40. The interaction between nonallelic genes at different loci in which one gene masks the phenotypic expression of another is called:
[SSC CHSL Tier-I, 5 July 2024 (Shift 1)]
(a) Epistasis
(b) Linkage
(c) Incomplete inheritance
(d) Complete inheritance
Answer: (a) Epistasis
Explanation: Epistasis describes an interaction in which one gene alters or masks the expression of another gene at a different locus. It can modify classic Mendelian ratios. Linkage instead concerns physical proximity of genes on the same chromosome and their tendency to be inherited together.
39. In which year did Wilhelm Johannsen coin the term “gene” for Mendelian units of heredity?
[SSC CHSL Tier-I, 4 July 2024 (Shift 2)]
(a) 1920
(b) 1910
(c) 1900
(d) 1909
Answer: (d) 1909
Explanation: Wilhelm Johannsen introduced the term gene in 1909, giving Mendel’s proposed hereditary factors a lasting name. Johannsen also distinguished genotype from phenotype. The terminology helped separate inherited constitution from observable traits and became foundational to modern genetics.
38. Mendel proposed the law of independent assortment on the basis of __________.
[SSC CHSL Tier-I, 1 July 2024 (Shift 4)]
(a) Dihybrid crosses
(b) Alleles
(c) Monohybrid crosses
(d) Genes
Answer: (a) Dihybrid crosses
Explanation: A dihybrid cross follows two different traits simultaneously. Mendel observed an approximately 9:3:3:1 phenotypic ratio among F2 offspring when the relevant alleles assort independently. The rule generally applies to unlinked genes, not necessarily genes tightly linked on one chromosome.
37. “Photo 51” refers to an image of:
[UPSC CDS General Knowledge, 21 April 2024]
(a) A crater on the Moon
(b) DNA molecules
(c) The virus responsible for COVID-19
(d) The virus responsible for the common cold
Answer: (b) DNA molecules
Explanation: Photo 51 is a famous X-ray diffraction photograph of DNA made by Rosalind Franklin and Raymond Gosling. Its characteristic pattern provided important structural evidence for the DNA double helix, the molecule that stores and transmits hereditary information.
36. In most prokaryotes, the chromosome number is:
[UPSC NDA General Ability Test, 21 April 2024]
(a) 4
(b) 3
(c) 2
(d) 1
Answer: (d) 1
Explanation: Many bacteria possess one principal circular chromosome carrying essential hereditary genes, although exceptions exist and some species have multiple replicons. Unlike eukaryotes, prokaryotes lack a membrane-bound nucleus; their chromosomal DNA generally occupies a nucleoid region.
35. Consider:
(1) DNA replication occurs when chromatin opens up;
(2) Chromatin condenses into rod-shaped chromosomes before cell division;
(3) Prokaryotes and eukaryotes divide by the same process.
Which statements are correct?
[UPSC NDA General Ability Test, 21 April 2024]
(a) 1 only
(b) 1 and 2 only
(c) 1, 2 and 3
(d) 3 only
Answer: (b) 1 and 2 only
Explanation: DNA becomes accessible for replication as chromatin reorganises, and it condenses into discrete chromosomes during eukaryotic division. Prokaryotes generally divide by binary fission, unlike mitosis and meiosis in eukaryotes. Thus, the first two statements are correct, but the third is not.
34. Among the following diseases, which one is NOT inherited?
[SSC CGL Tier-II, 26 October 2023]
(a) Down syndrome
(b) Cystic fibrosis
(c) Malaria
(d) Muscular dystrophy
Answer: (c) Malaria
Explanation: Malaria is a parasitic infection transmitted by mosquitoes and is not itself a genetic disease. Cystic fibrosis and many muscular dystrophies result from inherited mutations. Down syndrome is a chromosomal disorder, usually arising from a new nondisjunction event rather than direct parental inheritance.
33. Which process is called “reduction division” because it halves the chromosome number to form sperm and eggs?
[SSC MTS, 6 September 2023 (Shift 1)]
(a) Amitosis
(b) Meiosis
(c) Cytokinesis
(d) Karyokinesis
Answer: (b) Meiosis
Explanation: Meiosis reduces the chromosome complement from diploid to haploid when gametes form. Subsequent fertilisation reunites two haploid sets. Genetic recombination and independent assortment during meiosis also create variation, allowing sexually produced offspring to differ from their parents and siblings.
32. Chromosomes are composed of DNA and:
[UPSC CDS General Knowledge, 3 September 2023]
(a) Lipids
(b) Vitamins
(c) Proteins
(d) Carbohydrates
Answer: (c) Proteins
Explanation: Eukaryotic chromosomes consist of long DNA molecules wrapped around histone and other proteins. This packaging stabilises genetic material and helps regulate gene expression and chromosome segregation. Lipids and carbohydrates are not the principal partners forming the chromosome’s structural framework.
31. The two important features of sexual reproduction in higher organisms that create genetic diversity in offspring are:
[UPSC NDA-II General Ability, 3 September 2023]
(a) Mitosis and fertilisation
(b) Meiosis and fertilisation
(c) Mitosis and binary fission
(d) Meiosis and conjugation
Answer: (b) Meiosis and fertilisation
Explanation: Meiosis shuffles genes through crossing over and random chromosome assortment, producing genetically varied gametes. Fertilisation then combines genetic material from two parents in unpredictable pairings. Their combined effects explain much of the diversity seen among offspring produced sexually.
