X-Inactivation

Female mammals have two X chromosomes while males have one. Since the X chromosome contains over ten times more genes than the Y chromosome, females would produce twice as many X-linked proteins without compensation. X inactivation silences nearly all genes on one X chromosome during early embryonic development, balancing gene expression between sexes and preventing harmful protein overproduction.

A Barr body is the inactivated X chromosome condensed into a small, dense ball in the nuclear envelope. It forms when one X chromosome is randomly silenced through a process initiated by XIST RNA, which coats the chromosome and recruits protein partners. This condensed structure makes most X-linked genes inaccessible to transcription, effectively shutting down their expression.

XIST is a non-coding RNA produced from the X inactivation center that coats the chromosome from which it is transcribed. This coating recruits protein partners and triggers DNA reshaping, leading to Barr body formation. TSIX, transcribed in the opposite direction from the active chromosome, represses XIST activity through complementary binding, ensuring only one X chromosome remains active.

No. X inactivation is randomly determined in each cell during early development, with about half of embryonic cells inactivating the maternal X chromosome and half inactivating the paternal copy. This creates a mosaic pattern where different cell populations express different X chromosomes. Once established, the same inactivated X chromosome is passed to daughter cells during mitosis.

Calico and tortoiseshell cats are heterozygous for X-linked fur color genes with alleles for black and orange fur. Random X inactivation in different cell groups silences either the black or orange allele, creating patches of each color across their coat. The mosaic expression of these X-linked traits produces their distinctive mottled appearance.

X inactivation reduces the severity of conditions caused by extra X chromosomes. Males with Klinefelter syndrome form Barr bodies to inactivate their extra X chromosome. Females with Triple X syndrome form additional Barr bodies for their excess X chromosomes, allowing them to maintain relatively normal gene expression levels despite chromosomal abnormalities.

X inactivation is very stable and persists through every mitotic cell division during an organism's lifetime. However, it is not completely permanent. Notably, Barr bodies get reactivated in cells within the ovaries that become eggs, allowing female gametes to express both X chromosomes before meiosis occurs.