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What is Sex Chromatin?

Meaning of Sexual Chromatin

Sex chromatin is a structure formed in the nucleus of female mammalian cells that represents an inactivated X chromosome. Also known as Barr’s Corpuscle, this dense mass appears as a form of dosage compensation for the double genetic load of the X chromosome in females.

The Barr body can be most easily identified in the perinucleolar region of somatic cells during interphase, when the other chromosomes are not condensed.

The first researchers to observe the structure of sexual chromatin were Barr and Bertram, in 1949.

They reported a sexual dimorphism among the neurons of the cats, since in the female cells it was possible to notice a small and consistent body of a dark color near the nuclear envelope. , while the male cells did not show such a structure.

Then, in 1961, the geneticist Mary Lyon proposed that the X chromosomes in the somatic cells of mature women can be of two types. You remain active and fully express your genetic information; the other is inactive and does not serve as a source of genetic information.

The biological significance of silencing the functional activity of one of the X chromosomes in females is dose compensation, which makes the amount of gene products from the two copies of the X chromosome in females equivalent to the single dose of the chromosome. X in men.

In mammals it is important that the inactivation of the X chromosome occurs at an early stage of embryonic life (from day 13 to 16) and randomly in the cells, being able to inactivate both the X chromosome of paternal origin and that of maternal origin .

Thus, in some cells, X-linked genes inherited from the mother are expressed, while in other cells, X-linked genes inherited from the father are active.

For this condition, the somatic tissues of females are called mosaic, because they represent the contribution of different genes on the X chromosome.

X-chromosome inactivation is orchestrated by the XIST (X-inactivation-specific transcript) gene. It is expressed in cells where X is inactive and its expression determines the silencing of the other genes on this chromosome.

Some genes escape X inactivation, such as those in the pseudoautomatic region (homologous to the Y chromosome), the pericentromeric region, and about 30% of the genes on the short arm of X.

Failures in this process can be lethal, as it occurs when there is no inactivation of X in embryonic cells after day 13 to 16 of development.

There are cases where the inactivation of X does not occur randomly. In the process called selective Lyonization, the inactivation of the X chromosome occurs on the one with a mutation. Negative Lyonization, in turn, inactivates X that does not have a mutation, keeping the mutated X chromosome active.

And in cases of cells from extra-embryonic tissue, only the X chromosomes inherited from the father are inactivated.

The number of X chromosomes is directly related to the number of Barr bodies present in the nucleus.

This number is important because it is not only possible to differentiate male and female cells, but also to identify the appearance of syndromes or sexual chromosomal abnormalities.

Barr’s Corpuscle Table,

Thus, the number of Barr bodies = number of X chromosomes per cell – 1. In the following table, we will see examples.

Condition sex chromosomes Sex chromatin number (Barr’s corpuscles)
ordinary woman XX 1
regular male XY none
Trisomy X XXX two
Turner’s syndrome X none
Klinefelter syndrome XXY 1

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