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Class 12 · Tamil Nadu State Board

Samacheer Kalvi 12th Biology Chapter 9 Exercise Solutions

Tamil Nadu Class 12 Biology Chapter 9 Plant Breeding and Crop Improvement exercise solutions — hybridisation, mutation breeding, green revolution, biofortification, and exam tips.

Updated 25 May 202615 min read

Chapter 9 — Plant Breeding and Crop Improvement Exercise Solutions

This chapter connects genetics concepts to practical agriculture. Know specific variety names and the scientists associated with them.

How to Use This Page

1. Start with the Key Concepts table to quickly review definitions of every major term in the chapter. 2. Work through the MCQs to test recall of facts and variety names — these mirror 1-mark board questions. 3. Use Fill in the Blanks for precise terminology practice — every blank tests a word examiners look for. 4. True or False helps you spot common misconceptions; read the corrections carefully. 5. Match the Following cements paired associations (scientist–variety, method–example). 6. Answer Briefly sections are your 2-mark answer templates — keep answers within 3–5 sentences. 7. Answer in Detail sections are structured 5-mark answer guides — use the sub-point format in exams. 8. Common Mistakes to Avoid highlights exactly where students lose marks — review before the exam. 9. Exam Checklist — tick off each item once you can answer it from memory.

Key Concepts (Quick Recap)

TermDefinition
Plant breedingScience of improving crop plants through controlled manipulation of genetic characters
GermplasmTotal genetic material (genes) available in a species; collected from wild and cultivated varieties
HybridisationCrossing two genetically different plants to combine desirable traits in the offspring
Heterosis (Hybrid vigour)Superiority of F1 hybrid over both parents in growth, yield, and vigour
Inbreeding depressionReduction in vigour and fertility due to repeated self-pollination over generations
Mutation breedingUse of mutagens (X-rays, gamma rays, chemicals) to induce mutations and create new traits
PolyploidyCondition of having more than two sets of chromosomes (e.g. 3n, 4n)
ColchicineChemical that inhibits spindle fibre formation, inducing polyploidy during cell division
BiofortificationBreeding crops to increase nutritional value (vitamins, minerals, protein)
FortificationAdding nutrients to food during processing (different from biofortification)
Green RevolutionPeriod of dramatic increase in food production using HYV seeds, irrigation, fertilisers, pesticides
HYVHigh-Yielding Variety — crop varieties that produce significantly more grain per hectare
SonalikaHigh-yielding, rust-resistant wheat variety developed during India's Green Revolution
IR-8"Miracle rice" — high-yielding semi-dwarf rice variety developed at IRRI, Philippines
Somaclonal variationGenetic variation observed in plants regenerated from tissue culture

Steps in Plant Breeding

1. Collection of variability — germplasm collection from wild and cultivated varieties. 2. Evaluation and selection — identify plants with desirable traits. 3. Cross-hybridisation — cross selected parents to combine desirable traits. 4. Selection of superior hybrids — grow F2 and beyond, select best. 5. Testing, release, and commercialisation — field trials, regulatory approval.

Breeding Methods

MethodDescriptionUse
SelectionNatural or artificial; pure line selectionSelecting best from existing varieties
HybridisationCrossing two genetically different plantsCombining traits
Mutation breedingInducing mutations with mutagens (X-rays, colchicine)Creating new traits
Polyploid breedingInducing polyploidy with colchicineLarger fruits, seedless varieties
Tissue culture breedingSomaclonal variation, haploid productionUniform plants, homozygous lines

Important Varieties Developed

CropVarietyImprovement
WheatSonalika, Kalyan SonaHigh yield, rust-resistant (Green Revolution India)
RiceIR-8 (miracle rice), JayaHigh yield, semi-dwarf
SugarcaneCO-1148High sugar content, disease-resistant
MaizeHM-4High lysine content

Disease Resistant Varieties

  • Blight resistant wheat: Atlas 66
  • Virus resistant tobacco: Tobacco Mosaic Virus resistant varieties using RNAi
  • Bacterial blight resistant rice: Through crosses with wild rice species

Outbreeding and Hybrid Vigour

  • Hybrid vigour (heterosis): F1 hybrids show superior growth, yield, and vigour compared to parents.
  • Inbreeding depression: Repeated self-pollination reduces vigour and fertility.
  • Hybrid seeds must be purchased every year (F2 loses vigour).

I. Choose the Best Answer (MCQs)

1. The science of improving crop plants through controlled genetic manipulation is called: (a) Horticulture   (b) Plant breeding   (c) Agronomy   (d) Silviculture

Answer(b) Plant breeding

2. Which chemical is used to induce polyploidy in plants? (a) Ethylene   (b) Colchicine   (c) Gibberellin   (d) Cytokinin

Answer(b) Colchicine

3. Who is known as the father of the Green Revolution in India? (a) Norman Borlaug   (b) M.S. Swaminathan   (c) H.K. Jain   (d) William Gaud

Answer(b) M.S. Swaminathan

4. IR-8 is a high-yielding variety of: (a) Wheat   (b) Rice   (c) Maize   (d) Sugarcane

Answer(b) Rice

5. The first step in plant breeding is: (a) Cross-hybridisation   (b) Collection of variability   (c) Selection of hybrids   (d) Field testing

Answer(b) Collection of variability

6. Golden Rice is an example of: (a) Mutation breeding   (b) Biofortification   (c) Polyploid breeding   (d) Tissue culture

Answer(b) Biofortification

7. Heterosis refers to: (a) Reduction in vigour due to selfing   (b) Superiority of F1 hybrid over parents   (c) Induction of mutations   (d) Doubling of chromosomes

Answer(b) Superiority of F1 hybrid over parents

8. Which wheat variety was developed during India's Green Revolution for rust resistance? (a) IR-8   (b) Sonalika   (c) CO-1148   (d) HM-4

Answer(b) Sonalika

9. Colchicine acts by: (a) Inducing gene mutations   (b) Inhibiting spindle fibre formation   (c) Promoting cell elongation   (d) Increasing protein content

Answer(b) Inhibiting spindle fibre formation

10. Repeated self-pollination leading to reduced vigour is called: (a) Heterosis   (b) Inbreeding depression   (c) Polyploidy   (d) Somaclonal variation

Answer(b) Inbreeding depression

II. Fill in the Blanks

1. The total genetic material available in a species is called germplasm. 2. The high-yielding semi-dwarf rice variety developed at IRRI, Philippines is IR-8. 3. Norman Borlaug received the Nobel Peace Prize in 1970 for developing high-yielding wheat varieties. 4. The chemical colchicine is used to induce polyploidy by inhibiting spindle fibre formation. 5. Biofortification is the process of breeding crops to increase their nutritional value. 6. The superiority of F1 hybrids over both parents is called heterosis / hybrid vigour. 7. Sonalika is a high-yielding, rust-resistant wheat variety developed in India. 8. Golden Rice was developed to produce beta-carotene (pro-Vitamin A) to prevent Vitamin A deficiency. 9. Somaclonal variation is the genetic variation observed in plants regenerated from tissue culture. 10. The process of adding nutrients to food during processing is called fortification, not biofortification.

III. True or False (with corrections)

1.
AnswerFalse — Colchicine causes mutations in DNA. → Correction: Colchicine does not cause mutations; it inhibits spindle fibre formation, leading to polyploidy (doubling of chromosome sets).
2.
AnswerTrue — Heterosis is the superiority of F1 hybrids over both parents in growth and yield.
3.
AnswerFalse — Biofortification and fortification are the same process. → Correction: Biofortification increases nutrient content through breeding; fortification adds nutrients during food processing. They are different.
4.
AnswerTrue — Norman Borlaug is known as the father of the Green Revolution worldwide.
5.
AnswerFalse — Hybrid seeds can be saved and replanted each year with the same yield. → Correction: Hybrid seeds lose vigour in F2 generation (inbreeding depression). Farmers must purchase fresh hybrid seeds every year.
6.
AnswerTrue — M.S. Swaminathan is regarded as the father of the Green Revolution in India.
7.
AnswerFalse — IR-8 is a high-yielding wheat variety. → Correction: IR-8 is a high-yielding rice variety developed at the International Rice Research Institute (IRRI).
8.
AnswerTrue — Germplasm collection is the first step in any plant breeding programme.

IV. Match the Following

Column AColumn B
1. Norman Borlaug(c) High-yielding wheat / Green Revolution (world)
2. M.S. Swaminathan(d) Green Revolution in India
3. IR-8(f) Miracle rice / high-yielding rice variety
4. Sonalika(a) Rust-resistant wheat variety
5. Colchicine(g) Inhibits spindle fibres → induces polyploidy
6. Golden Rice(b) Biofortified rice with beta-carotene
7. Somaclonal variation(e) Genetic variation from tissue culture

V. Answer Briefly (2-mark)

1. What is germplasm? Why is its collection important in plant breeding?
Answer

Germplasm is the total genetic material (genes) available in a species, collected from wild relatives, cultivated varieties, and landraces. Its collection is important because it provides the raw material (genetic variability) from which desirable traits can be selected. Without sufficient germplasm, breeders cannot find genes for disease resistance, stress tolerance, or yield improvement.

2. Differentiate between biofortification and fortification.
Answer

Biofortification increases the nutrient content of crops through breeding or genetic modification (e.g., Golden Rice with beta-carotene). Fortification adds nutrients to processed food during manufacturing (e.g., iodised salt, fortified wheat flour). The key difference is that biofortification works at the crop level while fortification works at the processing level.

3. What is inbreeding depression? Give an example.
Answer

Inbreeding depression is the reduction in vigour, fertility, and yield that occurs when plants are repeatedly self-pollinated over several generations. This happens because harmful recessive alleles become homozygous. For example, if farmers save and replant hybrid seeds, the F2 generation shows significantly lower yield and weaker growth than the F1 hybrid.

4. List any two objectives of plant breeding.
Answer

Two objectives of plant breeding are: (1) Increase crop yield to meet the food demands of a growing population, and (2) Develop disease- and pest-resistant varieties to reduce dependence on chemical pesticides and ensure stable production.

5. What is mutation breeding? Name one mutagen used.
Answer

Mutation breeding is the process of inducing mutations in plants using physical or chemical mutagens to create new genetic variants with desirable traits. Gamma rays are a commonly used physical mutagen. Chemical mutagens include ethyl methane sulphonate (EMS) and nitrous acid. The induced mutations are random, so large populations must be screened to find useful variants.

VI. Answer in Detail (5-mark)

1. Describe the steps involved in plant breeding.
Answer

Plant breeding involves the following five steps:

  • Collection of variability: Germplasm is collected from wild relatives, landraces, and cultivated varieties. This provides the genetic base for selecting desirable traits. The entire collection is preserved in gene banks.
  • Evaluation and selection: The germplasm is evaluated for traits such as yield, disease resistance, and stress tolerance. Plants showing desirable characters are selected as parents for crossing.
  • Cross-hybridisation: Selected parent plants are crossed to combine desirable traits from both. For example, a high-yielding but disease-susceptible variety may be crossed with a low-yielding but disease-resistant variety. This step requires manual emasculation and pollination.
  • Selection of superior hybrids: From the F2 and subsequent generations, plants that combine the desired traits are identified and selected through repeated selfing and evaluation over several generations until homozygosity is achieved.
  • Testing, release, and commercialisation: The selected lines are tested in field trials across different locations and seasons (multi-location trials) for 2–3 years. After evaluating yield, quality, and adaptability, the best variety is released for commercial cultivation.
2. Explain hybridisation and heterosis with examples.
Answer

Hybridisation is the crossing of two genetically different plants to produce offspring (hybrids) that combine desirable traits from both parents.

  • Types: Intervarietal (cross between varieties of the same species), interspecific (cross between different species of the same genus), and intergeneric (cross between different genera).
  • Process: The female parent's anthers are removed (emasculation) before they dehisce, and pollen from the male parent is dusted onto the stigma. The resulting seeds produce F1 hybrids.
  • Heterosis (Hybrid vigour): F1 hybrids often show superiority over both parents in growth, yield, size, and resistance. This phenomenon is called heterosis.
  • Example: The rice hybrid IR-8 produced yields double that of traditional varieties.
  • Example: Hybrid maize shows 15–25% higher yield than inbred parents.
  • Limitation: Heterosis is maximum in the F1 generation. In F2, segregation and recombination reduce vigour, so farmers must buy fresh hybrid seeds each season.
  • Inbreeding depression: Repeated self-pollination of hybrids leads to reduced vigour and fertility, the opposite of heterosis.
3. Write about the Green Revolution and its impact on Indian agriculture.
Answer

The Green Revolution refers to the dramatic increase in food grain production during the 1960s–70s through the introduction of high-yielding variety (HYV) seeds, expanded irrigation, chemical fertilisers, and pesticides.

  • Global context: Norman Borlaug developed semi-dwarf, high-yielding, disease-resistant wheat varieties in Mexico. He received the Nobel Peace Prize in 1970 for this contribution.
  • Indian context: M.S. Swaminathan introduced Borlaug's wheat varieties to India. Indian scientists crossed these with local varieties to develop Sonalika and Kalyan Sona — rust-resistant, high-yielding wheat adapted to Indian conditions.
  • Rice revolution: IR-8, developed at IRRI (Philippines), was introduced in India and became known as "miracle rice" for its exceptional yields.
  • Positive impacts: India moved from food deficiency to food surplus; wheat production increased from 12 million tonnes (1965) to over 26 million tonnes (1975); farmer incomes rose; food grain availability improved.
  • Concerns: Heavy use of chemical fertilisers and pesticides caused soil degradation and water pollution; HYV crops required more water and inputs; traditional crop diversity declined; small farmers sometimes could not afford the inputs.
4. What is biofortification? Explain with examples.
Answer

Biofortification is the process of breeding crops to increase their nutritional value — specifically to raise the content of protein, vitamins, minerals, or beneficial fatty acids in the edible parts.

  • Objective: To address hidden hunger (micronutrient deficiencies) in populations that rely on staple crops for most of their calories, without requiring dietary changes.
  • Golden Rice: Rice variety engineered to produce beta-carotene (pro-Vitamin A) in the endosperm. Consumed in regions where Vitamin A deficiency causes childhood blindness and immune weakness.
  • Iron-rich bajra: Pearl millet varieties biofortified with higher iron content to combat anaemia.
  • Lysine-rich maize (Quality Protein Maize): Maize varieties bred to have higher lysine and tryptophan content, improving protein quality. Example: HM-4 variety.
  • Distinction from fortification: Biofortification works at the crop level through breeding or genetic modification. Fortification adds nutrients during food processing (e.g., iodised salt, fortified wheat flour). Biofortification is sustainable as the nutrient content is passed to the next generation of seeds.

Worked Examples

Example 1 — Steps of plant breeding (model answer)

1. Collect variability (germplasm). 2. Evaluate and select parents. 3. Cross-hybridise selected parents. 4. Select superior recombinants. 5. Test, release and commercialise the new variety.

Example 2 — Biofortification (model answer)

Biofortification is breeding crops to raise their nutrient (vitamin/mineral/ protein) content — e.g. Golden Rice (vitamin A), iron-rich bajra. It differs from fortification, where nutrients are added during processing.

Example 3 — Key contributions

Wheat Green Revolution (world) : Norman Borlaug Green Revolution (India) : M.S. Swaminathan Disease-resistant varieties via breeding reduce dependence on pesticides.

Common Mistakes to Avoid

  • Forgetting Norman Borlaug for wheat Green Revolution and M.S. Swaminathan for India.
  • Confusing biofortification with fortification — biofortification is through breeding, not adding nutrients during processing.
  • Not giving specific examples of variety names when asked.
  • Saying colchicine causes mutation — it inhibits spindle fibres to cause polyploidy, not mutation.
  • ❌ "Colchicine causes gene mutations" → ✅ "Colchicine inhibits spindle fibre formation, causing polyploidy (doubling of chromosome sets)"
  • ❌ "Biofortification means adding vitamins to food in factories" → ✅ "Biofortification means breeding crops to increase their nutrient content; adding nutrients during processing is fortification"
  • ❌ "Hybrid seeds can be reused every year with the same yield" → ✅ "F1 hybrid seeds lose vigour in F2 (inbreeding depression); fresh seeds must be purchased each season"
  • ❌ "IR-8 is a wheat variety" → ✅ "IR-8 is a high-yielding rice variety developed at IRRI, Philippines"
  • ❌ "Heterosis increases with each generation of selfing" → ✅ "Heterosis is maximum in F1; selfing leads to inbreeding depression, not increased vigour"

Exam Checklist

  • [ ] Can you list all five steps of plant breeding in order?
  • [ ] Can you explain the difference between biofortification and fortification with examples?
  • [ ] Do you know the contributions of Norman Borlaug and M.S. Swaminathan?
  • [ ] Can you name at least two HYV varieties (Sonalika, IR-8) and state which crop they belong to?
  • [ ] Can you explain colchicine's mechanism of action (spindle fibre inhibition → polyploidy)?
  • [ ] Can you define heterosis and inbreeding depression and explain why hybrid seeds must be bought each year?
  • [ ] Can you describe at least three breeding methods (selection, hybridisation, mutation breeding) with examples?
  • [ ] Can you explain mutation breeding and name a mutagen?
  • [ ] Do you know what germplasm is and why it is collected?
  • [ ] Can you discuss the positive and negative impacts of the Green Revolution in India?
  • [ ] Can you give an example of somaclonal variation from tissue culture?
  • [ ] Can you distinguish between polyploid breeding and mutation breeding?

Frequently Asked Questions

Plant breeding is the science of improving crop plants through controlled manipulation of genetic characters. Objectives: increase yield, improve quality (protein, vitamins), develop disease/pest resistance, stress tolerance (drought, salinity), widen adaptability, and develop early-maturing varieties.
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