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.

X chromosome Inactivation is random and occurs at an early point of development, however, about 10% of the genes on the inactivated X chromosome avoid being silenced.

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 .

To work out the number of barr bodies an individual has the formula: Xn-1 can be used.

Lyonization

Discovery of barr bodies leads to thousands more question for scientist. The changes that can lead to a permanent impact, is it formed randomly? And are they affecting all cells, or only a selective amount? Thus, Mary Lyon comes with her own ideology to answer these question, and her hypothesis is still accepted till today.

The Lyon Hypothesis:

• Inactivation is random at an early point in development
• Once inactivated, all progeny cells have the same X-chromosome inactivated