One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage.
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.
One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage.
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 Chromosomes, Turner and Klinefelter Syndromes, Sex-Linked Inheritance.
Tortoiseshell and calico cats show patches because the orange and black coat colour alleles are on different X chromosomes.
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.
The closest topics to X-Chromosome Inactivation are Sex Chromosomes, Turner and Klinefelter Syndromes, Sex-Linked Inheritance, Genomic Imprinting. Comparing them side by side is a good way to see how the ideas connect.
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.
X-Chromosome Inactivation: One X chromosome in each female cell is silenced as a Barr body to equalise gene dosage. Sex Chromosomes: X and Y chromosomes determine biological sex in humans; XX is typically female and XY male. Read both side by side and note what each one explains.
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 Chromosomes and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.
X and Y chromosomes determine biological sex in humans; XX is typically female and XY male.
🧬 GeneticsSex chromosome aneuploidies: Turner is XO and Klinefelter is XXY.
🧬 GeneticsGenes on the X chromosome show different patterns in males and females, as with red-green colour blindness.
🧬 GeneticsParent-of-origin-specific gene expression caused by epigenetic marks, as seen in IGF2 and H19.
🧬 Genetics