Biology · Principles of Inheritance and Variation · NEET
Thalassemia is an autosome-linked (autosomal) recessive disorder. "Autosomal" means the gene is on a normal chromosome, not on the X or Y sex chromosome. "Recessive" means a child shows the disease only when BOTH copies of the gene are faulty, so both parents must be carriers. This is exactly like sickle-cell anaemia. NEET often gives a wrong match saying "Thalassemia - X linked" (NEET 2018) - that is FALSE. It is autosomal recessive.
The difference is which globin chain is affected and which chromosome carries the gene. Alpha thalassemia = the alpha-globin chain is made too little; it is controlled by two closely linked genes, HBA1 and HBA2, on chromosome 16. Beta thalassemia = the beta-globin chain is made too little; it is controlled by a single gene, HBB, on chromosome 11. NEET 2024 tested exactly this: alpha -> chromosome 16, beta -> chromosome 11. Memory hook: "Beta = 11 letters? No - Beta pairs with sickle-cell, both on chromosome 11."
In thalassemia the globin chains that ARE made are normal in structure, but the body makes too FEW of them. Fewer globin molecules means less normal haemoglobin, which causes anaemia. That is a problem of quantity (how much), so it is called a quantitative defect. Compare this with sickle-cell anaemia, where the amount is fine but the globin has a WRONG amino acid (Glu changed to Val) - a quality problem, so it is a qualitative defect. NEET 2017 gave the answer as "Thalassemia is due to less synthesis of globin molecules."
Beta thalassemia and sickle-cell anaemia are BOTH linked to the beta-globin gene HBB on chromosome 11. Alpha thalassemia is different - its genes HBA1 and HBA2 are on chromosome 16. So do not assume all thalassemia is on chromosome 11: only beta is; alpha is on 16. This split is a favourite NEET matching trap (NEET 2024).
Both parents being carriers means each is heterozygous (one normal gene + one faulty gene). Cross them and the offspring ratio is 1 normal : 2 carriers : 1 affected. So there is a 1 in 4 (25%) chance the child is affected, a 2 in 4 (50%) chance the child is a carrier, and a 1 in 4 (25%) chance the child is fully normal. This is the standard autosomal recessive pattern, the same as for sickle-cell anaemia.
Thalassemia and sickle cell anemia are caused due to a problem in globin molecule synthesis. Select the correct statement.
Match List I with List II: A. Down's syndrome, B. alpha-Thalassemia, C. beta-Thalassemia, D. Klinefelter's syndrome | I. 11th chromosome, II. 'X' chromosome, III. 21st chromosome, IV. 16th chromosome.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a Mendelian disorder. It is caused by a change (mutation) in a single gene and is passed on following Mendel's rules, as an autosomal recessive trait. Chromosomal disorders (like Down's, Turner's, Klinefelter's) are instead caused by an extra or missing whole chromosome.
Yes. A carrier is heterozygous - one normal gene and one faulty gene. The one normal gene still makes enough globin, so the carrier is usually healthy and shows no or only mild symptoms. But two carrier parents can have an affected child.
Anaemia. Because too few normal globin chains are made, the body cannot make enough normal haemoglobin, so the blood carries less oxygen. This causes weakness and tiredness.
Thalassemia = too FEW globin molecules (quantitative defect). Sickle-cell anaemia = globin with a WRONG amino acid, Glu replaced by Val (qualitative defect). Both are autosomal recessive.
Alpha thalassemia = reduced alpha-globin chains (genes HBA1, HBA2 on chromosome 16). Beta thalassemia = reduced beta-globin chains (gene HBB on chromosome 11).
