X-inactivation is the process by which one of the two X chromosomes present in females is inactivated

  • This ensures that females do not produce twice as many X-linked gene proteins as males (who possess one X chromosome)

The silenced X chromosome is packaged as transcriptionally inactive heterochromatin, forming a structure called a Barr body

  • Once an X chromosome is inactivated, it will remain permanently inactive in the cell (and all cells derived from it)

X-inactivation occurs randomly in mammals, meaning both X chromosomes may be active in different cells

  • This allows for a heterozygous phenotype to occur in the female as a whole

X-Inactivation and the Formation of a Barr Body

barr body