Lyon’s Hypothesis and X-inactivation Method

Mary Lyon’s hypothesis, also known as Lyonization, explains the process of X chromosome inactivation in females. Lyon’s Hypothesis was proposed by Mary Lyon in 1961, and it provides a molecular explanation for the dosage compensation observed between males (XY) and females (XX). The key points of Lyon’s Hypothesis are:

• Random X Inactivation
  • Lyon proposed that early in embryonic development, one of the two X chromosomes in each somatic cell of a female mammal becomes randomly inactivated.
  • This inactivation occurs independently in each cell, leading to a mosaic pattern of X chromosome activity among the cells of an individual female.
• Formation of Barr Bodies
  • The inactivated X chromosome undergoes structural changes, transforming from euchromatin to heterochromatin.
  • The condensed, inactive X chromosome is visualized as a dark-staining structure called a Barr body in the nucleus.
• Balancing Gene Dosage
  • XCI balances the gene dosage of X-linked genes between males and females.
  • In males, who have only one X chromosome, all X-linked genes are expressed.
  • In females, one X chromosome is inactivated in each cell, resulting in roughly equal expression levels of X-linked genes compared to males.
• Selection of Active X Chromosome
  • Lyon’s Hypothesis suggests that the choice of which X chromosome to inactivate is random and occurs early in development.
  • Once the X chromosome is inactivated in a particular cell, the same X chromosome remains inactive in all descendant cells.
• Maintenance of XCI during Cell Division
  • XCI is maintained during cell division. If a cell undergoes division, the inactivated X chromosome is passed on to daughter cells.
  • This ensures that the dosage compensation mechanism is maintained throughout the life of the individual.
• Escape Genes
  • Lyon’s Hypothesis states that not all genes on the inactivated X chromosome are completely silenced. Some genes, known as escape genes, may continue to be expressed to varying degrees.

Also Read: Meaning, Diagram and Phases of Cell Cycle 

The Barr body is a condensed, inactivated X chromosome present in the cells of females due to X-chromosome inactivation, a process essential for dosage compensation. This process, known as lyonization, occurs randomly in each cell during early development.

The Barr body in the cell nucleus is absent in males. Its formation helps equalize gene expression between males and females and prevents overexpression of X-linked genes in females. In this article, we will learn about the structure, formation, and function of Barr body.