Barr Body: Overview, Formation, Function And Examples

A Barr body is the inactivated X chromosome in the nuclei of female mammalian cells. The inactivation ensures females, carrying two X chromosomes, have the same degree of X-linked gene expression as males, carrying only one X chromosome. This is an important topic covered in the class 12 chapter Principles of Inheritance and Variation in Biology. Questions on barr body meaning, diagram and Lyon hypothesis are often asked in competitive exams like NEET.

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This Story also Contains

1. What is a Barr Body?
2. Formation of Barr Bodies
3. Lyon Hypothesis
4. Structure of the Barr Body
5. What is the Function of the Barr Body?
6. X-Inactivation Example and Barr Body

What is a Barr Body?

A Barr body is defined as a compact, dormant X chromosome that is, in normal conditions, normally observable inside the nuclei of cells, though usually only in the female. The compact form was first discovered by Dr. Murray Barr and Ewart Bertram in 1949. It was named after Dr. Barr. In biological terms, a Barr body is an inactive X chromosome that is silenced, so that only one X chromosome can be active per cell.

Formation of Barr Bodies

The process of X-inactivation forms the Barr body. It involves one of the two X chromosomes in female cells becoming condensed and nonsensitive. The inactivated X chromosome compacts into a dense structure known as the Barr body, thus making gene expression between males and females well-balanced since a cell always has just one active X chromosome.

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• MCQ Practice on Barr Body
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• Molecular Basis of Inheritance

Lyon Hypothesis

The understanding of the existence of a Barr body commences with the Lyon hypothesis. The Lyon hypothesis is a hypothesis first proposed by geneticist Mary Lyon. According to the Lyon hypothesis, any cell in female mammals that has one of the two X chromosomes randomly becomes inactivated during the early stages of embryonic development. The inactivated X chromosome becomes the Barr body. The process is termed as X-inactivation and ensures that females, like males, do not have more than one functional X chromosome per cell. The Lyon hypothesis explains why the Barr body is necessary for maintaining the balance of the genetic makeup of a female to that of a male.

Structure of the Barr Body

The Barr body is a small, dense structure found in the nucleus of female cells. It results from the inactivation of one of the two X chromosomes. In a microscope, it presents as a dark spot. DNA in the Barr body is highly compact, which means that its genes are not active. This makes sure that females and males have equal numbers of active X-chromosome genes.

Barr Body and its Location

The Barr body exists in the nuclei of the cells of females, and sometimes it may appear in males with related genetic conditions, such as Klinefelter syndrome. In this condition, there happens to be an extra X chromosome exposing an individual to various medical conditions, including X-linked recessive disorders (XXY). The presence of a Barr body in each somatic cell is found to occur in normal females. In the case of males who have a typical complement of XY chromosomes, it is not found in the nuclei of cells. The appearance of a Barr body, therefore indicates that the person has more than one X chromosome.

What is the Function of the Barr Body?

The inactive X chromosome is called the Barr body and ensures that females do not produce double the amount of protein from X-linked genes as males. Since females carry two copies of the X chromosome, one of the X chromosomes must be inactivated in every cell to form the Barr body. The inactivation of an X chromosome takes place relatively early during development and is random for each cell.

X-Inactivation Example and Barr Body

Females balance the number of active X chromosomes between males and females by X-inactivation. An example is with cats which have the fur-color gene on the X chromosome. If a female cat has one X chromosome with the gene for black fur and the other X with the gene for orange fur, X-inactivation simply means that some cells in her body will use the black gene while others will use the orange gene, creating black and orange patches, or tortoise shell.

An inactivated X chromosome will form a dense structure called a Barr body, visible in the cell's nucleus under a microscope. This means that X-inactivation has occurred.

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