“Barr body is the inactive X-chromosome in the somatic cells of mammalian females.”
What is a Barr Body?
Females have two X chromosomes. The somatic cells of females are not involved in sexual reproduction. Here one of the two X chromosomes is inactivated by lyonization. This inactive X chromosome is known as a Barr body.
The process of X-inactivation was discovered by Mary F. Lyon, a British geneticist. One X-chromosome is inactivated so that unnecessary information is not passed onto the next generation. The amount of expression of X-chromosome genes should be equal in both males and females.
The active X-chromosome is enclosed within euchromatin, whereas, the inactive X-chromosome is enclosed within heterochromatin. The inactive X-chromosome is compacted and is not accessible to the molecules involved in transcription.
In X-inactivation, the X chromosome is compacted to create a small, dense structure called Barr body.
Also Read: Genetics
Formation of Barr Bodies
The X-chromosomes have an X-inactivation center (XIC) which contains a gene called X-inactive specific transcript (Xist). There is yet another gene known as Tsix (Xist reversed).
Xist is responsible for the inactivation of X-chromosome, whereas Tsix prevents it. X-inactivation is a random process that occurs during embryo development.
Lyon’s hypothesis states the following postulates:
- In the female mammals, one of the two X-chromosomes in the somatic cell is inactive.
- The inactivation of X-chromosome is random.
- The inactivation occurs during development.
- The inactive X chromosome remains inactivated in all the generations of the cell.
X-Inactivation Example and Barr Body
If a female cat has black and tan colour alleles on the X chromosome, it inactivates its two Xs during the embryonic development. This results in a tortoiseshell coat pattern containing alternate patches of black and tan fur. The black patches are obtained from the X-chromosome with active black allele, whereas, the tan patches are obtained from X-chromosome with an active tan allele.
Also Read: Sex Determination
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