Located inside the nuclear envelope, it is a densely staining object that is an inactivated X chromosome in female mammalian cells. Most Barr body genes are not expressed. They are reactivated in gonadal cells that undergo meiosis to form gametes. Female mammals are a mosaic of two types of cells, those with an active maternal X and those with an active paternal X. Which of the two Xs will be inactivated is determined randomly in embryonic cells. After an X is inactivated, all mitotic descendants will have the same inactive X. As a consequence, if a female is heterozygous for a sex-linked trait, about half of her cells will express one allele and the other cells well express the alternate allele. Examples of this type of mosaicism are coloration in calico cats and normal sweat gland development in humans. A woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands. X chromosome inactivation is associated with DNA methylation. Methyl groups (-CH3) attach to cytosine, one of DNA’s nitrogenous bases. Barr bodies are highly methylated compared to actively transcribed DNA.
What determines which of the two X chromosomes will be methylated? – A recently discovered gene, XIST is active only on the Barr body. The product of the XIST gene, X-inactive specific transcript, is an RNA; multiple copies of XIST attach to the X chromosome inactivating it.
