Class 9 Science Chapter 6: Reproduction in Animals – Important Questions with Complete Solutions

Class 9 Science Chapter 6 consistently appears in board exams with 8–12 marks dedicated to reproduction in animals. The CBSE pattern tests three key competencies: (1) Knowledge of reproductive structures and processes in mammals and insects; (2) Conceptual understanding of sexual vs asexual reproduction, and fertilisation stages; (3) Application of metamorphosis in real-world examples like frogs and insects. These important questions are designed to mirror the exact difficulty level and wording used in previous board exams. One-mark MCQs test recall (e.g., 'How many gametes fuse during fertilisation?'). Two-mark questions demand brief explanations with diagrams (e.g., 'Distinguish between sperm and ovum'). Three-mark and five-mark questions require structured answers covering processes, diagrams, and biological reasoning (e.g., 'Explain the stages of human fertilisation' or 'Compare sexual and asexual reproduction with three points'). Practising these patterns builds both speed and accuracy, two critical factors in board success.

Multiple-choice questions test immediate recall and concept clarity. Each correct answer carries 1 mark. **Q1. During human reproduction, the male gamete fuses with the female gamete to form a:** A) Zygote B) Embryo C) Sperm D) Ovum **Answer: A) Zygote** – The fusion of sperm (male gamete) and ovum (female gamete) creates a zygote, which is the first stage of a new organism. **Q2. Which of the following is an example of asexual reproduction?** A) Binary fission in amoeba B) Fertilisation in humans C) Mating in dogs D) Pollination in flowers **Answer: A) Binary fission in amoeba** – Asexual reproduction requires only one parent. Amoeba divides by binary fission without requiring a second parent. **Q3. The transformation of tadpole into an adult frog is called:** A) Fertilisation B) Metamorphosis C) Germination D) Budding **Answer: B) Metamorphosis** – Metamorphosis is the process of dramatic structural change during the development of an organism, particularly in insects and amphibians like frogs. **Q4. In humans, the fertilised egg begins to divide and form an:** A) Zygote B) Embryo C) Sperm D) Placenta **Answer: B) Embryo** – After the zygote divides repeatedly, the developing organism becomes an embryo, which later becomes a fetus and then a baby. **Q5. Asexual reproduction in animals occurs through:** A) Fertilisation B) Budding and binary fission C) Pollination D) Mating **Answer: B) Budding and binary fission** – Asexual reproduction methods in animals include budding (as in Hydra), binary fission (as in amoeba), and fragmentation, all requiring only one parent.

Two-mark questions require brief explanations, often with a diagram or two-point justification. **Q1. Distinguish between sperm and ovum (any two points).** **Answer:** | Sperm | Ovum | |-------|------| | Male gamete produced in testes | Female gamete produced in ovary | | Motile (has flagellum for movement) | Non-motile; relatively stationary | | Smaller in size | Larger in size | | Produces many sperm continuously | Releases one ovum during ovulation | **Q2. Name any two organisms that reproduce asexually. How is asexual reproduction different from sexual reproduction (state one difference)?** **Answer:** Two asexual organisms: Amoeba (binary fission), Hydra (budding), Starfish (fragmentation). **Difference:** Asexual reproduction requires only one parent and produces genetically identical offspring, whereas sexual reproduction requires two parents and produces genetically varied offspring due to mixing of genes. **Q3. List the main reproductive organs in male and female humans.** **Answer:** **Male:** Testes (produce sperm), seminal vesicles, prostate gland, penis. **Female:** Ovaries (produce ova), oviducts (fallopian tubes), uterus (womb), vagina. The testes and ovaries are primary reproductive organs. **Q4. What is fertilisation? Explain in two sentences.** **Answer:** Fertilisation is the fusion of male gamete (sperm) and female gamete (ovum) to form a zygote. This process typically occurs in the oviduct (fallopian tube) of females and combines genetic material from both parents. **Q5. Define metamorphosis. Give one example.** **Answer:** Metamorphosis is the dramatic change in body form and structure that occurs during the development of certain organisms. **Example:** A tadpole transforms into an adult frog through metamorphosis, developing legs, lungs, and losing its tail over several weeks.

Three-mark questions demand structured answers explaining processes with logical flow and supporting details. **Q1. Explain the main differences between sexual and asexual reproduction with three points.** **Answer:** 1. **Number of parents:** Sexual reproduction requires two parents (male and female), whereas asexual reproduction requires only one parent. 2. **Genetic variation:** Sexual reproduction produces offspring with genetic variation due to mixing of parental genes. Asexual reproduction produces genetically identical offspring (clones). 3. **Gamete involvement:** Sexual reproduction involves fusion of two gametes (sperm and ovum). Asexual reproduction does not involve gamete formation or fusion; new organisms develop from somatic cells. **Q2. Describe the process of fertilisation in humans. (Draw a labelled diagram if possible.)** **Answer:** Fertilisation occurs through the following stages: 1. **Entry of sperm:** After intercourse, millions of sperms are deposited in the vagina. They swim through the cervix and uterus into the oviduct using their flagellum. 2. **Meeting of gametes:** In the oviduct, one sperm penetrates the cell membrane of the ovum. The nuclei of both gametes fuse, combining their genetic material. 3. **Formation of zygote:** The fusion of the sperm nucleus and ovum nucleus forms a zygote (2n chromosome number), which is the first cell of the new organism. The zygote then travels down the oviduct and implants in the uterine wall, beginning embryonic development. **Q3. Explain why offspring produced by asexual reproduction are identical to each other and to the parent.** **Answer:** 1. **Single genetic source:** Asexual reproduction uses DNA from only one parent organism. No mixing of genes occurs from two parents. 2. **Mitotic cell division:** The parent cell divides by mitosis, producing exact copies of chromosomes. Each daughter cell receives an identical set of chromosomes as the parent. 3. **No variation in meiosis:** Unlike sexual reproduction where meiosis creates genetic variety, asexual reproduction relies on exact duplication, ensuring offspring are clones with identical traits and genetic makeup. **Q4. Describe the stages of metamorphosis in a frog. What is the biological advantage of metamorphosis?** **Answer:** **Stages of frog metamorphosis:** 1. **Tadpole stage:** The larva has a tail, gills for aquatic respiration, and weak jaw structures. It feeds on algae and plants. 2. **Intermediate stage:** Hind limbs develop first, followed by front limbs. Lungs begin forming while gills gradually degenerate. 3. **Adult frog stage:** The tail shortens and eventually disappears. Lungs fully develop for terrestrial respiration. The frog can now hop and hunt insects. **Biological advantage:** Metamorphosis allows the organism to exploit different ecological niches (aquatic as tadpole, terrestrial as adult), reducing competition between larvae and adults for resources and living space.

Five-mark questions require comprehensive explanations with multiple sub-points, diagrams, and detailed reasoning. **Q1. What is sexual reproduction? Explain the key steps involved in human sexual reproduction, from production of gametes to implantation of the zygote.** **Answer:** **Definition:** Sexual reproduction is the process of producing offspring by fusion of two different gametes (sperm from male, ovum from female) from two different parents, resulting in genetic variation. **Key steps:** 1. **Gamete production:** In males, sperms are produced in the testes through spermatogenesis. In females, ova (eggs) are produced in the ovaries through oogenesis. Spermatogenesis is continuous after puberty; oogenesis releases one ovum per menstrual cycle. 2. **Coitus (intercourse):** During sexual intercourse, millions of sperms are ejaculated into the vagina. They swim through the cervical mucus into the uterus and then into the oviducts. 3. **Fertilisation:** In the oviduct (fallopian tube), typically near the ovary, one sperm fuses with the ovum. The sperm nucleus and ovum nucleus merge, forming a zygote with 46 chromosomes (2n). 4. **Early cell division:** The zygote undergoes mitotic divisions while moving down the oviduct, becoming a 2-cell, 4-cell, 8-cell, and finally a blastocyst (a hollow ball of cells). 5. **Implantation:** After 6–7 days, the blastocyst reaches the uterus and embeds itself in the endometrium (uterine lining), where it develops into an embryo and eventually a fetus over 9 months. **Q2. Compare sexual and asexual reproduction across five parameters. Why is sexual reproduction considered advantageous despite requiring more energy?** **Answer:** | Parameter | Sexual Reproduction | Asexual Reproduction | |-----------|-------------------|----------------------| | Number of parents | Two (male and female) | One | | Gamete fusion | Yes (sperm + ovum) | No | | Genetic variation | High (due to meiosis and gene mixing) | Zero (all offspring are clones) | | Examples | Humans, dogs, birds, fish | Amoeba (binary fission), Hydra (budding), Starfish (fragmentation) | | Speed of reproduction | Slow (requires courtship, fertilisation, gestation) | Fast (no courtship or long gestation needed) | **Advantages of sexual reproduction despite energy cost:** 1. **Genetic diversity:** Offspring have different combinations of genes, increasing the population's adaptability to environmental changes, diseases, and predators. 2. **Evolutionary potential:** Genetic variation provides raw material for natural selection, allowing species to evolve and improve over generations. 3. **Disease resistance:** Diverse genetic makeup means some offspring may resist pathogens better than clones, improving population survival. 4. **Reduced mutation accumulation:** In asexual reproduction, harmful mutations accumulate over generations (Muller's ratchet). Sexual reproduction allows harmful alleles to be diluted out. **Q3. Describe metamorphosis in insects (using the example of a butterfly). Explain the structural and physiological changes in each stage.** **Answer:** **Definition:** Metamorphosis is the dramatic transformation of body form and physiology during development, especially in insects and amphibians. **Stages of butterfly metamorphosis (complete metamorphosis):** 1. **Egg stage:** The adult female butterfly lays tiny eggs on the leaf of a host plant (e.g., milkweed). The egg is about 1 mm in diameter with a hard shell. Inside, the embryo develops for 3–5 days. 2. **Larva (caterpillar) stage:** The egg hatches into a larva (caterpillar), which looks completely different from the adult. **Characteristics:** Long, segmented body; chewing mouthparts; multiple pairs of legs; green or brown coloration for camouflage; very high metabolism and appetite. The caterpillar eats leaves continuously, growing rapidly. It sheds its exoskeleton (skin) 4–5 times (instars) over 3–5 weeks, each time increasing in size. 3. **Pupa (chrysalis) stage:** Once fully grown, the caterpillar stops eating and forms a protective hard casing called a chrysalis or pupa. Inside, **histolysis** (breakdown of larval tissues) and **histogenesis** (formation of adult tissues) occur. Most larval structures dissolve; adult structures (wings, compound eyes, six legs, proboscis) develop from dormant clusters of cells called imaginal discs. This stage lasts 1–2 weeks (or several months if overwintering). 4. **Adult (imago) stage:** The chrysalis cracks open, and a wet, crumpled butterfly emerges. Over hours, its wings expand and dry; colours develop. **Adult characteristics:** Two pairs of scaled wings; coiled proboscis (feeding tube); compound eyes; six jointed legs; sexual maturity. The adult feeds on flower nectar and lives 2–6 weeks, focusing on reproduction. **Physiological changes:** Complete metamorphosis involves dramatic changes in diet (leaf-eating caterpillar to nectar-sipping butterfly), locomotion (crawling to flying), sensory systems, and reproductive capacity. This allows different life stages to exploit different food sources and habitats, reducing competition. **Q4. Explain the biological significance of different reproductive strategies in animals. Use examples to support your answer.** **Answer:** **Biological significance of reproductive strategies:** **Sexual reproduction significance:** - **Survival in changing environments:** In variable habitats (e.g., forests, oceans), genetic diversity from sexual reproduction allows populations to adapt. For example, if a disease threatens birds, those with resistance genes survive and reproduce, improving population resilience. - **Gene repair:** Sex allows the mixing of genes, which can "hide" harmful recessive mutations and keep populations healthy. Asexual organisms accumulate mutations faster (genetic load), weakening populations over time. - **Evolutionary innovation:** Humans, driven by sexual reproduction and natural selection, have evolved complex brains, languages, and behaviours. **Asexual reproduction significance:** - **Rapid population growth:** Amoeba (binary fission) can double its population every 20 minutes under ideal conditions. This is crucial for microorganisms colonising new environments quickly. - **Clonal advantage:** In stable, ideal conditions (e.g., a petri dish with unlimited nutrients), genetically identical offspring are efficient and perfectly adapted, requiring no "cost" of finding mates. - **Vegetative reproduction in plants:** Potatoes reproduce asexually (from tubers), allowing rapid crop production with consistent traits. **Mixed strategies:** Some organisms use both. Hydra typically reproduces asexually (budding) when resources are abundant, but switches to sexual reproduction (male and female gonads develop) when starved, producing hardy eggs that survive harsh conditions. **Conclusion:** The choice of reproductive strategy reflects the organism's ecological niche and environmental pressures. This diversity of strategies reflects evolution's elegant fit between organism and habitat.

Higher-order thinking skill (HOTS) questions demand analysis, inference, and application beyond textbook knowledge. **Case Study: The Mysterious Decline of Frogs in a Local Pond** A conservation group noticed a dramatic decline in adult frogs in a local pond over three years. Scientists collected water samples and found high levels of a pesticide used in nearby agricultural fields. Tadpoles were still developing normally, but the metamorphosis process was severely delayed (taking 6–8 months instead of 3–4 months). Many tadpoles showed deformed limbs, and adult frogs that did emerge had lower reproductive success rates. **Questions:** 1. **Hypothesis formation:** The pesticide likely interferes with which stage of frog development, and why? **Answer:** The pesticide interferes with **metamorphosis** (specifically the pupal/tadpole-to-adult transition). Evidence: Tadpoles develop normally, but metamorphic changes (limb development, tail resorption, gill-to-lung transition) are delayed. The pesticide likely disrupts hormonal signals (thyroid hormones) that trigger metamorphosis or damages imaginal discs (cells that form adult structures). Deformed limbs suggest the genetic programming for limb formation is disrupted. 2. **Ecological consequence:** Explain how delayed metamorphosis affects population dynamics and ecosystem stability. **Answer:** **Effects:** - **Increased tadpole mortality:** Extended larval period exposes tadpoles to predators longer and depletes their energy reserves. - **Reduced reproductive window:** Adults reaching maturity later have a shorter breeding season, lowering fecundity (number of offspring per adult). - **Food web disruption:** Fewer adult frogs means reduced insect predation (frogs eat mosquitoes and insects), potentially increasing disease vector populations. - **Population crash:** Combined effects of low reproduction and high juvenile mortality cause population collapse, threatening ecosystem services (pest control, nutrient cycling). 3. **Intervention strategy:** Propose two evidence-based solutions to restore the frog population. **Answer:** - **Source control:** Advocate for reduced pesticide use in surrounding agriculture through policy or organic farming conversion. Monitor water pesticide levels monthly to track improvement. Introduce buffer zones (wetland vegetation) to filter agricultural runoff before it reaches the pond. - **Population augmentation:** Establish a breeding programme in pesticide-free laboratories, raising tadpoles under controlled conditions and releasing metamorphosed adults back into the pond once water quality improves. This maintains genetic diversity and population size while remediation occurs. 4. **Critical thinking:** Why might sexual reproduction (as opposed to asexual reproduction) be crucial for the long-term recovery of this frog species? **Answer:** Sexual reproduction generates genetic variation among offspring. Some frogs may carry alleles conferring pesticide tolerance or resistance, allowing them to survive even if pesticide levels remain slightly elevated. Over several generations, natural selection would increase the frequency of these tolerance alleles, leading to an adapted population. Asexual reproduction would produce genetically identical clones, all equally vulnerable to the pesticide, preventing adaptation and recovery. Additionally, genetic diversity maintains the population's ability to respond to future environmental stresses (disease, climate change).

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