Question

Barr body in mammals represents
A. The Y-chromosomes in somatic cells of male
B. All heterochromatin in male and female cells
C. All heterochromatin in female cells only
D. One of the two X-chromosomes in somatic cells of females.

Answer 1

Hint: Chromosomes are thread like structures which are located inside the nucleus of an animal and plant cells. There are two types of chromosomes - X and Y. The Barr body is a kind of X chromosome.

Complete Answer:
Option a: Barr Body is an inactivated, condensed X chromosome found in female cells. Barr bodies are not observed in XY males. Therefore this is an incorrect option.

Option b: Heterochromatin is a tightly packed form of DNA or condensed DNA, which comes in multiple varieties. It is present at the periphery of the nucleus or buried in the interior of a chromosomal domain. Therefore this is an incorrect option.

Option c: Barr body refers to the inactivated X-chromosome in the somatic cell of the female. Therefore this is an incorrect option.

Option d: Barr body is the inactivated X-chromosome in the somatic cell of the female. It is present as tightly coiled and visible heterochromatin. Dosage compensation in humans is accomplished by the inactivation of one of the X chromosomes in females. This is done so that both males and females have only one functional X chromosome per cell because human males have only one X chromosome, whereas females have two and hence twice the genes present on the X-chromosome. Therefore, this is the correct option.

Thus, the correct option is D, One of the two X-chromosomes in somatic cells of females.

Additional information:
- The process of the X(chromosome)-inactivation was discovered by Mary F. Lyon, who was a British geneticist.
- Inactivation of any one of the X-chromosomes in females, is random. It occurs during the embryo development.
- This inactive X-chromosome remains inactive in all the future generations of that cell.

Note: It is to be noted that XX females have one Barr body per cell while the XXX females have 2 Barr bodies per cell, and XXY Klinefelter males have one Barr body per cell.