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. The Barr body can be seen in the interphase nucleus as a darkly staining small mass in contact with the nucleus membrane. 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. For example, people with Klinefelter syndrome (47, XXY) have a single Barr body, and people with a 47, XXX karyotype have two Barr bodies.

The inactivation of the X-chromosome is random and occurs during development. The inactive X chromosome remains inactivated in all the generations of the cell. The function of the Barr body is to regulate the number of gene products of genes present on the X chromosome, as females possess two X chromosomes compared to one X chromosome in males. The inactivation of one of the X chromosomes helps to balance the gene expression between males and females.

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.