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. 2010 Jan;2(1):6–15. doi: 10.1002/emmm.200900053

Figure 2. In mammals X inactivation compensates the difference in X-linked gene dosage between males and females.

Figure 2

  1. It is thought that the mammalian sex chromosomes X and Y originated from an autosome pair, when a spontaneous mutation arose specifying male sex (blue line). Evolution of the sex chromosomes led to the loss of sequences from the Y chromosome leading to a gene dosage difference between XY males and XX females.
  2. For balancing gene dosage between the sexes random inactivation of one of the two X chromosomes in female cells is initiated by Xist (pink) and maintained by other mechanisms in differentiated cells (black).
  3. Experimental expression of Xist in male cells can be used for probing the epigenetic context. In cells expressing initiation factors, Xist mediated gene silencing is initiated and loss of X-linked gene expression then causes the death of the cells. Therefore, a phenotype can be observed depending on the epigenetic context of the cell.