Define X - chromosome inactivation?

Answer 1

Typically a cell will only keep one copy of the X chromosome active for gene expression. Because women are XX, their cells typically deactivate one X chromosome. It seems that the choice is random; shortly after differentiating, one X chromosome super condenses into a closed chromatin structure. This super condensed chromosome is called a Barr Body (even a good light microscope can sometimes see these at the periphery of a nucleus).

The most famous example of phenotypic evidence of X inactivation is a Calico cat. Coat color in cats in influenced by genes on the X chromosome, so if the two X chromosomes contain information for different coat colors, then different parts of the body will express different coat colors, yielding the Calico phenotype. Since males only have one X chromosome, they can never be Calico. I hope this helps you!