20. How many pairs of sex chromosomes are present in humans?
[RRB Group D, 16 September 2022 (Shift 3)]
(a) 2 pairs
(b) 1 pair
(c) 3 pairs
(d) 4 pairs
Answer: (b) 1 pair
Explanation: A normal human diploid complement has 23 chromosome pairs: 22 pairs of autosomes and one pair of sex chromosomes. Typical females have XX and typical males have XY. Sex chromosomes influence sexual development and carry numerous additional genes.
19. One of the contrasting pairs of characters studied by Mendel was __________.
[RRB Group D, 15 September 2022 (Shift 2)]
(a) Blue and white flowers
(b) Purple and white flowers
(c) Yellow and purple flowers
(d) White and yellow flowers
Answer: (b) Purple and white flowers
Explanation: Mendel examined purple versus white flower colour in garden peas as one of his seven pairs of contrasting traits. Purple flower colour was dominant over white in his experimental lines. Tracking such differences across generations revealed predictable inheritance patterns.
18. In Mendel’s experiment, when F1 plants were self-pollinated, what was the genotypic ratio in F2?
[RRB Group D, 2 September 2022 (Shift 1)]
(a) 1 : 2 : 3
(b) 1 : 2 : 1
(c) 2 : 2 : 1
(d) 3 : 2 : 1
Answer: (b) 1 : 2 : 1
Explanation: Selfing a heterozygous monohybrid, such as Tt × Tt, produces TT, Tt, Tt and tt combinations. Their genotypic ratio is 1:2:1, whereas with complete dominance the corresponding phenotypes show a 3:1 dominant-to-recessive ratio.
17. How is the chromosome number maintained from one generation to another in sexually reproducing organisms?
[RRB Group D, 30 August 2022 (Shift 2)]
(a) Halving of chromosomes during gamete formation
(b) Doubling of chromosomes during zygote formation
(c) Halving of chromosomes during zygote formation
(d) Doubling of chromosomes during gamete formation
Answer: (a) Halving of chromosomes during gamete formation
Explanation: Meiosis reduces the chromosome number by half when gametes form, preventing doubling in successive generations. Fusion of two haploid gametes at fertilisation restores the diploid set. The balancing of reduction and restoration maintains the typical species chromosome number.
16. In human males, sex chromosomes are XY. What is the difference between them?
[RRB Group D, 24 August 2022 (Shift 2)]
(a) There is no difference in size.
(b) Y has more genes than X.
(c) X is larger, whereas Y is smaller.
(d) Y is larger, whereas X is smaller.
Answer: (c) X is larger, whereas Y is smaller.
Explanation: The human X chromosome is considerably larger and contains many more genes than the Y chromosome. Although both participate in sex-chromosome inheritance, the Y carries genes important for male sexual development, including the SRY region, and is much smaller.
15. DNA barcoding can be a tool to:
(1) assess the age of a plant or animal,
(2) distinguish species that look alike,
(3) identify undesirable animal or plant material in processed foods.
Which statements are correct?
[UPSC Civil Services Preliminary Examination, 5 June 2022]
(a) 1 only
(b) 3 only
(c) 1 and 2 only
(d) 2 and 3 only
Answer: (d) 2 and 3 only
Explanation: DNA barcoding compares selected DNA sequences against reference libraries to identify organisms. It can distinguish similar-looking species and reveal biological ingredients in processed products. This technique does not directly measure an individual organism’s chronological age, making statements two and three correct.
14. Har Gobind Khorana won a Nobel Prize for his contribution in __________.
[RRB NTPC CBT-I, 19 March 2021 (Shift 2)]
(a) Mathematics
(b) Physics
(c) Chemistry
(d) Genetics
Answer: (d) Genetics
Explanation: Har Gobind Khorana shared the 1968 Nobel Prize in Physiology or Medicine for research deciphering the genetic code and its role in protein synthesis. His work clarified how nucleotide sequences specify amino acids, establishing a critical link between genes and proteins.
13. Change in the base sequence within a gene is called:
[UPPSC Preliminary Examination, 2020]
(a) Mutation
(b) Cloning
(c) Fission
(d) Breeding
Answer: (a) Mutation
Explanation: A mutation is a change in nucleotide sequence within DNA. A gene-level mutation can alter a protein, change regulation or have no noticeable effect. Such changes may arise during replication or following exposure to mutagens and can be inherited when present in germ cells.
12. Which characteristics of the genetic code are true?
(i) It is generally universal;
(ii) it uses triplets of nucleotide bases;
(iii) it is non-overlapping, unambiguous and comma-less;
(iv) it includes start and stop codons.
[MPPSC State Service Preliminary Examination, 2020]
(a) i, ii and iv only
(b) i, iii and iv only
(c) i, ii and iii only
(d) All of the above
Answer: (d) All of the above
Explanation: Codons are nucleotide triplets, read sequentially during translation. Most organisms use a nearly universal code with defined initiation and termination signals. Each codon specifies one amino acid or a stop signal, though multiple codons can specify the same amino acid.
11. Consider:
(1) Genetic changes can be introduced in the cells that produce eggs or sperm.
(2) A person’s genome can be edited before birth at an early embryonic stage.
(3) Human induced pluripotent stem cells can be injected into a pig embryo.
Which are correct?
[UPSC Civil Services Preliminary Examination, 2020]
(a) 1 only
(b) 2 and 3 only
(c) 2 only
(d) 1, 2 and 3
Answer: (d) 1, 2 and 3
Explanation: Modern biotechnology can alter germline precursor cells, edit early embryos and introduce human pluripotent cells into animal embryos for experimental research. These possibilities raise major ethical and regulatory questions. All three describe techniques or approaches considered biologically feasible.
