Which statement is not correct regarding to stickle cell anaemia :-

Options

A. This is autosomal linked recessive trait
B. Heterzygous `(Hb^(A)Hb^(S))` are resistant to malarial disease
C. Heterozygous `(Hb^(A)Hb^(S))` individuals are diseased phenotype
D. Defect is caused by the substitution of glutamic acid (Glu) by valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule

Answer

**Step-by-Step Solution:**

1. **Understanding Sickle Cell Anemia**: Sickle cell anemia is a genetic disorder characterized by the production of abnormal hemoglobin, known as hemoglobin S (HbS), which leads to the distortion of red blood cells into a sickle shape.

2. **Analyzing the Statements**:
- **Statement 1**: "This is an autosomal linked recessive trait."
- **Evaluation**: This statement is correct. Sickle cell anemia is indeed an autosomal recessive disorder, meaning the gene responsible for the condition is located on one of the autosomes (non-sex chromosomes) and two copies of the mutated gene are required for the disease to manifest.

- **Statement 2**: "Heterozygous HbA, HbS are resistant to malarial disease."
- **Evaluation**: This statement is also correct. Individuals who are heterozygous for sickle cell trait (HbA/HbS) have some protection against malaria, which is an advantage in regions where malaria is prevalent.

- **Statement 3**: "Heterozygous HbA, HbS individuals are disease phenotype."
- **Evaluation**: This statement is incorrect. Heterozygous individuals (HbA/HbS) are carriers and do not exhibit the disease phenotype. The disease phenotype occurs only in individuals who are homozygous for the sickle cell allele (HbS/HbS).

- **Statement 4**: "Defect is caused due to the substitution of glutamic acid by valine at the sixth position of beta globin chain of the hemoglobin molecule."
- **Evaluation**: This statement is correct. The mutation that causes sickle cell anemia is a point mutation where glutamic acid is replaced by valine at the sixth position of the beta globin chain, leading to the formation of HbS.

3. **Conclusion**: The statement that is not correct regarding sickle cell anemia is the third one: "Heterozygous HbA, HbS individuals are disease phenotype."

**Final Answer**: The incorrect statement is: "Heterozygous HbA, HbS individuals are disease phenotype."

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