In mammals, the potential imbalance of gene expression for the two X chromosomes in females is resolved by inactivating one X in all somatic tissues. In the embryo proper, the process of X inactivation is considered to be random between the maternal and paternal chromosomes. Thus, most females have mosaic expression of maternal and paternal alleles of X chromosome loci, with a contribution of about 50% from each chromosome. However, some females show a skewed ratio of X inactivation, which can be due to negative or positive selection, or to an underlying primary genetic process. Belmont (1996) observed familial clustering of females with highly skewed patterns of X inactivation and reviewed the genetic control of X inactivation.
Genetic Heterogeneity of Skewed X Inactivation
See also SXI2 (300179) for a locus that maps to chromosome Xq25-q26. [from
OMIM]