Number of Barr body present in a female child with down syndrome

Options

A. 0
B. 1
C. 2
D. All are correct

Answer

To determine the number of Barr bodies present in a female child with Down syndrome, we need to understand a few key concepts about Barr bodies and Down syndrome.

### Step-by-Step Solution:

1. **Understand Barr Bodies**:
- Barr bodies are inactivated X chromosomes found in females. Typically, for every additional X chromosome beyond one, there is one Barr body formed.
- The formula to calculate the number of Barr bodies is: Number of Barr bodies = (Number of X chromosomes - 1).

2. **Identify the Karyotype of Down Syndrome**:
- Down syndrome is caused by the presence of an extra chromosome 21, resulting in a total of 47 chromosomes instead of the normal 46.
- In females, the typical karyotype is 46,XX, but in the case of Down syndrome, the karyotype would be 47,XX,+21.

3. **Count the X Chromosomes**:
- In a female with Down syndrome, there are still only 2 X chromosomes (from the karyotype 47,XX,+21).
- Therefore, we have: Number of X chromosomes = 2.

4. **Calculate the Number of Barr Bodies**:
- Using the Barr body formula: Number of Barr bodies = (Number of X chromosomes - 1).
- Substituting the values: Number of Barr bodies = (2 - 1) = 1.

5. **Consider the Age Factor**:
- The question specifies a "female child." In children, particularly in early development, the inactivation of X chromosomes may not be fully established, leading to the possibility of no visible Barr bodies.

6. **Final Conclusion**:
- While theoretically, there should be 1 Barr body based on the number of X chromosomes, in a female child with Down syndrome, it is possible that no Barr bodies are visible. Thus, the answer is 0.

### Final Answer:
The number of Barr bodies present in a female child with Down syndrome is 0.