A Barr body, also known as X-chromatin, is an inactive X chromosome. In species with XY sex-determination, females typically have two X chromosomes, and one is rendered inactive in a process called lyonization. Errors in chromosome separation can also result in the formation of Barr bodies. Key points about Barr bodies include:

• Appearance: Barr bodies can be seen in the interphase nucleus as a darkly staining small mass in contact with the nucleus membrane. They are condensed, inactivated X chromosomes that are typically found exclusively in female mammals.

• Number: In humans with euploidy, a genotypical female (46, XX karyotype) has one Barr body per somatic cell nucleus, while a genotypical male (46, XY) has none. In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes.

• Mechanism: It is thought that the inactivation of one X chromosome in females occurs through a process of choice, allowing downstream processes to establish the compact state of the Barr body. These changes include histone modifications, such as histone H3 methylation and histone H2A ubiquitination, as well as direct modification of the DNA itself, via the methylation of CpG sites. These changes help inactivate gene expression on the inactive X-chromosome.

Barr bodies can be found in various biological samples such as hair, buccal cells, and blood. Detecting the presence or absence of a Barr body is a simple and noninvasive method to determine the sex of an unknown sample of forensic significance.