X-inactivation

(redirected from X chromosome inactivation)
Also found in: Acronyms.

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 ?
Another researcher states that the study confirmed a long-standing theory that females had an additional risk factor for neural tube defects and that a breakdown in the associated X chromosome inactivation process could help to explain why females were more likely than males to have neural tube-related birth defects.
Jacobs, "Age- and tissue-specific variation of X chromosome inactivation ratios in normal women," Human Genetics, vol.
Benvenisty, "Meta-analysis of the heterogeneity of X chromosome inactivation in human pluripotent stem cells," Stem Cell Research, vol.
Recent advances in the X chromosome inactivation. Curr Opin Cell Biol 2004; 16: 247-255.
X chromosome inactivation sparked by non-coding RNAs.
X chromosome inactivation and development patterns in mammals.
Patterns of X chromosome inactivation in sporadic basal cell carcinomas: evidence for clonality.
Two new reports add to a growing body of evidence that an unusual gene called Xist may hold the key to X chromosome inactivation. While most genes encode proteins, Xist produces a short strand of RNA, a nucleic acid similar to DNA.
While this new research has begun to reveal the secrets behind X chromosome inactivation, scientists must still explain how a cell counts Xist genes and how the Xist RNA silences the thousands of genes on an X chromosome.