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]

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
References in periodicals archive ?
The values of relative X-inactivation for the normal allele in Case 4 was 67.0%, and the values of mothers of all four patients were 65.0% (Case 1 mother), 62.0% (Case 2 mother), and 58.0% (Case 3 and 4 mother), respectively (Figure 3).
Willard, "X-inactivation profile reveals extensive variability in X-linked gene expression in females," Nature, vol.
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).
Here, we established iPS cells from one female DMD-manifesting carrier and one female DMD carrier with three X chromosomes and high serum creatine kinase (CK) levels by using an all-in-one retroviral vector or Sendai viral (SeV) vector and examined their X-inactivation status.
Genetic characterization in symptomatic female DMD carriers: lack of relationship between X-inactivation, transcriptional DMD allele balancing and phenotype.
Much of the research included in the book involves humans (e.g., twin studies), rats, cats (X-inactivation), and agouti mice.
The relocation of male-biased genes to the autosomes may be due to a selective advantage favoring genes that move off the X chromosome and therefore avoid X-inactivation during meiosis.
While males carry one X and one Y-chromosome, females' two X-chromosomes could result in a potentially toxic double dose of genes-except for a unique evolutionary mechanism whereby one of the two X's is silenced in a process known as "X-inactivation."
"Together with Job Dekker, who has pioneered chromosome conformation capture technologies, we have discovered these principles by studying a critical region of the X chromosome, the X-inactivation center," said Heard.
Interestingly, in premutation female #2, the absence of a methylated NL allele (absence of a Met-PCR-generated blue peak in the NL size range) and corresponding absence of a nonmethylated PM allele (absence of a nonMet-PCR-generated green peak in the PM size range) are indicative of skewed X-inactivation in this individual, i.e., her NL FMR1 allele is carried predominantly on the active X chromosome and her PM allele is on the inactive X chromosome.
The purpose of this initiative is to support new stud les on the genes and genetic mechanisms influencing sex determination, human fertility, the role of differential expression of parental alleles (i.e., genomic imprinting and X-inactivation) in reproduction, and other topics in reproductive genetics.