Q10. Which condition is also known as trisomy 18?
[RRB Staff Nurse – 20 Jul 2019, Shift 1]
(A) Turner syndrome
(B) Patau syndrome
(C) Edwards syndrome
(D) Down syndrome
Answer: (C) Edwards syndrome
Explanation: Edwards syndrome results from an additional chromosome 18 in most affected cells. It often causes severe growth and developmental problems. Down syndrome involves chromosome 21, while Patau syndrome involves chromosome 13; Turner syndrome commonly involves a missing X chromosome.
Q9. In humans, trisomy of chromosome number 21 is responsible for which condition?
[RPSC RAS Prelims – 2018]
(A) Haemophilia
(B) Klinefelter syndrome
(C) Down syndrome
(D) Turner syndrome
Answer: (C) Down syndrome
Explanation: Trisomy 21 causes the common chromosomal form of Down syndrome. Affected individuals have three copies of chromosome 21, often as a result of meiotic nondisjunction. Klinefelter and Turner syndromes involve sex chromosomes, while haemophilia is usually monogenic.
Q8. A Barr body is absent in the typical somatic cells of which individual?
[WBCS Prelims – 2017]
(A) An individual with Klinefelter syndrome
(B) An individual with triple-X syndrome
(C) An individual with Turner syndrome
(D) A typical XX female
Answer: (C) An individual with Turner syndrome
Explanation: Barr bodies are inactivated X chromosomes; a common rule is X chromosome count minus one. A typical 45,X Turner cell has only one X and therefore no Barr body. Classic XXY cells contain one and XXX cells two.
Q7. Sickle-cell anaemia is due to which cause?
[UPPSC GIC Lecturer (Zoology) – 2017]
(A) Hormones
(B) Viruses
(C) Bacteria
(D) Gene mutation
Answer: (D) Gene mutation
Explanation: Sickle-cell anaemia results from a mutation in the beta-globin gene, not from infection or hormone imbalance. The classical variant produces haemoglobin S. Under reduced oxygen conditions, its polymerisation deforms red cells and contributes to anaemia and vascular obstruction.
Q6. Identify the correct statements:
(1) Haemophilia is X-linked recessive.
(2) Down syndrome involves aneuploidy.
(3) Phenylketonuria is autosomal recessive.
(4) Sickle-cell anaemia is X-linked recessive.
[AIPMT/NEET – 2016]
(A) 1 and 4 only
(B) 2 and 4 only
(C) 1, 3 and 4 only
(D) 1, 2 and 3 only
Answer: (D) 1, 2 and 3 only
Explanation: The first three statements are correct. Haemophilia is generally X-linked recessive, Down syndrome commonly involves trisomy 21, and classical phenylketonuria is autosomal recessive. Sickle-cell anaemia is autosomal recessive, so statement four is incorrect.
Q5. A colour-blind woman and a man with normal colour vision have one son and one daughter. Under classical X-linked recessive inheritance, which is the expected outcome?
[UPSC CSE Prelims – 2009]
(A) Both children colour-blind
(B) Only the daughter colour-blind
(C) Neither child colour-blind
(D) Son colour-blind; daughter unaffected but a carrier
Answer: (D) Son colour-blind; daughter unaffected but a carrier
Explanation: A colour-blind mother has two affected X chromosomes and transmits an affected X to both children. Her son receives a Y from his unaffected father and is colour-blind. Her daughter receives a normal paternal X and is generally an unaffected carrier.
Q4. Both sickle-cell anaemia and Huntington chorea are examples of which category?
[AIPMT (Prelims) – 2006]
(A) Bacteria-related diseases
(B) Genetic disorders present from conception
(C) Pollutant-induced disorders
(D) Virus-related diseases
Answer: (B) Genetic disorders present from conception
Explanation: Both conditions result from inherited or newly arising gene variants. Sickle-cell disease generally shows autosomal recessive inheritance, whereas Huntington disease is autosomal dominant. Huntington symptoms usually appear later in life even though its genetic variant has been present since conception.
Q3. Which of the following is an example of a sex-linked inherited condition?
[AIPMT – 2002]
(A) AIDS
(B) Red–green colour blindness
(C) Syphilis
(D) Gonorrhoea
Answer: (B) Red–green colour blindness
Explanation: The common red–green colour-vision deficiencies are X-linked recessive traits, so they are more frequently expressed in males. AIDS is caused by HIV; syphilis and gonorrhoea are bacterial infections. Infectious diseases are not inherited through a Mendelian sex-linked allele.
Q2. Which disorder provides a classic example of pleiotropy?
[AIPMT – 2002]
(A) Haemophilia
(B) Thalassaemia
(C) Sickle-cell anaemia
(D) Colour blindness
Answer: (C) Sickle-cell anaemia
Explanation: Pleiotropy occurs when one gene influences multiple phenotypic features. A beta-globin mutation underlying sickle-cell disease can cause red-cell sickling, haemolytic anaemia, painful vaso-occlusive episodes and organ damage. These diverse manifestations arise from a single molecular defect.
Q1. Which disease–chromosome association is correctly matched?
[AIPMT – 2002]
(A) Down syndrome — chromosome 21
(B) Sickle-cell anaemia — X chromosome
(C) Haemophilia — Y chromosome
(D) Parkinson disease — X and Y chromosomes
Answer: (A) Down syndrome — chromosome 21
Explanation: Down syndrome most often involves an additional copy of autosome 21. The HBB gene responsible for classic sickle-cell disease is on chromosome 11, while haemophilia A and B usually involve genes on the X chromosome. Parkinson disease is genetically heterogeneous.
