## **Core Concept**
The Barr body, also known as the sex chromatin body, is a compact, inactivated X chromosome found in the cells of female mammals that have more than one X chromosome. Typically, females have two X chromosomes (XX), and one of them is inactivated to form a Barr body to avoid a doubling of gene expression. This process is crucial for dosage compensation between males (XY) and females (XX).

## **Why the Correct Answer is Right**
The correct answer, **Turner syndrome (45,X)**, is right because individuals with Turner syndrome have only one X chromosome. Since they have only one X chromosome, there is no second X chromosome to inactivate and form a Barr body. Therefore, Barr bodies are absent in females with Turner syndrome.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, in a normal female (46,XX), one of the two X chromosomes is inactivated to form a Barr body. Thus, a Barr body is present.
- **Option B:** This option is incorrect because, although the number of X chromosomes can vary, the presence of more than one X chromosome typically leads to the formation of Barr bodies equal to the number of X chromosomes minus one. For example, in Triple X syndrome (47,XXX), two Barr bodies are present.
- **Option C:** This option is incorrect because, similar to option B, the presence of an extra X chromosome usually results in the formation of Barr bodies.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the presence or absence of Barr bodies can be used as a diagnostic tool in certain genetic conditions. For instance, Turner syndrome (45,X) is characterized by the absence of Barr bodies, while conditions with extra X chromosomes (like Triple X syndrome) have an increased number of Barr bodies.

## **Correct Answer: B. Turner syndrome (45,X)**