Barr Body

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

Since females posses two X chromosomes and males have one X chromosome and a Y chromosomes, Barr bodies are essential to regulate the amount of X-linked gene product 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 becomes very condensed - 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 (where the genotype is 47XXY), the number of Barr bodies would also be 1.