Sickle cell anemia is due to which type of mutation?
**Core Concept**
Sickle cell anemia is a genetic disorder caused by a mutation in the HBB gene, which codes for the beta-globin subunit of hemoglobin. This mutation affects the structure of hemoglobin, leading to the formation of abnormal hemoglobin S (HbS) that polymerizes under low oxygen conditions, causing red blood cells to become misshapen and sickle-shaped.
**Why the Correct Answer is Right**
The mutation responsible for sickle cell anemia is a point mutation at the 6th codon of the HBB gene, resulting in the substitution of glutamic acid (Glu) with valine (Val) at position 6 of the beta-globin chain. This substitution leads to the formation of HbS, which has a higher affinity for deoxyhemoglobin and polymerizes under low oxygen conditions, causing the characteristic sickling of red blood cells. The polymerization of HbS within the red blood cell leads to the formation of rigid fibers that cause the cell to lose its biconcave disk shape and become sickle-shaped.
**Why Each Wrong Option is Incorrect**
* **Option A:** No information provided.
* **Option B:** This option is incorrect because the mutation responsible for sickle cell anemia is a point mutation, not a frameshift mutation.
* **Option C:** This option is incorrect because the mutation responsible for sickle cell anemia is a specific substitution of glutamic acid with valine, not a deletion of a nucleotide.
**Clinical Pearl / High-Yield Fact**
Sickle cell anemia is an autosomal recessive disorder, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to express the disease. This is why individuals with sickle cell trait, who have one normal and one mutated gene, are usually asymptomatic but can pass the mutated gene to their offspring.
**Correct Answer:** C. Point mutation.