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.
Which gene controls β thalassemia, and on which chromosome is it present?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → HBB on chromosome 11
Thalassemia is grouped by the globin chain whose production is affected. In α thalassemia the α globin chain is affected, and in β thalassemia the β globin chain. The two forms are controlled by different genes on different chromosomes; the passage does not give these genes, so they come from NCERT.
Option (1) HBA1 on chromosome 11 — Incorrect. HBA1 is one of the two genes for the α globin chain, and it lies on chromosome 16, not 11. It controls α thalassemia.
Option (2) HBB on chromosome 16 — Incorrect. HBB is the right gene for β thalassemia, but it lies on chromosome 11. Chromosome 16 carries the α globin genes.
Option (3) HBA1 and HBA2 on chromosome 16 — Incorrect. HBA1 and HBA2 on chromosome 16 are the two closely linked genes that control α thalassemia, not β thalassemia.
Option (4) HBB on chromosome 11 — Correct. β thalassemia is controlled by a single gene, HBB, on chromosome 11 of each parent. It occurs due to mutation of one or both of these genes.
NCERT Extract:
Mendelian Disorders – “… Thalassemia can be classified according to which chain of the haemoglobin molecule is affected. In α Thalassemia, production of α globin chain is affected while in β Thalassemia, production of β globin chain is affected. α Thalassemia is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16 of each parent and it is observed due to mutation or deletion of one or more of the four genes. The more genes affected, the less alpha globin molecules produced. While β Thalassemia is controlled by a single gene HBB on chromosome 11 of each parent and occurs due to mutation of one or both the genes.”