In mammalian development, the repression of one of the two X chromosomes in the somatic cells of females as a method of dosage compensation. At an early embryonic stage in the normal female, one of the two X chromosomes undergoes inactivation, apparently at random. From this point on, all descendant cells will be clonal in that they will have the same X chromosome inactivated as the cell from which they arose. Thus, the mammalian female is a mosaic composed of two types of cells—one that expresses only the paternal X chromosome, and another that expresses only the maternal X chromosome. In some cells and tissues, the inactivated X chromosome can be seen as a dense body in the nucleus (referred to as a Barr body or sex chromatin). In abnormal cases where more than two X chromosomes are present, only one X remains active and the others are inactivated. In marsupials, the paternal X is selectively inactivated during female development. In the somatic cells of human females the “inactivated” X chromosome is not completely silenced. Fifteen percent of all X-linked genes escape inactivation, and another 10% vary in their expression. Therefore over 15% of the genes on the X are transcribed at higher levels in women than in men. See Chronology, 1949, Barr and Bertram; 1961, Lyon and Russell; 1962, Beutler et al.; 1963, Russell; 1998, Lyon; Cattanach translocation, human X chromosome, XG, XIST.
Subjects: Genetics and Genomics.