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The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome (see dosage compensation).
Males with Klinefelter syndrome typically have one extra copy of the X chromosome in each cell, for a total of two X chromosomes and one Y chromosome (47,XXY). It is less common for affected males to have two or three extra X chromosomes (48,XXXY or 49,XXXXY) or extra copies of both the X and Y chromosomes (48,XXYY) in each cell.
Xist (X-inactive specific transcript) is a non-coding RNA transcribed from the X chromosome of the placental mammals that acts as a major effector of the X-inactivation process. [5] It is a component of the Xic – X-chromosome inactivation centre [ 6 ] – along with two other RNA genes ( Jpx and Ftx ) and two protein genes ( Tsx and Cnbp2 ).
•List of human protein-coding genes page 2 covers genes EPHA1–MTMR3 •List of human protein-coding genes page 3 covers genes MTMR4–SLC17A7 •List of human protein-coding genes page 4 covers genes SLC17A8–ZZZ3 NB: Each list page contains 5000 human protein-coding genes, sorted alphanumerically by the HGNC-approved gene symbol.
The SHOX gene in the PAR1 region is the gene most commonly associated with and well understood with regards to disorders in humans, [17] but all pseudoautosomal genes escape X-inactivation and are therefore candidates for having gene dosage effects in sex chromosome aneuploidy conditions (45,X, 47,XXX, 47,XXY, 47,XYY, etc.).
X chromosome reactivation (XCR) is the process by which the inactive X chromosome (the Xi) is re-activated in the cells of eutherian female mammals. Therian female mammalian cells have two X chromosomes, while males have only one, requiring X-chromosome inactivation (XCI) for sex-chromosome dosage compensation .
The X chromosome makes a very small percentage of the total human genome and the epigenetics of this chromosome is a major contributor to certain diseases. Thus sex-limited epigenetic traits may have played a pivotal role in the evolution of mammals and other species, particularly as a mechanism to ameliorate intralocus conflict between the sexes.
The X-chromosome carries a larger number of genes in comparison to the Y-chromosome. In humans, X-chromosome inactivation enables males and females to have an equal expression of the genes on the X-chromosome since females have two X-chromosomes while males have a single X and a Y chromosome. X-chromosome inactivation is random in the somatic ...