Sickle cell anemia is

Options

A. caused by a change in a single base pair of DNA
B. characterized by elongated sickle like RBCs with a nucleus
C. an autosomal linked dominant trait
D. caused by substitution of valine by glutamic acid in the beta globin chain of haemoglobin

Answer

**Step-by-Step Solution:**

1. **Understanding Sickle Cell Anemia:**
- Sickle cell anemia is an inherited blood disorder characterized by the abnormal shape of red blood cells (RBCs). In this condition, RBCs become rigid and shaped like a sickle or crescent, which can lead to various health complications.

2. **Cause of Sickle Cell Anemia:**
- The disorder is caused by a mutation in the hemoglobin gene. Specifically, it is due to a change in a single base pair of DNA, which leads to the production of abnormal hemoglobin known as hemoglobin S (HbS).

3. **Genetic Basis:**
- Sickle cell anemia is classified as a recessive genetic disease. This means that an individual must inherit two copies of the mutated gene (one from each parent) to exhibit the disease.

4. **Specific Mutation:**
- In sickle cell anemia, the specific mutation involves the substitution of glutamic acid (a normal amino acid) with valine in the beta globin chain of hemoglobin. This substitution alters the properties of hemoglobin, causing the RBCs to deform into a sickle shape.

5. **Analyzing the Options:**
- **Option A:** "Caused by a change in a single base pair of DNA." - This is correct as it accurately describes the genetic mutation responsible for sickle cell anemia.
- **Option B:** "Characterized by elongated sickle-like RBCs with a nucleus." - This is incorrect because mature RBCs do not have a nucleus.
- **Option C:** "An autosomal linked dominant trait." - This is incorrect because sickle cell anemia is a recessive trait, not dominant.
- **Option D:** "Caused by substitution of valine by glutamic acid in the beta globulin chain of hemoglobin." - This is incorrect as it reverses the actual mutation; it should be glutamic acid substituted by valine.

6. **Conclusion:**
- The correct answer is **Option A**, as it accurately describes the cause of sickle cell anemia.

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