Class 12 Biology Notes Chapter 18 (Answers to Descriptive Ques) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 18 of your NCERT Exemplar for Class 12 Biology. Now, remember, this chapter itself doesn't introduce new biological concepts. Instead, it provides the model answers to the descriptive questions asked in the previous chapters (Chapters 1 to 16).

Studying these answers is crucial, especially for competitive government exams, because they show you:

1. How to structure your answers: The expected level of detail, use of keywords, and logical flow.
2. Key concepts emphasized: Which topics are considered important enough to warrant descriptive questions.
3. Application of knowledge: How to apply theoretical concepts to specific scenarios or problems.
4. Diagrammatic representation: Where diagrams are necessary and how they should be drawn and labelled.

Think of this chapter as a guide to mastering the art of writing effective, high-scoring answers based on your NCERT knowledge. Let's break down the key takeaways by consolidating the types of concepts tested across the syllabus, as reflected in the answers provided in Chapter 18.

Consolidated Notes based on Concepts Tested in Exemplar Descriptive Questions (Reflected in Ch 18 Answers):

Unit VI: Reproduction

• Key Areas: Gametogenesis (Spermatogenesis, Oogenesis), Menstrual Cycle (Hormonal regulation, phases), Fertilization, Implantation, Embryonic Development (Key stages, formation of germ layers), Parturition, Lactation, Reproductive Health (STIs, Birth control methods, ARTs like IVF, ZIFT, GIFT).
• Types of Questions & Answer Insights (from Ch 18):
  • Process Explanation: Expect detailed step-by-step explanations (e.g., spermatogenesis, oogenesis, fertilization). Answers emphasize specific cell types, ploidy changes, and hormonal roles.
  • Differentiation: Questions often ask to differentiate between related terms (e.g., spermatogenesis vs oogenesis, ZIFT vs GIFT). Answers highlight key distinguishing features in a comparative manner.
  • Hormonal Control: Focus on the roles of hormones like GnRH, LH, FSH, Estrogen, Progesterone, hCG, hPL, Relaxin. Answers clearly link hormones to specific events.
  • Diagrams: Structure of sperm, ovum, blastocyst, Graafian follicle are important. Answers often require labelled diagrams.
  • Application: Questions on ARTs or contraception require explaining the principle and application.

Unit VII: Genetics and Evolution

• Key Areas: Mendel's Laws (Dominance, Segregation, Independent Assortment), Incomplete Dominance, Co-dominance, Multiple Allelism, Chromosomal Theory of Inheritance, Linkage & Recombination, Sex Determination, Pedigree Analysis, Genetic Disorders (Mendelian & Chromosomal), DNA structure, RNA types, Replication, Transcription, Translation, Genetic Code, Gene Regulation (Lac Operon), Human Genome Project (Goals, Methodology, Salient features), DNA Fingerprinting, Theories of Evolution (Darwinism, Lamarckism - though less emphasized), Hardy-Weinberg Principle, Evidences of Evolution (Paleontological, Embryological, Comparative Anatomy - Homologous & Analogous organs), Adaptive Radiation, Human Evolution.
• Types of Questions & Answer Insights (from Ch 18):
  • Problem Solving: Genetic crosses (monohybrid, dihybrid) require showing Punnett squares and calculating phenotypic/genotypic ratios. Answers demonstrate clear steps.
  • Conceptual Explanation: Defining terms, explaining laws, describing processes (replication, transcription, translation). Answers are precise and use correct terminology.
  • Mechanism Description: Lac operon functioning requires explaining the roles of inducer, repressor, operator, promoter, and structural genes under different conditions.
  • Significance/Application: Questions on HGP, DNA fingerprinting focus on their importance and applications. Answers highlight societal or scientific relevance.
  • Comparison: Differentiating homologous and analogous organs, or different types of genetic disorders.
  • Hardy-Weinberg: Explaining the principle and the factors affecting it (gene flow, genetic drift, mutation, recombination, natural selection).

Unit VIII: Biology and Human Welfare

• Key Areas: Human Diseases (Pathogens, transmission, symptoms, prevention/control of common diseases like Malaria, Typhoid, Pneumonia, Ascariasis, Filariasis, AIDS, Cancer), Immunity (Innate vs Acquired, Active vs Passive, Humoral vs Cell-mediated, Vaccination, Allergies, Autoimmunity), Microbes in Human Welfare (Household products, Industrial products - antibiotics, enzymes, beverages; Sewage treatment - primary/secondary treatment, BOD; Biogas production; Biocontrol agents; Biofertilizers), Strategies for Enhancement in Food Production (Plant breeding techniques, Tissue culture, Single Cell Protein - SCP, Animal Husbandry, Animal breeding techniques - inbreeding, outbreeding, cross-breeding, MOET).
• Types of Questions & Answer Insights (from Ch 18):
  • Disease Cycle/Causation: Explaining the life cycle of pathogens (e.g., Plasmodium) or the mechanism of diseases like AIDS or Cancer. Answers require specific details about the pathogen/process.
  • Microbial Roles: Describing the role of specific microbes in various processes (e.g., LAB in curd, Yeast in fermentation, Methanogens in biogas, specific bacteria/fungi as biocontrol/biofertilizers). Answers name the microbe and explain its function.
  • Process Description: Explaining sewage treatment stages or biogas production steps. Answers focus on the sequence and biological agents involved.
  • Techniques: Describing plant/animal breeding techniques or tissue culture. Answers explain the steps and objectives.
  • Immunity Concepts: Differentiating types of immunity or explaining antibody structure/function.

Unit IX: Biotechnology and Its Applications

• Key Areas: Principles (Genetic engineering tools - restriction enzymes, cloning vectors, ligase; Process of rDNA technology), PCR, Gel Electrophoresis, Bioreactors, Downstream Processing. Applications in Agriculture (Bt crops, pest resistance, herbicide tolerance), Medicine (Genetically engineered insulin, Gene therapy, Molecular diagnostics - ELISA, PCR), Transgenic Animals, Ethical Issues (Bioethics, Biopiracy).
• Types of Questions & Answer Insights (from Ch 18):
  • Tool Function: Explaining the role of restriction enzymes, vectors, ligase, PCR. Answers focus on the specific action and purpose of each tool/technique.
  • Process Outline: Describing the steps involved in creating a recombinant DNA molecule or performing gene therapy. Answers follow a logical sequence.
  • Application Examples: Explaining how biotechnology is used to create Bt cotton, produce insulin, or diagnose diseases. Answers link the technology to a specific outcome.
  • Ethical Considerations: Discussing the ethical dilemmas associated with GMOs or gene manipulation. Answers require a balanced perspective.

Unit X: Ecology and Environment

• Key Areas: Organism & Environment (Adaptations to abiotic factors), Population Attributes (Density, birth/death rate, age pyramids, sex ratio), Population Interactions (Mutualism, Competition, Predation, Parasitism, Commensalism, Amensalism), Ecosystem Structure & Function (Components, Productivity - GPP/NPP, Decomposition, Energy flow - food chains/webs, Ecological pyramids), Nutrient Cycling (Carbon, Phosphorus), Biodiversity (Levels, Patterns, Importance, Loss - causes like Habitat loss, Over-exploitation, Alien species invasion, Co-extinctions), Biodiversity Conservation (In-situ & Ex-situ strategies), Environmental Issues (Air pollution & control - ESP, Scrubber; Water pollution & control, Eutrophication, Biomagnification; Solid waste management; Agrochemicals; Ozone depletion; Deforestation; Greenhouse effect & Global warming).
• Types of Questions & Answer Insights (from Ch 18):
  • Interaction Examples: Defining and giving examples of different population interactions. Answers need clear definitions and appropriate examples.
  • Ecosystem Processes: Explaining decomposition steps, energy flow limitations (10% law), or differences between GPP and NPP. Answers focus on mechanisms and quantitative aspects where relevant.
  • Ecological Pyramids: Describing different types and explaining why some can be inverted.
  • Biodiversity Concepts: Explaining causes of biodiversity loss or differentiating between in-situ and ex-situ conservation. Answers require specific causes and examples of strategies (National Parks, Seed banks etc.).
  • Environmental Issues: Explaining phenomena like biomagnification, eutrophication, greenhouse effect, or the working of pollution control devices. Answers focus on causes, effects, and sometimes, mitigation.

Key takeaway for Exam Preparation: Don't just read the answers in Chapter 18. Understand the underlying concepts from the respective chapters (1-16). Use these model answers to practice writing your own responses, focusing on clarity, precision, use of technical terms, diagrams where needed, and addressing all parts of the question.

Multiple Choice Questions (MCQs) based on Concepts Highlighted:

1. Which hormone is primarily responsible for maintaining the corpus luteum during the initial stages of pregnancy?
   (a) LH (Luteinizing Hormone)
   (b) FSH (Follicle Stimulating Hormone)
   (c) hCG (Human Chorionic Gonadotropin)
   (d) Prolactin

2. In the Lac operon model, the repressor protein binds to which region in the absence of the inducer?
   (a) Promoter region
   (b) Operator region
   (c) Structural genes
   (d) Regulator gene

3. The technique used to amplify a specific DNA segment manifold is:
   (a) Gel Electrophoresis
   (b) ELISA
   (c) Polymerase Chain Reaction (PCR)
   (d) DNA Fingerprinting

4. Which of the following represents an example of Commensalism?
   (a) Lichens (Algae and Fungi)
   (b) Ticks on a dog
   (c) Orchid growing on a mango tree branch
   (d) Cuscuta growing on a hedge plant

5. Biochemical Oxygen Demand (BOD) is a measure of:
   (a) Industrial pollution in water bodies
   (b) Amount of oxygen needed by aerobic bacteria to decompose organic matter
   (c) Amount of dissolved oxygen present in a water body
   (d) Oxygen released by aquatic plants during photosynthesis

6. Down's Syndrome is a genetic disorder caused by:
   (a) Trisomy of chromosome 21
   (b) Monosomy of X chromosome
   (c) Presence of an extra Y chromosome
   (d) Trisomy of chromosome 18

7. Which of the following is an example of an ex-situ conservation method?
   (a) National Park
   (b) Wildlife Sanctuary
   (c) Sacred Grove
   (d) Cryopreservation in a gene bank

8. Golden Rice, a transgenic crop, is enriched with:
   (a) Vitamin C
   (b) Vitamin A (Beta-carotene precursor)
   (c) Iron
   (d) Essential amino acids

9. The process of conversion of nitrogen gas (N2) into ammonia (NH3) by microorganisms is called:
   (a) Nitrification
   (b) Denitrification
   (c) Nitrogen Fixation
   (d) Ammonification

10. Which type of immunity is provided by the transfer of antibodies from mother to fetus through the placenta?
    (a) Active Natural Immunity
    (b) Passive Natural Immunity
    (c) Active Artificial Immunity
    (d) Passive Artificial Immunity

Answer Key for MCQs:

1. (c) hCG
2. (b) Operator region
3. (c) Polymerase Chain Reaction (PCR)
4. (c) Orchid growing on a mango tree branch
5. (b) Amount of oxygen needed by aerobic bacteria to decompose organic matter
6. (a) Trisomy of chromosome 21
7. (d) Cryopreservation in a gene bank
8. (b) Vitamin A (Beta-carotene precursor)
9. (c) Nitrogen Fixation
10. (b) Passive Natural Immunity

Remember to correlate these concepts back to the detailed descriptive answers provided in Chapter 18 of your Exemplar. Good luck with your preparation!