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Theoretically, X-inactivation should eliminate the differences in gene dosage between affected individuals and individuals with a typical chromosome complement. In affected individuals, however, X-inactivation is incomplete and the dosage of these non-silenced genes will differ as they escape X-inactivation, similar to an autosomal aneuploidy.
Not all random X-inactivation is entirely random. Some alleles, generally mutations in the X-inactivation center on the X-chromosome have been demonstrated to confer a bias towards inactivation for the chromosome on which they sit. [1] Truly random X-inactivation may also appear to be non-random if one X-chromosome carries a deleterious mutation.
Barr bodies can be seen in neutrophils at the rim of the nucleus. In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes. For example, people with Klinefelter syndrome (47, XXY) have a single Barr body, and people with a 47, XXX karyotype have two ...
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 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.).
The mosaic pattern of X inactivation may also determine how penetrant a disease is, if the disease allele is present on one X-chromosome and not the other. The organism may have few cells in which the diseased allele has not been condensed, leading to little expression of the disease allele. This is referred to as skewed X-chromosome inactivation.
X-inactivation. The inactivation of one X chromosome takes place during the early development of mammals (see Barr body and dosage compensation). In placental mammals, the inactivation is random as between the two Xs; thus the mammalian female is a mosaic in respect of her X chromosomes. In marsupials it
Skewed X-inactivation has medical significance due to its impacts on X-linked diseases. X-chromosome skewing has an ability to amplify diseases on the X chromosome. In wildtype women, recessive diseases on the X chromosome are often unexpressed due to the roughly even inactivation process, which prevents mutated alleles from becoming heavily ...