X inactivation


Also found in: Wikipedia.

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

The silencing of the genes on one of a woman's paired X chromosomes, allowing the other chromosome's genes to be expressed phenotypically. The process is not universal but affects 75% or more of the genes on the chromosome. Genes that are found near the short end of the chromosome are more likely than the rest to be expressed phenotypically, i.e., to escape inactivation.
References in periodicals archive ?
Scientists haven't worked out all the details of X inactivation, and a new study prolongs the puzzlement.
They speculated that Tsix might also control the random X inactivation of women.
Migeon speculates that Tsix controls imprinted, but not random, X inactivation.
The study looked at human cells that were far past the period of X inactivation, says Harvard researcher Jeannie T.
Migeon's finding may help explain why women don't have imprinted X inactivation, says Andrew Feinberg, a biologist from Johns Hopkins who wasn't involved in the study.
Scientists first recognized the phenomenon of X inactivation 4 decades ago.
Another new study offers an intriguing explanation for that finding and suggests a mechanism for how X inactivation occurs.
The new work doesn't come close to resolving all the questions surrounding X inactivation, but optimistic investigators contend that they're closing in on a better understanding of the puzzling event.
Ultimately, it'll look a lot more simple and elegant," says Neil Brockdorff, who studies X inactivation at the Imperial College School of Medicine and Hammersmith Hospital, both in London.
While some critics challenged Lyon's hypothesis, it quickly garnered enough support that X inactivation was called--and still is, by some scientists--Lyonization.
Although researchers are still not sure exactly why this occurs, many assert that X inactivation initially arose to prevent the genetic inequity that would result if females had a double dose of active X chromosome genes.
Willard at Stanford University and another led by Neil Brockdorff of the Medical Research Council Clinical Research Center in Harrow, England, have isolated two forms of a gene that may play a role in X inactivation in females.