I. Introduction

• Female mammals, including humans, inherit two X chromosomes
• The number of X chromosomes is twice the number inherited by males

II. X Chromosome Inactivation

• One of the X chromosomes in each cell of a female mammal becomes inactivated during early embryonic development
• The inactive X chromosome condenses into a compact object called a Barr body
• Most genes of the X chromosome that forms the Barr body are not expressed
• In the ovaries, Barr body chromosomes are reactivated in cells that give rise to eggs

III. Selection of Active X Chromosome

• The selection of which X chromosome will form the Barr body occurs randomly and independently in each embryonic cell
• Females consist of a mosaic of two types of cells: those with the active X derived from the father and those with the active X derived from the mother
• If a female is heterozygous for a sex-linked trait, about half of her cells will express one allele, while the others will express the alternate allele

IV. X Inactivation Process

• Inactivation of an X chromosome involves modification of DNA and proteins bound to it
• A particular region of each X chromosome contains several genes involved in the inactivation process
• One of the genes, called XIST, becomes active only on the chromosome that will become the Barr body
• RNA products of the XIST gene attach to the X chromosome on which they are made, initiating X inactivation

V. X Inactivation in Humans

• Mosaicism can be observed in humans with recessive X-linked mutations
• A woman who is heterozygous for a certain recessive X-linked mutation has patches of normal skin and patches of skin lacking sweat glands

VI. Conclusion

• X inactivation in female mammals ensures that cells of females and males have the same effective dose of most X-linked genes.