1. Sex chromatin bodies ((Barr body
Practical Of Genetics
Sex chromatin bodies ((Barr body

2. History
1940’s two Canadian scientists (Murray L. Barr and E. G. Bertram) noticed a dark staining mass in the nuclei of cat brain cells.
Found these dark staining spots in female but not males.
This held for cats and humans.
They thought the spot was a tightly condensed X chromosome .
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3. Mary Lyon
The Lyon Hypothesis (Lyonization)
definition: condensation of X chromosome mechanism for inactivating the genes on the chromosome
Named inactive X chromosome a Barr body

4. Barr Bodies
Barr bodies represent the inactive X chromosome and are normally found only in female somatic cells.

5. Mechanism of X-chromosome Inactivation
A region of X chromosome near the centromere called the X-inactivation center (XIC) is the control unit.
Contains 12 genes
7 genes code for proteins
5 genes code for untranslated RNA
XIC

6. Xi   Xist expression Xa   Xist expression
This region contains two genes actively involved in inactivating an X chromosome
Xist (X-inactive specific transcript )
Tsix
Antagonistic roles
Xi   Xist expression
 Tsix expression
Xa   Xist expression
 Tsix expression
Xa = active X chromosome
Xi = inactive X chromosome

7. Xist and Tsix
The Xist gene is the only gene which is expressed from the Xi but not from the Xa.
X chromosome lacking Xist gene cannot be inactivated

8. Xist gene (pronounced “exist”)
Encodes a large RNA molecule
Coats Xi from the XIC near the centromere outward along the X chromosome

9. Mechanism for compacting Xi (Barr body)
The Xi has high levels DNA methylation (CH3) locks the chromosome in the inactive state. (associated with gene silencing )

10. A woman with the chromosome constitution 47, XXX should have 2 Barr bodies in each cell.
XXY individuals are male, but have a Barr body.
XO individuals are female but have no Barr bodies.

11. 46, XX 1 Barr body
45, X 0 Barr body
47, XXY 1 Barr body
47, XXX 2 Barr bodies
48, XXXX 3 Barr bodies
# Barr bodies = N-1 (N = # X cs. present)

12. Gene Representation
Males and females have equal dose of gene representation
Levels of enzymes or proteins encoded by genes on the X chromosome are the same in both males and females
Even though males have 1 X chromosome and females have 2.

13. G6PD, glucose 6 phosphate dehydrogenase, gene is carried on the X chromosome
This gene codes for an enzyme that breaks down sugar which helps red blood cells work properly.
Females produce the same amount of G6PD enzyme as males
XXY and XXX individuals produce the same about of G6PD as anyone else

14. The process is independent from cell to cell.
In cells with more than two X chromosomes, only one X remains genetically active and all the others become inactivated.
The process is independent from cell to cell.
A maternal or paternal X is randomly chosen to be inactivated.