Question
Regarding hemoglobinopathies, identify the correct statements:
a. Hemoglobin S (HbS) is caused by substitution of valine for glutamic acid at the 6th position of the β-globin chain.
b. Hemoglobin H (HbH) disease results from deletion of three α-globin genes.
c. Hemoglobin Bart’s (Hb Bart’s) results from deletion of all four α-globin genes.
d. Hemoglobin C (HbC) is caused by substitution of lysine for glutamic acid at the 6th position of the β-globin chain.
| A. | a and b are true; c and d are false |
| B. |
b and c are true; a and d are false |
| C. |
b and d are true; a and c are false |
| D. |
a, b, c, and d are true |
Show Answer
|
Correct Answer » D Explanation |
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a. HbS — True
- HbS is caused by a point mutation in the β-globin gene.
- The mutation occurs at the 6th position of the β-globin chain.
- Glutamic acid is replaced by valine.
Mutation:
β6 Glu → Val
Original statement error:
- It states lysine for glutamine, which is incorrect.
- The correct substitution is valine for glutamic acid.
b. HbH disease — True
- Normally, there are four α-globin genes.
- Deletion of three α-globin genes causes HbH disease.
- Excess β-globin chains form tetramers.
HbH:
β₄
Clinical features:
- Microcytic anemia
- Hemolytic anemia
- Splenomegaly
c. Hb Bart’s — True
- Hb Bart’s occurs due to deletion of all four α-globin genes.
- During fetal life, excess γ-globin chains form tetramers.
Hb Bart’s:
γ₄
Clinical consequence:
- Severe anemia
- Hydrops fetalis
- Usually fatal without treatment
Original statement error:
- It states deletion of four β-globin genes.
- The correct abnormality is deletion of all four α-globin genes.
d. HbC — True
- HbC is caused by a point mutation in the β-globin gene.
- The mutation occurs at the 6th position.
- Glutamic acid is replaced by lysine.
Mutation:
β6 Glu → Lys