7. The number of “Barr Bodies” present in a somatic cell of a woman suffering from Turner syndrome is ____.

(A) 0

(B) 1

(D) 3

Barr Bodies in Turner Syndrome: X-Chromosome Inactivation Explained

Introduction

Sex chromosomes determine biological sex in humans and also present an important challenge in maintaining balanced gene expression between males and females. Females normally possess two X chromosomes (46,XX), whereas males possess one X chromosome and one Y chromosome (46,XY). Since the X chromosome contains thousands of genes, having two active X chromosomes would result in excessive gene expression in females compared to males. To overcome this imbalance, mammals employ a process known as X-chromosome inactivation, also called Lyonization.

During early embryonic development, one of the two X chromosomes in every somatic cell of a normal female becomes transcriptionally inactive. This condensed, inactive X chromosome appears as a dark-staining structure at the nuclear periphery known as the Barr body. The number of Barr bodies present in a cell depends entirely on the number of X chromosomes and follows a simple rule:

Number of Barr Bodies = Number of X Chromosomes − 1

Correct Answer

Correct Option: (A) 0

Detailed Explanation

Turner syndrome is a chromosomal disorder caused by the complete or partial absence of one X chromosome. The most common karyotype is 45,X, also represented as XO. Individuals with Turner syndrome possess only a single X chromosome instead of the normal two X chromosomes found in females.

Since Barr bodies are formed by the inactivation of extra X chromosomes, an individual must possess at least two X chromosomes to form one Barr body. In Turner syndrome, there is only one X chromosome, and this chromosome remains active because it is essential for normal cellular function. There is no additional X chromosome available for inactivation.

Therefore, individuals with Turner syndrome do not possess any Barr body in their somatic cells.

Formula for Barr Body Calculation

The standard formula is:

Number of Barr Bodies = Number of X Chromosomes − 1

For Turner syndrome:

Number of X chromosomes = 1

Barr Bodies = 1 − 1 = 0

Step-by-Step Calculation

Parameter	Value
Karyotype	45,X (XO)
Number of X Chromosomes	1
Barr Body Formula	X − 1
Number of Barr Bodies	0

Explanation of Each Option

Option (A): 0

This option is correct. Turner syndrome individuals have only one X chromosome, and therefore no X chromosome undergoes inactivation. As a result, no Barr body is formed.

Option (B): 1

This option is incorrect. One Barr body is present only in normal females with the karyotype 46,XX.

Option (C): 2

This option is incorrect. Two Barr bodies are found in individuals with three X chromosomes, such as 47,XXX.

Option (D): 3

This option is incorrect. Three Barr bodies occur only when four X chromosomes are present, such as in 48,XXXX individuals.

Barr Body Calculation in Different Karyotypes

Karyotype	Number of X Chromosomes	Barr Bodies
46,XY	1	0
45,X (Turner Syndrome)	1	0
46,XX	2	1
47,XXY (Klinefelter Syndrome)	2	1
47,XXX	3	2
48,XXXX	4	3

Comparison of Common Sex Chromosome Disorders

Disorder	Karyotype	Barr Bodies
Normal Male	46,XY	0
Normal Female	46,XX	1
Turner Syndrome	45,X	0
Klinefelter Syndrome	47,XXY	1
Triple X Syndrome	47,XXX	2

Biological Significance

X-chromosome inactivation is an excellent example of dosage compensation in mammals. By inactivating all but one X chromosome, females maintain gene expression levels comparable to males. The condensed inactive X chromosome forms the Barr body, which serves as an important cytogenetic marker for diagnosing sex chromosome abnormalities. Barr body analysis has historically been used in clinical genetics, prenatal diagnosis, forensic biology, and chromosome studies.

Final Answer

Turner syndrome has the karyotype 45,X, meaning only one X chromosome is present.

Using the formula:

Barr Bodies = Number of X Chromosomes − 1

= 1 − 1 = 0