Barr Body

The term “Barr Body” refers to a condensed, inactive X chromosome found in the nuclei of cells. It was named after Murray Barr, the scientist who discovered it in 1949, and it is primarily observed in the cells of female mammals.

Formation

The Barr Body is formed during embryonic development when one of the two X chromosomes in each female cell becomes inactivated. This process, known as X chromosome inactivation or lyonization, ensures that females with two X chromosomes have the same dosage of X-linked genes as males with only one X chromosome.

Appearance and Function

The Barr Body appears as a small, darkly stained mass-like structure within the nucleus of the cell. It represents the condensed, tightly coiled X chromosome that is transcriptionally inactive. This means that the genes on the inactivated X chromosome are not actively expressed or involved in the cell’s functioning.

Significance

The presence of a Barr Body allows for equal gene expression between females (XX) and males (XY), despite the difference in the number of X chromosomes. The inactivation of one X chromosome in females helps prevent an “overdose” of X-linked gene products. Additionally, the detection of a Barr Body during laboratory testing can be used to determine the sex of an individual, specifically to confirm the presence of two X chromosomes.

Exceptions

While most cells in female mammals exhibit a Barr Body, there are exceptions to this rule. For instance, certain cells in the ovaries and some immune cells do not undergo X chromosome inactivation and, therefore, do not have a Barr Body present.