Q30. In an X-linked recessive disorder, an affected father and unaffected non-carrier mother ordinarily have which outcome?
(A) All sons affected
(B) All daughters affected
(C) All daughters carriers and no sons affected
(D) All children affected
Answer: (C) All daughters carriers and no sons affected
Explanation: An affected father gives his X chromosome to every daughter and his Y chromosome to every son. With a non-carrier mother, daughters receive one mutant X and become carriers, while sons inherit a normal maternal X and remain unaffected.
Q29. The HBB gene associated with sickle-cell disease is located on which human chromosome?
(A) Chromosome 7
(B) Chromosome 11
(C) Chromosome 21
(D) X chromosome
Answer: (B) Chromosome 11
Explanation: The HBB gene on chromosome 11 codes for the beta-globin component of haemoglobin. Certain mutations produce haemoglobin S, which can polymerise when deoxygenated. Sickle-cell disease is inherited in an autosomal recessive pattern, not as an X-linked trait.
Q28. Which amino-acid substitution characterises classic sickle haemoglobin (HbS)?
(A) Valine replaced by glycine
(B) Glutamic acid replaced by valine
(C) Lysine replaced by alanine
(D) Tyrosine replaced by proline
Answer: (B) Glutamic acid replaced by valine
Explanation: Classic HbS results from a missense change in beta-globin that replaces glutamic acid with valine at position six of the mature chain. This small molecular change alters haemoglobin behaviour and promotes aggregation when oxygen levels become low.
Q27. Under which condition does haemoglobin S particularly promote sickling of red blood cells?
(A) High oxygen tension
(B) Low oxygen tension
(C) Excess vitamin C
(D) High calcium intake
Answer: (B) Low oxygen tension
Explanation: Deoxygenated haemoglobin S tends to polymerise into rigid fibres. These distort red blood cells into sickle shapes, reduce their flexibility and contribute to vessel blockage and haemolysis. Reoxygenation can reverse early sickling, but repeated injury damages cell membranes.
Q26. Which gene is commonly mutated in cystic fibrosis?
(A) CFTR
(B) HBB
(C) PAH
(D) DMD
Answer: (A) CFTR
Explanation: Cystic fibrosis arises from pathogenic variants in CFTR, which encodes an epithelial chloride and bicarbonate channel. Impaired salt transport produces abnormally thick secretions, particularly affecting the respiratory and digestive systems. Most classic cases follow autosomal recessive inheritance.
Q25. Deficiency of the lysosomal enzyme hexosaminidase A is a feature of which inherited disorder?
(A) Tay–Sachs disease
(B) Haemophilia A
(C) Turner syndrome
(D) Thalassaemia
Answer: (A) Tay–Sachs disease
Explanation: Tay–Sachs disease is an autosomal recessive lysosomal storage disorder commonly involving HEXA mutations. Deficient hexosaminidase A causes GM2 ganglioside accumulation, especially in neurons. Severe infantile forms lead to progressive neurological deterioration and loss of developmental abilities.
Q24. Phenylketonuria primarily results from deficiency of which enzyme?
(A) Lactase
(B) Phenylalanine hydroxylase
(C) Hexosaminidase A
(D) DNA ligase
Answer: (B) Phenylalanine hydroxylase
Explanation: Most classical phenylketonuria cases involve deficient phenylalanine hydroxylase, which normally converts phenylalanine to tyrosine. Untreated phenylalanine accumulation can harm brain development. Newborn screening and an appropriately controlled dietary regimen greatly reduce preventable complications.
Q23. Which genetic disorder is a common example of autosomal dominant short-limbed dwarfism?
(A) Achondroplasia
(B) Cystic fibrosis
(C) Haemophilia A
(D) Phenylketonuria
Answer: (A) Achondroplasia
Explanation: Achondroplasia is commonly caused by activating variants in FGFR3 and usually shows autosomal dominant inheritance. It affects cartilage growth at the growth plates, producing disproportionately short limbs. Many cases result from new mutations rather than an affected parent.
Q22. Huntington disease is most commonly inherited in which manner?
(A) Autosomal recessive
(B) X-linked recessive
(C) Autosomal dominant
(D) Mitochondrial
Answer: (C) Autosomal dominant
Explanation: Huntington disease is an autosomal dominant neurodegenerative disorder caused by expansion of CAG repeats in HTT. Inheritance of one disease-associated allele can confer risk. Symptoms often develop in adulthood and may involve movement, cognition and behaviour.
Q21. Haemophilia A is usually caused by a deficiency of which coagulation factor?
(A) Factor VIII
(B) Factor IX
(C) Factor X only
(D) Fibrinogen only
Answer: (A) Factor VIII
Explanation: Haemophilia A is generally an X-linked inherited bleeding disorder caused by pathogenic variants affecting coagulation factor VIII. Deficiency reduces the ability to form stable fibrin clots. Recurrent joint bleeding can occur, especially in more severe cases.
