Zoology 2023 Paper II 50 marks Explain

Q4

(a) Explain the following: (10×2=20 marks) (i) Haemophilia (ii) Red-green colour blindness (b) Describe various stages of prophase of meiosis-I in an animal cell. (15 marks) (c) Explain phenomenon of natural selection taking examples of peppered moth and sickle cell anaemia. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) निम्नलिखित की व्याख्या कीजिए: (10×2=20 अंक) (i) हीमोफीलिया (ii) लाल-हरी वर्णांधता (b) एक प्राणिकोशिका में अर्धसूत्री विभाजन-I के प्रोफेज के विभिन्न चरणों का वर्णन कीजिए। (15 अंक) (c) पेपर्ड शलभ (मोथ) एवं हँसिया कोशिका अरक्तता (सिकल सेल एनीमिया) का उदाहरण लेते हुए प्राकृतिक चयन की घटना की व्याख्या कीजिए। (15 अंक)

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Directive word: Explain

This question asks you to explain. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'explain' demands clear causal reasoning and mechanistic clarity across all parts. Allocate approximately 40% of time/words to part (b) [15 marks] covering leptotene through diplotene with diagrams; 30% to part (a) [20 marks] contrasting X-linked recessive inheritance patterns; and 30% to part (c) [15 marks] linking industrial melanism and heterozygote advantage to selection mechanisms. Structure: brief introduction defining genetic variation sources → systematic treatment of each sub-part with integrated diagrams → synthesis on how molecular events scale to evolutionary outcomes.

Key points expected

  • For (a)(i): Haemophilia A (FVIII deficiency) vs B (FIX deficiency), X-linked recessive inheritance, Royal pedigree (Queen Victoria), clotting cascade disruption
  • For (a)(ii): Red-green opsin genes on Xq28, protanopia/deuteranopia vs protanomaly/deuteranomaly, Ishihara test, gene duplication origin of trichromacy
  • For (b): Sequential stages—leptotene (chromosome condensation, bouquet formation), zygotene (synapsis, SC assembly), pachytene (crossing over, recombination nodules), diplotene (chiasmata visible, SC dissolution), diakinesis (terminalization, nuclear envelope breakdown)
  • For (c): Peppered moth (Biston betularia)—Kettlewell's predation experiments, carbonaria morph frequency shift, lichen cover correlation; Sickle cell—HbS point mutation, malaria-endemic regions (Africa, India: tribal populations), heterozygote advantage (AS phenotype)
  • Synthesis: Connection between meiotic recombination (part b) generating variation upon which selection (part c) acts, and how X-linked disorders (part a) persist due to male hemizygosity and selection dynamics

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11Accurately distinguishes haemophilia A from B with specific clotting factors; correctly identifies Xq28 locus for colour blindness; precisely orders meiotic substages with molecular markers (SYCP3, MLH1); correctly defines heterozygote advantage vs directional selectionMixes up haemophilia A/B or omits specific factors; conflates colour blindness types; stages listed but out of sequence or missing key events (e.g., omits chiasmata); conflates balancing selection with other modesFundamental errors (e.g., calls haemophilia autosomal, misidentifies meiosis I as mitosis, describes Lamarckian mechanism for moth color change)
Diagram / labelling18%9Clear labelled diagrams for: (a) pedigree showing X-linked inheritance with generations; (b) synaptonemal complex cross-section and chromosome configurations at each prophase-I stage; (c) peppered moth camouflage contrast and sickle cell RBC morphology; all with accurate annotationsDiagrams present but incomplete labelling, missing scale/context, or conflating stages; pedigree lacks generation numbers; moth/sickle cell descriptions text-onlyNo diagrams where essential (meiosis stages), or seriously misleading representations (e.g., showing mitosis instead); illegible or unlabelled sketches
Examples & nomenclature20%10Cites: Royal family pedigree for haemophilia; Ishihara test for colour blindness; Biston betularia and Kettlewell 1950s experiments; HbS/HbA alleles, malaria-endemic Indian tribes (e.g., Gond, Bhil); uses standard genetic symbols (X^h, X^c), SC componentsGeneric examples without specificity (e.g., 'a family' instead of Royal pedigree); omits Kettlewell or Indian context; uses informal terminologyNo named examples, invented pedigrees, or confused nomenclature (e.g., 'haemoglobin C' for sickle cell); irrelevant examples
Process explanation22%11For meiosis: explains molecular machinery (cohesins, DSB formation by Spo11, resolution); for selection: explains frequency-dependent predation and parasite-mediated selection mechanisms with quantitative reasoning; for disorders: explains why males exclusively show severe phenotypesLists events without mechanistic causation; describes 'what happens' without 'how/why'; superficial treatment of selection pressuresDescriptive only with no process logic; confuses cause and effect; incoherent sequence of events
Evolutionary / applied context18%9Synthesizes: how meiotic recombination generates variation for selection; why X-linkage affects evolutionary dynamics (faster fixation of recessive beneficial alleles in males); applied relevance—gene therapy (haemophilia), CRISPR prospects, conservation genetics of Indian moth populations, public health implications for malaria controlBrief mention of evolution without integration; lists applications without connecting to mechanisms; no Indian contextNo evolutionary framework; purely descriptive with no applied or synthetic thinking; ignores synthesis between parts

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