Question

How are Barr bodies formed?

Answer 1

Hint: The genetic constitution of females is XX that is having two X chromosomes. The somatic cells are not involved in sexual reproduction. One of the two X chromosomes is inactivated by the process of lyonization. This inactivated X chromosome is known as a Barr body.

Complete answer:
Mary F. Lyon, a British geneticist, discovered the process of X-inactivation. One of the two X-chromosomes is inactivated so that unnecessary information does not pass onto the next generation. The amount of expression of genes on the X-chromosome should be equal in both males and females.
Euchromatin encloses the active X-chromosome, whereas heterochromatin encloses the inactive X-chromosome. The inactive X-chromosome is not accessible to the molecules involved in transcription.
During X-inactivation, the X chromosome is compacted and condensed to create a small, dense structure called the Barr body. An X-inactivation center (XIC) present on the X-chromosome contains a gene called X-inactive specific transcript (Xist). Tsix is also a gene that prevents X-inactivation. X-chromosome inactivation is a random process that occurs during embryo development.

Lyonisation is the term coined by Lyon and was derived from Lyon’s hypothesis which states the following postulates-
-One of the two X-chromosomes in the somatic cell of female mammals is inactive.
-The X-chromosome inactivation is a random process.
-This inactivation occurs during embryonic development.
-The inactive X chromosome remains inactivated throughout all the generations of the cell.

Note: The example of a female Calico cat is very famous here.
-If a female cat consists of black and tan color alleles on the X chromosome, it inactivates its two X chromosomes during embryonic development.
-This results in a tortoiseshell coat pattern with alternate patches of black and tan fur.
-The black patches are obtained from the X-chromosome with an active black allele, whereas, the tan patches are obtained from the X-chromosome with an active tan allele.