Template:Checked Inactivation of the X chromosome' or lyonization occurs in the early stages of development (approximately at the embryo stage of 100-200 cells) in the event that [[Human karyotype|karyotype] ] contains more than one X chromosome (most often in the case of the normal female karyotype 46,XX; however, it also occurs in males with Klinefelter syndrome - karyotype 47,XXY and in other pathological karyotypes with by more than one X chromosome so that in the final state there is only one active X chromosome in the cell). The inactivation of the X chromosome is random in every cell of the embryo, but also permanent, since all other cells arising from the division of this cell will already have the same inactivated chromosome, whether of maternal or paternal origin. The inactivated X chromosome in this way represents a deposit of highly condensed chromatin, visible as a so-called Barr body' or sex chromatin. Individuals with monosomy 45,X, like 46,XY males, do not have Barr bodies. The inactivation process is controlled by a regulatory region known as the X-inactivation center (XIC). Among other things, the gene for non-coding RNA XIST (X inactive specific transcript (non-protein coding); Xq13.2; is located in this region /entry/314670 OMIM: *314670) and several of its regulators including the ''TSIX' gene (TSIX transcript, XIST antisense RNA; Xq13.2; org/entry/300181 OMIM: *300181). It is the RNA product of the XIST gene that induces changes in the conformation of the X chromosome, which ultimately lead to its inactivation.
The inactivation of the X chromosome is also called the ``Lyonization process in honor of the British geneticist Mary Frances Lyon (1925-2014), who first described this process in 1961.
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ŠIPEK, Antonín. Genetics [online]. [feeling. 29/05/2009]. < http://www.genetika-biologie.cz/karyotyp-cloveka >.