The term "barr body" refers to a condensed X chromosome that is present in female cells. The spelling of this term, /bɑr ˈbɒdi/, reflects the IPA phonetic transcription, where the first syllable is pronounced with the "ah" sound found in "father," and the second syllable is pronounced with a short "o" sound. The term is named after Canadian cytologist Murray Barr who was the first to observe the structure in the mid-20th century. Today, its detection is used in laboratory diagnosis of certain genetic disorders.
A barr body, also known as a Barr chromatin body or sex chromatin body, refers to a dense, inactive X chromosome that is visible within the nuclei of female mammalian cells. It is named after Murray Barr, the Canadian-Australian physician who first described it in 1949. The presence of a barr body is a result of X chromosome inactivation, a process that occurs in females to balance gene expression between the sexes.
In female mammals, both X chromosomes contain a similar set of genes, but only one of them is active in each cell. To achieve dosage compensation, one of the two X chromosomes is randomly inactivated during early development, resulting in the formation of a condensed, sex chromatin body - the barr body.
Under a light microscope, a barr body appears as a small, dark-staining clump located at the periphery of the nucleus. It consists of tightly coiled DNA, along with various proteins, which help facilitate transcriptional silencing of the inactive X chromosome. The formation of a barr body ensures that males and females have an equal expression of X-linked genes, despite females having two X chromosomes.
The presence of a barr body has been widely used in cytogenetics and histopathology to determine the sex of an individual, particularly if there are chromosomal abnormalities or intersex conditions present. However, it is important to note that the formation of a barr body is not exclusive to females, as certain individuals with chromosomal disorders or anomalies may also possess barr bodies.
The term "barr body" is named after Murray Barr, a Canadian anatomist and geneticist who first observed this structure in the nuclei of cells. In 1949, Murray Barr and his colleague Ewart Bertram discovered a darkly staining, densely compacted structure inside the nuclei of female mammalian cells. They found that this structure was absent in male cells and concluded that it was related to an X chromosome phenomenon.
The Barr body is a condensed, inactive X chromosome that has been randomly chosen to be silenced in female cells through a process called X chromosome inactivation. This inactivation mechanism compensates for the double dose of X chromosome genes in females compared to males.
Thus, the term "barr body" was coined to honor Murray Barr's contribution to the understanding of X chromosome inactivation in female cells.