Zoology 2025 Paper II 50 marks Explain

Q2

(a)(i) Describe composition of plasma membrane. (10 marks) (a)(ii) Differentiate between facilitated and passive diffusion across the membrane with examples. (10 marks) (b) Explain how mutations affect variations in population and natural selection. (15 marks) (c) Draw pedigrees for autosomal recessive and sex-linked inheritance using examples from human. (15 marks)

हिंदी में प्रश्न पढ़ें

(a)(i) प्रद्रव्य झिल्ली की संरचना का वर्णन कीजिए । (10 अंक) (a)(ii) प्रद्रव्य झिल्ली के आर-पार होने वाले सुकृत एवं निष्क्रिय विसरण के बीच सौदाहरण विभेदन कीजिए । (10 अंक) (b) उत्परिवर्तन, जनसंख्या एवं प्राकृतिक चयन में विविधताओं को कैसे प्रभावित करता है, इसकी व्याख्या कीजिए । (15 अंक) (c) मानव उदाहरणों का उपयोग करते हुए अप्रभावी अलिंगसूत्री एवं लिंग-सहलग्निक वंशागति के लिए वंशावलियों का रेखाचित्र बनायें । (15 अंक)

Evaluate your answer to this question → See all 2025 Zoology questions

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 elaboration of mechanisms. Structure your answer with a brief integrated introduction, then allocate approximately 20% time/words to (a)(i) on membrane composition, 20% to (a)(ii) comparing diffusion types, 30% to (b) on mutations and natural selection, and 30% to (c) with two complete pedigrees. Conclude by linking molecular mechanisms to evolutionary outcomes.

Key points expected

  • (a)(i) Fluid mosaic model: phospholipid bilayer structure, integral and peripheral proteins, cholesterol, carbohydrate chains (glycocalyx); mention Singer-Nicolson model
  • (a)(ii) Distinction between simple diffusion (passive, no protein, down gradient, e.g., O2, CO2) and facilitated diffusion (protein-mediated, passive, specific, e.g., GLUT transporters, aquaporins); include channel vs carrier proteins
  • (b) Types of mutations (gene, chromosomal, genome), sources (spontaneous, induced), role in generating genetic variation; Hardy-Weinberg equilibrium reference; natural selection acting on variation with examples (antibiotic resistance, industrial melanism in Biston betularia)
  • (c) Autosomal recessive pedigree: consanguinity, equal sex ratio, skipped generations; example: sickle cell anemia, thalassemia (Indian context) or albinism
  • (c) Sex-linked inheritance pedigree: X-linked recessive showing criss-cross inheritance, more males affected, no male-to-male transmission; example: hemophilia, color blindness, or Duchenne muscular dystrophy
  • Pedigree symbols: squares (male), circles (female), shaded (affected), half-shaded (carrier), lines showing relationships and generations numbered with Roman numerals

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Accurately describes fluid mosaic model with lipid bilayer amphipathic nature; correctly distinguishes passive vs facilitated diffusion energy requirements; explains mutation types and selection mechanisms without errors; pedigree symbols follow standard conventionsBasic membrane structure correct but misses cholesterol or glycocalyx; diffusion distinction present but confused on energy/gradient aspects; mutations described superficially; pedigrees mostly correct with minor symbol errorsIncorrect membrane model (outdated); confuses active and passive transport; fundamental errors in mutation-selection relationship; pedigrees incorrectly drawn or symbols misused
Diagram / labelling20%10Clear membrane cross-section diagram with labelled phospholipid heads/tails, protein types, cholesterol; TWO complete pedigrees (autosomal recessive and X-linked) with 3+ generations, proper symbols, shading, and generation numbersMembrane diagram present but incomplete labelling; pedigrees drawn but missing one type or lacking generational depth, minor labelling issuesNo diagram for membrane; pedigrees absent, incomplete, or fundamentally wrong with incorrect inheritance pattern shown
Examples & nomenclature20%10Specific examples: aquaporins/GLUT for facilitated diffusion; Indian examples like thalassemia, sickle cell for autosomal recessive; hemophilia/color blindness for X-linked; uses correct genetic terms (carrier, proband, consanguinity, allele frequency)Generic examples given; some Indian context missing; terminology mostly correct but inconsistent use of genetic termsNo specific examples or incorrect examples (e.g., calling Huntington's recessive); poor or absent genetic terminology
Process explanation20%10Clear stepwise explanation of how mutations create variation (mutation → allele frequency change → selection); explains membrane protein conformational changes in facilitated diffusion; logical pedigree interpretation showing probability calculationsProcesses described but mechanistic details lacking; selection explanation generic; pedigree description narrative without probabilityNo process explanation; jumps between topics without causal links; cannot explain how pedigree pattern indicates inheritance type
Evolutionary / applied context20%10Links mutation-variation-selection to speciation; discusses heterozygote advantage (sickle cell/malaria in Indian tribal populations); mentions genetic counseling applications from pedigree analysis; connects membrane transport to medical relevance (cystic fibrosis, diabetes)Some evolutionary context present but superficial; applied aspects mentioned without development; missing heterozygote advantage explanationNo evolutionary or applied context; answer ends with description without synthesis; misses medical/social relevance entirely

Practice this exact question

Write your answer, then get a detailed evaluation from our AI trained on UPSC's answer-writing standards. Free first evaluation — no signup needed to start.

Evaluate my answer →

More from Zoology 2025 Paper II