**Core Concept**
Sickle cell anaemia is a genetic disorder caused by a mutation in the haemoglobin gene, leading to the production of abnormal haemoglobin subunits. This mutation affects the polymerization of haemoglobin, resulting in the characteristic sickling of red blood cells.

**Why the Correct Answer is Right**
The defect in sickle cell anaemia is due to a point mutation in the HBB gene, which codes for the beta-globin subunit of haemoglobin. This mutation leads to the substitution of glutamic acid with valine at position 6 of the beta-globin chain (HbS). The abnormal haemoglobin subunits have a higher affinity for deoxygenated states, causing red blood cells to polymerize and become sickled.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the defect in sickle cell anaemia is not in the alpha-globin chain, which is responsible for thalassemia major.
**Option B:** This option is incorrect because the defect in sickle cell anaemia is not in the gamma-globin chain, which is responsible for fetal haemoglobin production.
**Option C:** This option is incorrect because the defect in sickle cell anaemia is not in the delta-globin chain, which is not a component of the haemoglobin tetramer.

**Clinical Pearl / High-Yield Fact**
Sickle cell anaemia is more common in individuals of African descent due to the presence of a founder effect, where a single mutation became widespread in a population.

**Correct Answer:** C. Beta chain.