Summary
One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage.
In depth
Early in female embryos, one X in each cell is randomly silenced by XIST RNA coating, then it condenses into a Barr body. Which X is inactivated is random per cell and then inherited by daughter cells, so females are mosaics.
Some genes escape inactivation.
The process starts at the X inactivation centre. XIST RNA coats the chromosome in cis and recruits repressive complexes, producing heterochromatin; some genes escape, about 15 percent, which helps explain features of Turner syndrome. The pattern is kept through cell divisions, giving females a mosaic of cells expressing the maternal or paternal X.
Tortoiseshell and calico cats are the classic visible example, since the orange gene is on the X and patches show which X is active; such cats are almost always female. In mice, the paternal X is inactivated in early embryos, and in humans the choice is random.
Example: female carriers of haemophilia may have reduced clotting factor if more cells inactivate the normal X (skewed inactivation).
Frequently asked questions
What is X-Chromosome Inactivation?
One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage.
Which area of biology covers X-Chromosome Inactivation, and how advanced is it?
X-Chromosome Inactivation is one of the 54 topics in Genetics. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Sex-Linked Inheritance, Turner and Klinefelter Syndromes, Nondisjunction.
Can you give an example of X-Chromosome Inactivation?
Tortoiseshell and calico cats show patches because the orange and black coat colour alleles are on different X chromosomes.
What should I learn before X-Chromosome Inactivation?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to X-Chromosome Inactivation. Advanced topics assume the Intermediate vocabulary.
What topics are related to X-Chromosome Inactivation?
The closest topics to X-Chromosome Inactivation are Sex-Linked Inheritance, Turner and Klinefelter Syndromes, Nondisjunction, Sex Chromosomes. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand X-Chromosome Inactivation?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for X-Chromosome Inactivation. To practise the whole of Genetics, use the Genetics quiz.
How is X-Chromosome Inactivation different from Sex-Linked Inheritance?
X-Chromosome Inactivation: One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage. Sex-Linked Inheritance: Genes on the X chromosome show different patterns in males and females, as with red-green colour blindness. Read both side by side and note what each one explains.
How should I study X-Chromosome Inactivation step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Sex-Linked Inheritance and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.