X-inactivation


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ly·on·i·za·tion

(lī'on-i-zā'shŭn),
The normal phenomenon that wherever there are two or more haploid sets of X-linked genes in each cell all but one of the genes are inactivated apparently at random and have no phenotypic expression. Lyonization is usual but not invariable for all loci. Its randomness explains the more variable espressivity of X-linked traits in women than in men. Lyonization occurs in men with the Klinefelter (XXY) karyotype.
See also: gene dosage compensation.
[M. Lyon]

X-inactivation

ly·on·i·za·tion

(lī'on-ī-zā'shŭn)
The normal phenomenon whereby wherever there are two or more haploid sets of X-linked genes in each cell, all but one of the genes are inactivated apparently at random and have no phenotypic expression. Its randomness explains the more variable expressivity of X-linked traits in women than in men.
See also: gene dosage compensation
Synonym(s): Lyon hypothesis, X-inactivation.
[M. Lyon]

X-inactivation

The normal failure of expression of one of the two X chromosomes in females. Early in development some cells switch off the paternal X chromosome, other cells switch off the maternal one. Inactivated chromosomes remains so in all subsequent daughter cells. Most women have a mixture of two different cells populations each expressing a different X chromosome. The inactivated chromosome is visible microscopically as the Barr body. This effect, sometimes called Lyonization after the British geneticism Mary Frances Lyon (1925-), who proposed it in 1961, accounts for a number of observed phenomena in genetics.

Lyon,

Mary F., English cytogeneticist, 1925–.
Lyon hypothesis - Synonym(s): lyonization
lyonization - the normal phenomenon that wherever there are two or more haploid sets of X-linked genes in each cell, all but one of the genes are inactivated, apparently at random, and have no phenotypic expression. Synonym(s): Lyon hypothesis; X-inactivation

X-inactivation

see the lyon hypothesis, dosage compensation.
References in periodicals archive ?
In normal female fetuses with 2 large gene-rich X chromosomes, random X-inactivation occurs in somatic cells early in embryogenesis and, therefore, transcriptionally silences the majority of the genes on 1 of the 2 X chromosomes (Lyon 1961).
Most X; autosome carriers may present with a variable phenotype, developmental delay, and recognizable X-linked syndrome due to nonrandom X-inactivation.
Genetic characterization in symptomatic female DMD carriers: lack of relationship between X-inactivation, transcriptional DMD allele balancing and phenotype.
Therefore, this new assay will not accurately detect skewed X-inactivation unless the skewing is extreme.
Brockdorff's team used a similar procedure to isolate a candidate X-inactivation gene from mouse cells.
A phenomenon called X-inactivation, which randomly silences one member of every X chromosome pair, creates a mosaic of affected and unaffected nerve cells in the brain.
X-inactivation status at the human androgen receptor locus was tested in all patients' mothers as described previously (5).
584T>C) mutation identified in this study was not found in 200 apparently healthy females; moreover, this amino acid belongs to the zinc finger domain, was conserved during evolution (data not shown), and is segregated in a 3-generation pedigree with a classic ATR-X phenotype and a skewed X-inactivation status in carrier females.
Often, scientists explain the basis for X-inactivation to be the need of male and female cells to produce the same levels of proteins whose genes are on the X chromosome, although the male cells have only one copy and the female cells have two copies.
A prerequisite for all methods of clonality analysis based on X-inactivation is the ability to distinguish the paternally derived X-chromosome from the maternally derived one.
However, 21-31% of healthy women have a skewing pattern of X-inactivation (or lyonization) with techniques based on DNA polymorphism, leading to a potential limitation for X-inactivation clonality assays because constitutional skewing mimics clonal derivation of cells, rendering clonality results uninterpretable [7,13].