Lyon hypothesis


The Barr body identifies the inactive X.

X-inactivation , also called lyonization, is a process by which one of the copies of the X chromosome is inactivated in female.

The inactive X chromosome is silenced by it being transcriptionally inactive structure called heterochromatin.

The choice of which X chromosome will be inactivated is random, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism.

X-inactivation is part of the activation cycle of the X chromosome throughout the female life.

Inactivation event is irreversible during the lifetime of the individual, with the exception of the germline.

In the female germline before meiotic entry, X-inactivation is reversed, so that after meiosis all haploid oocytes contain a single active X chromosome.

When the egg is fertilized by a sperm a diploid zygote forms, and the X chromosome undergoes the following changes:

The descendants of each cell which inactivated a particular X chromosome will also inactivate that same chromosome.

Normal females possess two X chromosomes, and in any given cell one chromosome will be active (designated as Xa) and one will be inactive (Xi).

In cells with more than two X chromosomes there is still only one Xa, and all the remaining X chromosomes are inactivated.

The default state of the X chromosome in females is inactivation, but one X chromosome is always selected to remain active.

X-chromosome inactivation is a random process, as maternal and paternal X chromosomes have an equal probability of inactivation.

One would suggest that women would be expected to suffer from X-linked disorders approximately 50% as often as men.

But in actuality the occurrence of these X-linked disorders in females is much lower than that, due to the fact 12–20% of genes on the inactivated X chromosome remain expressed, thus providing women with added protection against defective genes coded by the X-chromosome.

Inactivation occurs in the epiblast phase, which gives rise to the embryo.

Inactivation occurs on a cellular level, resulting in a mosaic expression.

Inactivation results in patches of cells with inactive maternal X-chromosome, while other patches have an inactive paternal X-chromosome.

A female heterozygous for hemophilia, an X-linked disease, would have about half of her liver cells functioning properly, enough to ensure normal blood clotting.

Genetic differences on the chromosome may also render one X-chromosome more likely to undergo inactivation.

Males only have one copy of the X chromosome, all expressed X-chromosomal genes are located on that copy of the chromosome.

Females, primarily express the genes or alleles located on the X-chromosomal copy that remains active.

On average, each X chromosome is inactivated in half of the cells.

The inactive X forms a discrete body within the nucleus called a Barr body, which is generally located on the periphery of the nucleus, is late replicating within the cell cycle.

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