Zoology 2021 Paper II 50 marks Describe

Q3

(a) What is chromosome mutation ? Describe various types of polyploidy with suitable examples. Add a note on phenotypic effects of polyploidy. 20 (b) What is chromosome theory of linkage ? Describe the methods for determination of linkage using suitable examples. 15 (c) What is Mendel's dihybrid cross ? Discuss the mechanism of independent assortment using suitable example. 15

हिंदी में प्रश्न पढ़ें

(a) गुणसूत्र उत्परिवर्तन क्या है ? विभिन्न प्रकार की बहुगुणिता (पॉलीप्लॉइडी) का वर्णन उपयुक्त उदाहरणों के साथ कीजिए । बहुगुणिता के लक्षण प्रकृति प्रभावों पर एक टिप्पणी भी लिखिए । 20 (b) सहलग्नता (लिंकेज) का गुणसूत्र सिद्धांत क्या है ? उपयुक्त उदाहरणों का प्रयोग करते हुए सहलग्नता के निर्धारण की विधियों का वर्णन कीजिए । 15 (c) मैंडल का द्विसंकर (डाइहाइब्रिड) क्रॉस क्या है ? स्वतंत्र अपव्यूहन (इंडिपेंडेंट असॉर्टमेंट) की क्रियाविधि की विवेचना उपयुक्त उदाहरण के साथ कीजिए । 15

Evaluate your answer to this question → See all 2021 Zoology questions

Directive word: Describe

This question asks you to describe. 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 'describe' demands detailed, systematic exposition of chromosome mutations, polyploidy types, linkage theory, and dihybrid mechanisms. Allocate approximately 40% of effort to part (a) given its 20 marks, with 30% each to parts (b) and (c). Structure with brief definitions, followed by detailed descriptions with diagrams, and conclude with applied significance for each sub-part.

Key points expected

  • Clear definition of chromosome mutation distinguishing it from gene mutation; classification of polyploidy into autopolyploidy (autotriploid, autotetraploid) and allopolyploidy with specific examples like Triticum aestivum, Saccharum officinarum, and Raphanobrassica
  • Phenotypic effects of polyploidy: gigantism, increased cell size, delayed flowering, and evolutionary significance in plant speciation
  • Chromosome theory of linkage: Sutton and Boveri's postulates, complete vs incomplete linkage, and relationship with chromosome structure
  • Methods for linkage determination: two-point and three-point test crosses, calculation of recombination frequency, map units, and construction of genetic maps with Drosophila or maize examples
  • Mendel's dihybrid cross: 9:3:3:1 ratio derivation, parental and recombinant phenotypes, and statistical validation
  • Mechanism of independent assortment: metaphase I orientation, random segregation of homologous pairs, and cytological basis with annotated diagram
  • Distinction between linkage and independent assortment as contrasting mechanisms of inheritance
  • Applied significance: polyploidy in crop improvement (wheat, cotton), linkage in breeding programs, and independent assortment in hybrid vigor

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11Precise definitions of chromosome mutation vs gene mutation; accurate distinction between auto- and allopolyploidy; correct statement of chromosome theory of linkage including Sutton-Boveri postulates; flawless 9:3:3:1 ratio derivation with correct gamete combinations for dihybrid crossBasic definitions present but some confusion between chromosome and gene mutation; incomplete classification of polyploidy; partial statement of linkage theory; arithmetic errors in dihybrid ratio or incorrect gamete identificationFundamental misconceptions: treats polyploidy as gene mutation, confuses linkage with crossing over, or states incorrect ratios like 3:1 for dihybrid cross; omits key theoretical components
Diagram / labelling18%9Three clear, well-labelled diagrams: (a) meiotic origin of polyploidy or chromosome pairing in autotetraploid; (b) test cross showing parental and recombinant types with recombination frequency calculation; (c) dihybrid cross Punnett square or independent assortment at metaphase I with proper chromosomal representationTwo diagrams present but with incomplete labelling or minor errors in chromosomal representation; missing one required diagram or poorly scaled Punnett squareSingle diagram or none; illegible sketches without labels; diagrams that misrepresent biological processes (e.g., showing independent assortment in linked genes)
Examples & nomenclature20%10Specific, accurate examples: for (a) Triticum aestivum (hexaploid wheat), Saccharum officinarum, Oryza sativa or Indian crop examples like Brassica juncea; for (b) Drosophila eye color/body color or maize kernel traits with gene symbols; for (c) Mendel's seed color/shape with correct parental genotypes (YYRR × yyrr)Generic examples without species names; correct organisms but wrong ploidy levels; incomplete gene symbols or missing parental genotypes in dihybrid crossNo specific examples; invented organisms; confusion of Indian examples (e.g., calling rice autotetraploid); failure to provide any gene symbols or parental genotypes
Process explanation22%11Stepwise mechanisms: unreduced gamete formation for polyploidy; detailed test cross procedure with recombinant frequency formula (RF = recombinants/total × 100); clear explanation of metaphase I orientation possibilities and probability basis of 9:3:3:1 ratio; mathematical working shownProcesses described in general terms without stepwise breakdown; correct formula stated but calculation not shown; incomplete explanation of how chromosome orientation produces ratioNo procedural explanation; confused sequence of events; incorrect formula application; failure to explain how independent assortment produces phenotypic ratios
Evolutionary / applied context18%9Integrated applied significance: polyploidy in Green Revolution wheat varieties (Norin-10, Kalyan Sona), cotton improvement; linkage in marker-assisted selection and QTL mapping; independent assortment in hybrid seed production and genetic diversity maintenance; brief note on Haldane's rule or evolutionary implicationsMention of agricultural applications without specific varieties; generic statement about breeding without linkage/independent assortment distinction; missing evolutionary perspectiveNo applied or evolutionary context; irrelevant applications; failure to connect mechanisms to practical breeding or speciation

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 2021 Paper II