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.
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[1][2].
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