As per Lyon's hypothesis one of the two x - chromosomes undergoes heterochromatisation and is called

Options

A. Barr body
B. Karyotypic body
C. Genotypic body
D. Phenotypic body

Answer

### Step-by-Step Solution:

1. **Understanding Lyon's Hypothesis**:
- Lyon's hypothesis states that in females (who have two X chromosomes), one of the X chromosomes is randomly inactivated in each cell. This process is known as X-inactivation or dosage compensation.

2. **Purpose of X-Inactivation**:
- The purpose of this inactivation is to ensure that females, like males (who have only one X chromosome), have only one functional X chromosome per cell. This balances the gene dosage between the sexes.

3. **Process of Heterochromatization**:
- The inactivated X chromosome undergoes a process called heterochromatization, where it becomes tightly coiled and condensed, making it less accessible for transcription.

4. **Terminology**:
- The inactivated X chromosome in females is specifically referred to as a "Barr body." This term is used to describe the visible manifestation of the inactivated X chromosome in somatic cells.

5. **Conclusion**:
- Therefore, according to Lyon's hypothesis, one of the two X chromosomes undergoes heterochromatization and is called a Barr body.

### Final Answer:
As per Lyon's hypothesis, one of the two X chromosomes undergoes heterochromatization and is called a **Barr body**.

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