The most striking example of point mutation is found in a disease called

Options

A. thalassemia
B. night blindness
C. Down's syndrome
D. sickle-cell anaemia

Explanation

**Step-by-Step Text Solution:**

1. **Understand the Concept of Point Mutation:**
- A point mutation refers to a change in a single nucleotide base in the DNA sequence. This can occur as a substitution, insertion, or deletion of a nucleotide.

2. **Identify the Diseases Listed:**
- The options provided are thalassemia, night blindness, down syndrome, and sickle cell anemia.

3. **Analyze Each Disease:**
- **Thalassemia:** This is a blood disorder caused by mutations in the genes that produce hemoglobin, but it is not primarily caused by a point mutation.
- **Night Blindness:** This condition can be caused by various factors, including vitamin A deficiency or genetic mutations, but it is not specifically a point mutation.
- **Down Syndrome:** This is caused by an extra copy of chromosome 21 (trisomy 21) and is not related to point mutations.
- **Sickle Cell Anemia:** This disease is specifically caused by a point mutation in the beta-globin gene of hemoglobin.

4. **Explain the Mechanism of Sickle Cell Anemia:**
- In sickle cell anemia, at the sixth position of the beta-globin chain, the normal amino acid glutamic acid is replaced by valine due to a point mutation. This single nucleotide change leads to the production of abnormal hemoglobin (HbS), which causes red blood cells to become sickle-shaped.

5. **Conclude with the Correct Answer:**
- Based on the analysis, the most striking example of point mutation is found in sickle cell anemia.

**Final Answer: Sickle Cell Anemia**

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