30. In a sexually reproducing organism, which statement is appropriate for both parent and offspring?
[UPSC NDA-II General Ability, 3 September 2023]
(a) Chromosome number increases but DNA content remains constant.
(b) Both chromosome number and DNA content remain constant.
(c) Chromosome number decreases but DNA content remains constant.
(d) Both chromosome number and DNA content decrease.
Answer: (b) Both chromosome number and DNA content remain constant.
Explanation: Meiosis halves the chromosome number in gametes, and fertilisation restores the normal diploid complement in each new generation. For a species with stable ploidy, parents and offspring ordinarily retain the same characteristic chromosome number and broadly similar total nuclear DNA content.
29. Match: (i) Gaucher disease, (ii) Hunter syndrome, (iii) Tay–Sachs disease, (iv) phenylketonuria
with (a) phenylalanine hydroxylase, (b) hexosaminidase A, (c) iduronate-2-sulfatase, (d) glucocerebrosidase.
[SSC CGL Tier-I, 26 July 2023 (Shift 4)]
(a) i-a, ii-b, iii-c, iv-d
(b) i-b, ii-c, iii-a, iv-d
(c) i-d, ii-c, iii-b, iv-a
(d) i-b, ii-a, iii-d, iv-c
Answer: (c) i-d, ii-c, iii-b, iv-a
Explanation: Each genetic metabolic disorder follows deficiency of a characteristic enzyme: Gaucher disease involves glucocerebrosidase, Hunter syndrome iduronate-2-sulfatase, Tay–Sachs disease hexosaminidase A, and phenylketonuria phenylalanine hydroxylase. The specified order is therefore d, c, b, a.
28. In which year did Joe Hin Tjio publish the finding that 2n = 46 is the exact number of human chromosomes?
[SSC CGL Tier-I, 20 July 2023 (Shift 4)]
(a) 1950
(b) 1952
(c) 1956
(d) 1960
Answer: (c) 1956
Explanation: The correct human somatic chromosome count of 46 was reported in 1956 through work by Joe Hin Tjio and Albert Levan. It overturned the earlier assumption of 48 and became crucial for clinical cytogenetics and the recognition of chromosomal disorders.
27. The process of determining the relative positions of genes on a chromosome is called:
[BPSC Teacher Recruitment Examination (TRE-1), August 2023]
(a) Genome walking
(b) Chromosome walking
(c) Gene mapping
(d) More than one of the above
Answer: (c) Gene mapping
Explanation: Gene mapping identifies the position and relative order of genes on chromosomes. Genetic linkage, recombination frequencies and molecular markers can help locate genes. Such maps are useful in understanding inherited diseases, plant breeding and distinguishing chromosome regions associated with particular traits.
26. The human ABO blood-group system is an example of:
[MPPSC Forest Service Examination, 2023]
(a) Epistatic genes
(b) Multiple alleles
(c) Complementation
(d) Polygenic inheritance
Answer: (b) Multiple alleles
Explanation: The ABO locus has three common alleles in populations: Iá´¬, Iá´® and i. An individual inherits only two, but the existence of more than two allele forms at the same gene locus demonstrates multiple allelism. Iá´¬ and Iá´® additionally show codominance.
25. What would have happened if Mendel had NOT studied the F2 generation?
[RRB Group D, 11 October 2022 (Shift 3)]
(a) He would not have studied multiple characters.
(b) He would not have identified dominant traits.
(c) He would not have obtained hybrid plants.
(d) He would not have discovered that recessive traits are not lost.
Answer: (d) He would not have discovered that recessive traits are not lost.
Explanation: Recessive characters disappear phenotypically in a standard F1 monohybrid generation but reappear among F2 offspring after hybrid selfing. Seeing the F2 result established that hidden hereditary factors persisted rather than being destroyed or permanently blended in the first generation.
24. Identify the true statement about chromosomes carried by human sperm.
[RRB Group D, 6 October 2022 (Shift 2)]
(a) X-bearing sperm have 22 chromosomes and Y-bearing sperm have 23.
(b) X-bearing sperm have 23 chromosomes and Y-bearing sperm have 22.
(c) Both X-bearing and Y-bearing sperm have 23 chromosomes.
(d) Both X-bearing and Y-bearing sperm have 22 chromosomes.
Answer: (c) Both X-bearing and Y-bearing sperm have 23 chromosomes.
Explanation: A normal human sperm contains 22 autosomes and one sex chromosome, either X or Y, making 23 in total. The identity of the sex chromosome changes, not the total number. An egg also carries 23 chromosomes, including an X.
23. Which statement about Mendel’s pea-plant experiments is INCORRECT?
[RRB Group D, 29 September 2022 (Shift 2)]
(a) Tallness and shortness reappearing in F2 showed both traits were inherited by F1.
(b) All plants in the F2 generation obtained from F1 tall plants were tall.
(c) Mendel studied contrasting traits such as round and wrinkled seeds.
(d) Crossing pure tall and pure dwarf plants gave tall F1 offspring.
Answer: (b) All plants in the F2 generation obtained from F1 tall plants were tall.
Explanation: When heterozygous tall F1 pea plants self-pollinate, recessive dwarf offspring return in F2. The expected phenotypic ratio is approximately three tall to one dwarf, not all tall. This observation helped establish segregation of paired hereditary factors.
22. A normal human body cell has 46 chromosomes. How many chromosomes does a normal human gamete have?
[RRB Group D, 20 September 2022 (Shift 3)]
(a) 22
(b) 44
(c) 46
(d) 23
Answer: (d) 23
Explanation: Human somatic cells are generally diploid, containing 23 chromosome pairs. Meiosis generates haploid sperm or eggs with one member of each pair, totalling 23. Fertilisation then combines two haploid gametes to restore 46 chromosomes in the zygote.
21. Which of the following statements is INCORRECT?
[RRB Group D, 19 September 2022 (Shift 2)]
(a) All children inherit a Y chromosome from their mother.
(b) Human chromosomes do not all exist as identical matching pairs.
(c) In some animals, including certain snails, individuals can change sex.
(d) Sex can be determined by different mechanisms in different species.
Answer: (a) All children inherit a Y chromosome from their mother.
Explanation: Human mothers normally have XX sex chromosomes and contribute X-bearing eggs. Consequently, they do not transmit Y chromosomes to children. A Y chromosome, if present, is contributed by the father. Other species may have different sex-determining systems.
