Read the passage and answer the following questions:
Genetic disorders may be grouped into two categories – Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sicklecell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive. Thalassemia : This is also an autosome-linked recessive blood disease transmitted from parents to the offspring when both the partners are unaffected carrier for the gene (or heterozygous). The defect could be due to either mutation or deletion which ultimately results in reduced rate of synthesis of one of the globin chains (α and β chains) that make up haemoglobin.
How does thalassemia differ from sickle-cell anaemia?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Thalassemia is a quantitative problem of synthesising too few globin molecules.
The passage states that in thalassemia the defect “ultimately results in reduced rate of synthesis of one of the globin chains (α and β chains) that make up haemoglobin.” Too little globin is made, so thalassemia is a quantitative problem. In sickle-cell anaemia the amount is normal but the globin itself is faulty, so that is a qualitative problem.
Option (1) Thalassemia is a qualitative problem of synthesising an incorrectly functioning globin. — Incorrect. An incorrectly functioning globin is the qualitative problem of sickle-cell anaemia, not of thalassemia.
Option (2) Thalassemia is a quantitative problem of synthesising too few globin molecules. — Correct. A reduced rate of synthesis means too few globin molecules are made. This quantitative problem is what sets thalassemia apart from sickle-cell anaemia.
Option (3) Thalassemia is caused by the substitution of glutamic acid by valine in the β globin chain. — Incorrect. The substitution of glutamic acid by valine at the sixth position of the β globin chain is the cause of sickle-cell anaemia. Thalassemia is due to mutation or deletion that lowers globin production.
Option (4) Thalassemia is a quantitative problem of synthesising too many globin molecules. — Incorrect. Thalassemia is a quantitative problem, but the passage speaks of a reduced rate of synthesis, so the globin molecules are too few, not too many.