Barr Body Definition
A Barr body is an inactive X chromosome found in the somatic cells of female mammals. Females have two X chromosomes, but only one is active in each cell, and the other becomes a Barr body. It is also known as sex chromatin and can be identified by its dense staining with nuclear dyes.
What is a Barr Body?
A Barr body is an inactive X chromosome found in female somatic cells. Females typically have two X chromosomes, while males have one. In females, one of the X chromosomes is randomly inactivated to ensure that the amount of gene expression from the X chromosome is equal in both sexes.
The Barr body is packaged in heterochromatin, which means it is condensed and not easily accessible to molecules involved in transcription.
The term “Barr body” was named after Murray L. Barr, a Canadian physician who discovered it in 1949. It appears as a small, well-defined body that stains intensely with nuclear dyes. In humans, someone with two X chromosomes has only one Barr body per somatic cell, while someone with one X chromosome has none.
Formation of Barr Bodies
Barr bodies are inactivated, condensed X chromosomes found in female cells. Females have two X chromosomes, and one of them is randomly inactivated during embryonic development to ensure that the amount of X-linked gene product being transcribed is similar between males and females.
The inactivation process is initiated from the X inactivation center (Xic), usually found near the centromere, which contains a gene called X-inactive specific transcript (Xist). Another gene known as Tsix (Xist reversed) regulates the expression of Xist.
The formation of Barr bodies within the nucleus of female cells was the first description of facultative heterochromatin, representing a transcriptionally inactive chromatin state. The importance of Barr bodies can be considered in terms of their efficacy in detecting physiological abnormalities.
Barr bodies are found in the lobes of neutrophils in female cells and can be used as a genetic femaleness test. They are also critical for reporting any abnormalities.
Which X Chromosome Becomes the Barr Body?
In females, one of the two X chromosomes is randomly inactivated during embryonic development to prevent an overdose of X-linked gene products.
The inactivated X chromosome then condenses into a compact structure called a Barr body, which is stably maintained in a silent state. Females typically have one Barr body per somatic cell, while males have none.
The specific X chromosome that becomes the Barr body is random and varies from cell to cell. In individuals with more than two X chromosomes, each extra X chromosome will be converted to a Barr body.
For example, triple X females neutralize their extra Xs by forming additional Barr bodies – there would be two Barr bodies in a cell from an XXX female or XXXY male.
Mechanism of X-inactivation
X-inactivation is a process by which one of the two X chromosomes in female mammals is randomly inactivated during development, resulting in the silencing of one of the X chromosomes.
The inactive X chromosome is silenced to prevent an overexpression of genes that would result from having two active X chromosomes.
The process of X-inactivation occurs when RNA transcribed from the Xist gene on the X chromosome from which it is expressed spreads to coat the whole X chromosome.
Prior to inactivation, both X chromosomes weakly express Xist RNA from the Xist gene. During the inactivation process, the future active (Xa) ceases to express Xist, whereas the future inactive (Xi) dramatically increases its production.
There are two types of X-inactivation: imprinted and random. During imprinted X-inactivation, the paternal (father’s) X chromosome is preferentially silenced in eutherian mammals’ placenta and earlier embryonic cells.
In contrast, random inactivation occurs during embryonic development and affects either maternal or paternal chromosomes with equal probability.
The mechanism of choice for which chromosome will be silenced involves a region on the chromosome named “X-inactivation center” (Xic), which contains several genes that play a role in regulating this process. One such gene is Tsix, which antagonizes the action of another gene called Xist.
Tsix expression from one chromosome prevents its silencing by repressing expression of its counterpart’s Tsix and promoting expression of its own Tsix and other genes that counteract silencing by recruiting chromatin remodeling factors.
X-Inactivation Example and Barr Body
calico cats
One example of X-inactivation and the formation of a Barr body is in the coat-color patterning of tortoiseshell or calico cats. In these cats, the gene for coat color is located on the X chromosome. Since females have two X chromosomes, they can have two different alleles for coat color. During embryonic development, one of the X chromosomes becomes inactivated and forms a Barr body, resulting in patches of fur with different colors.
Triple X females or Klinefelter males
Another example is in Triple X females or Klinefelter males with more than two X chromosomes who neutralize their extra Xs by forming additional Barr bodies. The inactive X chromosome forms a discrete body within the nucleus called a Barr body, which is generally located on the periphery of the nucleus and is late replicating.
What is the function of the Barr Body?
The Barr body is an inactivated X chromosome found in female cells. Females possess two X chromosomes, while males have one X and one Y chromosome.
To ensure that the amount of X-linked gene product being transcribed is similar between males and females, one of the X chromosomes in a female cell is inactivated and forms a compact structure called a Barr body.
Barr body maintains gene equality in male (XY) and female (XX) cells by silencing transcription on the inactive X chromosome.
In species with XY sex-determination, such as humans, most females have one Barr body per somatic cell, while most males have none. However, individuals with certain sex chromosome genotypes may form additional Barr bodies to neutralize extra X chromosomes.
For example, triple X females or Klinefelter males with more than two X chromosomes may form additional Barr bodies.
Overall, the function of the Barr body is to regulate the expression of genes on the inactive X chromosome in order to maintain gene dosage balance between males and females.
Genetic Disorders and the Barr Body
The Barr body is a small structure found at the rim of the nucleus in female somatic cells between divisions. It is formed when one of the two X chromosomes in females is randomly inactivated during embryonic development.
Most human females have only one Barr body per somatic cell, while most human males have none because they have only one X chromosome.
The loss of the Barr body may be due to genetic loss or an epigenetic instability of the repressed states on the Xi and transcriptional reactivation.
Recent studies suggest that there may be a functional relationship between the loss of the Barr body and disproportional expression of X-linked genes in malignancy development. However, more research is needed to fully understand this relationship.
Some genetic disorders are associated with abnormal numbers of Barr bodies. For example, people with Klinefelter syndrome (47, XXY) have a single Barr body, while people with a 47, XXX karyotype have two Barr bodies.