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. Lyon Hypothesis
3. Barr Body Formation
4. Structure of the Barr Body
5. Barr Body and its Location
6. Tips, Tricks and Strategies to Study Barr Bodies for Exams
7. Types of Questions and Weightage of Barr Body in Exams
8. Recommended Video on Barr Bodies

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.

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• Principles of Inheritance and Variation
• Genetics
• Chromosomal Abnormalities
• 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.

Barr Body Formation

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.

Structure of the Barr Body

A Barr body is usually represented as a compact, small body located near the nuclear envelope in the cell nucleus. The formation of a Barr body is associated with the condensation of an inactive X chromosome. The condensed chromosome, thus, is no longer transcriptionally active and therefore, expression of its genes is not possible. Students can make it simpler by drawing a diagram of the Barr body and analyzing the structure and spatial positioning inside the cell nucleus.

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-predisposing 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 is, therefore indicative that the person possesses more than one X chromosome.

Barr Body and Clinical Relevance

The study of Barr bodies also holds clinical relevance as the presence or absence of a Barr body may be useful in diagnosis. For instance, the presence of a Barr body can be applied in sex determination tests as well as in cases related to diseases related to abnormal numbers of chromosomes X. For instance, the absence of Barr bodies is a characteristic of Turner syndrome, where one has a single X chromosome (XO). The third is Klinefelter syndrome, in which an extra X chromosome is present so that male cells contain Barr bodies.

Tips, Tricks and Strategies to Study Barr Bodies for Exams

Here are some tips and tricks to study Barr body:

"L.B.I.S." - Lyon Barr Inactive Silent

• L: Lyon Hypothesis
• B: Barr body
• I: Inactivation of X chromosome
• S: Silent chromosome (inactive X)

"One X, Other Compressed" (O.X.O.C.)

• O: One active X chromosome
• X: X-inactivation
• O: Other X is silenced
• C: Condensed as Barr body

"F.A.C.T." - Female Active, Condensed, Transcription Silent

• F: Found in Females
• A: One X is Active
• C: Condensed chromosome (Barr body)
• T: Transcription is Silent

Types of Questions and Weightage of Barr Body in Exams

The table below indicates the weightage and types of questions asked from barr body in different exams:

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Recommended Video on Barr Bodies