Barr Body

A Barr Body is an inactivated. condensed X chromosome found in female cells.

Since females poses two X chromosomes and males one X chromosome and a Y chromosomes, barr bodies are essential to regulate the amount of X-linked genes being transcribed. To ensure that X-linked gene product doses are kept similar between males and females, one of the X chromosomes in a female is crammed into a small space which condenses (the Barr Body). This results in the genetic information on the chromosome being inaccessible to proteins that cause gene transcription. This is called dosage compensation.

Inactivation of the X chromosome is random and occurs at an early point of development.

The number of Barr bodies in a cell is one less than the number of X chromosomes. For example:

• In a normal female with the genotype 46XX , the number of Barr bodies would be 1.
• In a normal male with the genotype 46XY, the number of Barr bodies would be 0.

However in a male with Klinefelter's syndrome and the genotype 47XXY, the number of Barr bodies would also be 1.

Reference

Alberts, B. Johnnson, A. Lewis, J. Raff, M. Roberts, K and Walter, P. (2008) Molecular Biology of The Cell 5th Edition, New York:Garland Science. (page 473)