Q.66 Which of the following statement is true for Barr body?
1. It is a highly condensed chomosome.
2. It is an active X chromosome.
3. It is equivalent to the single Y chromosome of the male.
4. It is equivalent to the two X chromosomes of the female.
The Barr body is a highly condensed, inactive X chromosome in female somatic cells. This structure arises from X-chromosome inactivation (lyonization), ensuring dosage compensation between XX females and XY males.
Barr Body Formation
Barr body forms via random X-inactivation early in female embryogenesis; XIST RNA coats one X chromosome, converting it to heterochromatin.
One Barr body per cell in XX females; males (XY) and XO (Turner) have none; XXX females have two.
Visible as dark-staining nuclear mass near periphery; persists through cell divisions.
Correct Answer: 1. It is a highly condensed chromosome
Barr body represents the inactivated X chromosome, highly condensed as facultative heterochromatin, transcriptionally silent.
Euchromatin surrounds active X; heterochromatin defines Barr body structure.
Option Breakdown
| Option | Statement | Explanation | True? |
|---|---|---|---|
| 1. Highly condensed chromosome | Describes inactivated X structure. | Yes; heterochromatin condensation. | Yes |
| 2. Active X chromosome | Active X remains transcriptionally active. | No; Barr body = inactive X. | No |
| 3. Equivalent to single Y of male | Y carries few genes; Barr body silences entire X. | No; dosage compensation matches one active X. | No |
| 4. Equivalent to two X chromosomes of female | Females have one active + one Barr body X. | No; represents single inactivated X. | No |
Dosage Compensation Role
X-inactivation equalizes X-gene expression: females express one X (like males); mosaic pattern from random choice per cell.
Disorders: extra Barr bodies in XXX; PFO/stroke risk from incomplete inactivation.
