once served as a simple readout of how many X chromosomes a cell carries beyond the first active one. A typical female cell shows one Barr body; cells with extra X chromosomes show more, which historically helped flag certain chromosomal conditions.

Today, direct DNA and chromosome analysis has largely replaced Barr-body counting in the clinic, but the Barr body remains a clear teaching example of an epigenetic state made visible.