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A Barr body is an inactive X chromosome found in the nucleus of female mammal cells. Since females have two X chromosomes but only need one to be active, the other one becomes inactive. This process, called lyonization, ensures that females don’t make twice the amount of X-linked genes as males, who have only one X chromosome. The inactive X chromosome forms a dense structure known as a Barr body, which can be seen under a microscope near the edge of the cell’s nucleus. This process helps keep the genetic activity balanced between males and females.
Barr Body Diagram
Barr Body Diagram is as follows:
Barr Body Formula
The Barr body formula helps find out how many Barr bodies are in a cell. It is:
Number of Barr bodies = Number of X chromosomes – 1
For example, a typical female with two X chromosomes (46XX) has one Barr body. In contrast, a male with one X and one Y chromosome (46XY) has no Barr bodies.
What is Lyon’s Hypothesis?
Lyon’s Hypothesis, also called X-inactivation or Lyonization, was introduced by geneticist Mary Lyon in 1961. It explains how female mammals, which have two X chromosomes, don’t end up with double the amount of X chromosome gene products compared to males, who have only one X chromosome.
According to this hypothesis, in each female cell, one of the two X chromosomes gets randomly inactivated early in development. This inactive X chromosome forms a structure called a Barr body. This inactivation stays the same in all the cells that come from that original cell, so females, like males, have only one active X chromosome in each cell.
This process helps equalize the expression of genes on the X chromosome between males and females.
Where Barr Body is found?
A Barr body is found inside the nucleus of female body cells. It looks like a dense, compact mass near the edge of the nucleus. This structure is an inactive X chromosome. It helps ensure that females with two X chromosomes don’t produce twice as many X-linked gene products as males, who have only one X chromosome.
Barr Body in Male
A Barr body is an inactive X chromosome usually found in female cells. Males, who typically have one X and one Y chromosome, don’t have Barr bodies because their single X chromosome stays active. However, in certain genetic conditions like Klinefelter syndrome (47, XXY), where males have an extra X chromosome, a Barr body can appear. This happens because one of the extra X chromosomes gets inactivated to keep the balance of X-linked gene activity.
Barr Body in Female
In female cells, one of the two X chromosomes becomes inactive through a process called lyonization. This prevents females from producing twice the amount of X-linked genes compared to males, who have only one X chromosome. The inactive X chromosome forms a Barr body, which appears as a dark spot near the edge of the cell’s nucleus when viewed under a microscope.
Barr Body Formation
A Barr body is an inactive X chromosome found in the body cells of female mammal. This happens through a process called X-chromosome inactivation, or lyonization, named after geneticist Mary Lyon.
Formation of Barr Bodies:
- Random Inactivation: Early in development, one of the two X chromosomes in each cell is randomly switched off. This makes sure females, like males, have only one active X chromosome in each cell.
- X-Inactivation Center (XIC): The X chromosome has a region called the X-inactivation center (XIC) that includes the Xist gene. The Xist gene creates RNA that covers the X chromosome and leads to its inactivation.
- Compaction: The inactivated X chromosome becomes tightly packed into a structure called a Barr body, which can be seen in the cell’s nucleus.
Barr Body Functions
Barr bodies are inactive X chromosomes found in female mammals. They are tightly packed structures seen in the nucleus of most female cells. Although once thought to be inactive, new research shows that Barr bodies are important for regulating gene expression and balancing X chromosome gene products in females.
Functions of Barr Bodies:
Dosage Compensation:
- Females have two X chromosomes, while males have one. This difference could cause an imbalance in gene expression between males and females.
- To prevent this, one X chromosome in females becomes inactive, forming a Barr body. This process, called X inactivation, happens randomly early in development and continues in future cell divisions.
- Because of this, females have a mix of cells that express either the mother’s or father’s X chromosome.
Gene Expression Regulation:
- Most genes on the inactive X chromosome are switched off, but a few escape inactivation. These genes are important for many cellular functions and are active in both males and females.
- The process that controls which genes remain active is complex, involving DNA changes, protein modifications, and certain RNAs.
Cellular Differentiation:
- Barr bodies might also influence how cells develop and differentiate. Some research suggests that the choice of which X chromosome is inactivated can affect how a cell functions.
X Chromosome Aneuploidy:
- In individuals with abnormal numbers of X chromosomes (such as Turner syndrome with one X chromosome or Klinefelter syndrome with XXY), the number of Barr bodies matches the number of inactivated X chromosomes.
- This information is used to diagnose disorders related to abnormal X chromosome numbers.
Barr Body Examples
A Barr body is an inactive X chromosome found in the body cells of female mammals. Here are some examples of where Barr bodies can be seen:
- Calico Cats: The unique fur patterns of calico cats result from X-inactivation. Female cats have two X chromosomes, and the inactivation of one X chromosome in different cells creates patches of different colors on their fur.
- Human Cells: In humans, Barr bodies can be found in samples like hair, cheek cells, and blood. In typical females (46, XX), one X chromosome becomes inactive and forms a Barr body.
- Klinefelter Syndrome: Males with Klinefelter syndrome (47, XXY) have an extra X chromosome. One of the X chromosomes becomes inactive, forming a Barr body.
- Neutrophils: Barr bodies can sometimes be seen at the edge of the nucleus in neutrophils, a type of white blood cell.
- Epithelial Cells: These are cells that line surfaces in the body, like the skin and the digestive tract. In females, Barr bodies can be found in the nucleus of epithelial cells.
- Fibroblasts: These are cells in connective tissues that make collagen. In females, Barr bodies are visible in the nucleus of fibroblasts.
These examples show how Barr bodies help regulate gene expression from the X chromosome in females and individuals with more than one X chromosome.
FAQs on Barr Body
What is a Barr body and what is its function?
A Barr body is an inactivated X chromosome found in the somatic cells of females. Its function is to ensure dosage compensation by balancing the expression of X-linked genes between males and females.
Why do only females have a Barr body?
Females have two X chromosomes, and one of them is inactivated to prevent overexpression of X-linked genes, forming a Barr body. Males have only one X chromosome, so they don't form Barr bodies.
How many Barr bodies are in XXY?
Individuals with Klinefelter syndrome (XXY) have one Barr body since one of their two X chromosomes is inactivated.
Why double Barr body in Klinefelter syndrome?
In individuals with Klinefelter syndrome and more than two X chromosomes (e.g., XXXY), additional X chromosomes are inactivated, resulting in two Barr bodies.
How many Barr bodies are in trisomy 21?
In females with trisomy 21 (47,XX,+21), there is one Barr body. In males with trisomy 21 (47,XY,+21), there are no Barr bodies.
Where is the Barr body found?
The Barr body is found in the nucleus of somatic cells, appearing as a dense, dark-staining spot at the periphery of the nucleus during interphase.
