## **Core Concept**
Sickle cell anemia is a genetic disorder caused by a specific type of mutation in the HBB gene that codes for the beta-globin subunit of hemoglobin. This mutation leads to the production of abnormal hemoglobin, known as sickle hemoglobin or hemoglobin S. The disorder is characterized by the sickling of red blood cells under certain conditions.

## **Why the Correct Answer is Right**
The correct answer, , refers to a point mutation. In sickle cell anemia, a point mutation occurs in the HBB gene where glutamic acid (coded by GAG) is replaced by valine (coded by GTG) at the sixth position of the beta-globin chain. This change from GAG to GTG is a classic example of a point mutation, specifically a missense mutation, because it results in a codon that codes for a different amino acid.

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option is incorrect because a frameshift mutation involves the insertion or deletion of nucleotides that alters the reading frame of the genetic sequence, leading to the synthesis of an abnormal protein. This is not the type of mutation seen in sickle cell anemia.
- **Option B:** - This option is incorrect because a chromosomal mutation involves larger scale changes such as deletions, duplications, inversions, or translocations of genetic material. Sickle cell anemia is caused by a much smaller scale change.
- **Option C:** - This option might seem plausible because it implies a type of genetic alteration, but it is not specific or accurate in the context of sickle cell anemia.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for students to remember is that sickle cell anemia is an autosomal recessive disorder. This means that a person must inherit two copies of the mutated gene (one from each parent) to express the disease. Carriers, who have one normal and one mutated gene, typically have a milder condition known as sickle cell trait.

## **Correct Answer: B. point mutation**