title: [X chromosome inactivation--a biological phenomenon of clinical significance for women].
abstract: In female mammals one of the two X chromosomes is inactivated in early embryonic life. Females are therefore mosaics for two cell types, cells with the maternal X as the active X chromosome, and cells with the paternal X as the active X chromosome. The process of X inactivation is random and permanent for each cell. A skewed X chromosome inactivation pattern may be defined as a pattern where the ratio between the two cell types is 80:20 or more. Skewed X inactivation may have several causes. Since X inactivation is a stochastic event, a skewed pattern may be the result of a chance occurrence. It may also be the result of genetic factors, or be the result of a selection leading to a growth advantage for one of the two cell types. A selection mechanism may be the reason for the increase in the frequency of skewed X inactivation found in normal elderly females. An important consequence of skewed X inactivation is the manifestation of X-linked disease in female carriers.
authors.affiliation: Avdeling for medisinsk genetikk Ullevål sykehus, Oslo.
authors.forename: K H
authors.initials: KH
authors.lastname: Orstavik
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002455:Cell Division / Q000235:genetics; D004303:Dosage Compensation, Genetic; D005260:Female; D008040:Genetic Linkage; D006801:Humans; D008875:Middle Aged; D009030:Mosaicism / Q000235:genetics; D014960:X Chromosome