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Barr Body

“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

Lyon’s hypothesis states the following postulates:

  1. In the female mammals, one of the two X-chromosomes in the somatic cell is inactive.
  2. The inactivation of X-chromosome is random.
  3. The inactivation occurs during development.
  4. The inactive X chromosome remains inactivated in all the generations of the cell.

X-Inactivation Example and Barr Body

Calico cat

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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