What is a Barr body?

A Barr body is an inactivated X chromosome present in female somatic cells, visible as a dense spot at the cell's nucleus periphery.

In more detail, the Barr body is named after the scientist Murray Barr who discovered it in 1949. It is a fascinating aspect of female genetics and is a crucial part of the process known as X-chromosome inactivation. This process is a mechanism that has evolved in female mammals to balance the dosage of X-linked genes with males, who only have one X chromosome.

In female mammals, one of the two X chromosomes in each cell is randomly inactivated during embryonic development. This inactivated X chromosome condenses into a compact structure called a Barr body. The inactivation is random, meaning that in some cells, the X chromosome inherited from the mother is inactivated, and in other cells, the X chromosome from the father is inactivated. This leads to a mosaic pattern of gene expression in female mammals, which can have various effects, such as the calico pattern in cats.

The Barr body is typically located at the periphery of the nucleus and can be identified under a microscope as a dense, dark-staining spot. It remains inactivated throughout the lifetime of the cell and its descendants in the organism. Despite being inactivated, not all genes on the Barr body are silent. Some genes escape inactivation and are expressed from both X chromosomes in the cell.

The concept of the Barr body and X-chromosome inactivation is also significant in understanding certain genetic disorders. For example, in Turner syndrome, where there is only one X chromosome, there is no Barr body. In contrast, in Klinefelter syndrome, where there is an extra X chromosome (XXY), there is one Barr body.

In summary, the Barr body is a vital component in the genetic makeup of female mammals. It plays a crucial role in ensuring the balance of gene expression from the X chromosome, contributing to the diversity and complexity of female genetics.