Samacheer Kalvi 12th Biology Chapter 11 Exercise Solutions

Asexual reproduction involves a single parent and no gamete fusion; offspring are genetically identical (clones). Examples: binary fission in Amoeba, budding in Hydra. Sexual reproduction involves two parents, gamete fusion, and offspring show genetic variation due to recombination. Example: human reproduction. Sexual reproduction provides variation which is the raw material for evolution.

Parthenogenesis is the development of an unfertilised egg into a new organism without fertilisation. Examples: (1) Honey bee drones develop from unfertilised eggs and are haploid. (2) Aphids reproduce by parthenogenesis during summer months. It can also be induced artificially by temperature shock or chemical stimulation.

Oviparous organisms lay eggs outside the body and the young develop externally (e.g., birds, reptiles, amphibians). Viviparous organisms give birth to live young after internal development, often with placental nourishment (e.g., most mammals). Ovo-viviparous is an intermediate condition where eggs hatch inside the mother's body.

External fertilisation is the fusion of male and female gametes outside the female body, typically in water. The organism releases large numbers of gametes to increase the chance of fertilisation. Examples: frogs, most bony fish. A disadvantage is that the zygote and embryos are exposed to predators and environmental hazards.

Regeneration is the ability of an organism to regrow lost or damaged body parts, which can sometimes result in complete organism formation. In Planaria, if the body is cut into pieces, each piece can regenerate into a complete organism. Regeneration differs from fragmentation because the organism does not intentionally break apart — only lost parts regrow.

Sexual reproduction involves the fusion of gametes from two parents, producing offspring with genetic variation through recombination and crossing over. This variation is the raw material for natural selection and enables populations to adapt to changing environments. It also helps eliminate harmful mutations over generations.

• Binary fission: The parent organism divides into two nearly equal daughter cells. Based on the plane of division: (i) Irregular binary fission — Amoeba (division can occur in any plane); (ii) Transverse binary fission — Paramoecium (division across the longitudinal axis); (iii) Longitudinal binary fission — Euglena. The nucleus divides first (mitosis), followed by cytoplasmic division (cytokinesis).
• Multiple fission: The parent nucleus divides repeatedly, producing many daughter cells simultaneously. Occurs in Plasmodium (the malaria parasite) inside the human liver and RBCs as part of its life cycle (schizogony). Also occurs in Amoeba under unfavourable conditions (encysted form).
• Budding: An outgrowth (bud) develops on the parent body, grows, and eventually separates as a new individual. Seen in Hydra (freshwater cnidarian) and yeast (Saccharomyces). In Hydra, the bud develops a mouth and tentacles before detaching.
• Fragmentation: The body breaks into two or more pieces, and each fragment grows into a complete organism. Observed in Spirogyra (filamentous alga), starfish, and some flatworms. Each fragment must contain enough cells to regenerate.
• Regeneration: The ability to regrow lost body parts. In Planaria, a cut piece regenerates its missing half. In Hydra, a small body piece can grow into a complete organism. Regeneration involves mitotic cell division and is not the same as fragmentation.
• Sporulation: Spores are produced that germinate into new organisms. Common in fungi (Rhizopus) and ferns. Spores are resistant to environmental conditions.
• Parthenogenesis: Development of an unfertilised egg into a new organism. Honey bee drones (males) are haploid and develop from unfertilised eggs. Aphids reproduce by parthenogenesis during summer.

Advantages of sexual reproduction:

• Genetic variation: Fusion of gametes from two parents and crossing over during meiosis produce offspring that are genetically different from both parents and from each other.
• Adaptation: Variation allows populations to adapt to changing environments — those with favourable traits survive and reproduce (natural selection).
• Evolution: Genetic variation is the raw material for evolution. Without variation, all individuals would be identical and equally vulnerable to a single threat.
• Elimination of harmful mutations: In sexual reproduction, harmful recessive alleles can be masked by dominant alleles, and deleterious combinations can be eliminated over generations.
• Disease resistance: Genetic diversity makes it harder for pathogens to wipe out an entire population, as individuals vary in their susceptibility.

Sexual reproduction involves three phases: pre-fertilisation, fertilisation, and post-fertilisation.

Pre-fertilisation events:

• Gametogenesis: Formation of male gametes (sperms in animals, pollen in plants) and female gametes (ova in animals, ovules in plants). Gametes are haploid (n) produced by meiosis.
• Gamete transfer: Bringing male and female gametes together. In aquatic animals (fish, frogs), both gametes are released in water (external fertilisation). In terrestrial animals, sperm is transferred to the female reproductive tract (internal fertilisation). In flowering plants, pollination transfers pollen to the stigma.

Fertilisation (syngamy):

• Fusion of male and female gametes to form a diploid zygote (2n).
• External fertilisation: Occurs outside the body in water. Large numbers of gametes are released to compensate for high predation risk. Example: frogs, bony fish. Disadvantage: zygotes are vulnerable to predators and environmental hazards.
• Internal fertilisation: Occurs inside the female body. Fewer gametes are needed because fertilisation is more secure. Example: birds, reptiles, mammals, flowering plants.

Post-fertilisation events:

• Zygote formation: The diploid zygote is the first cell of the new organism.
• Embryogenesis: The zygote undergoes mitotic divisions (cleavage) to form an embryo. Cell differentiation leads to tissues and organs.
• In oviparous animals, the zygote develops inside a calcareous egg shell (birds, reptiles) or gelatinous egg mass (frogs).
• In viviparous animals, the embryo develops inside the uterus with placental nourishment (mammals).
• In ovo-viviparous animals, eggs develop and hatch inside the mother's body; young are born live (sea horse, some sharks).

Parthenogenesis is the development of an unfertilised egg into a new organism without fertilisation. It is a form of asexual reproduction that occurs naturally in some species.

Natural parthenogenesis:

• Honey bees (Haplodiploidy): Queen bee lays both fertilised and unfertilised eggs. Fertilised eggs develop into diploid females (queen and workers). Unfertilised eggs develop into haploid males (drones) by parthenogenesis.
• Aphids: During summer, female aphids reproduce by parthenogenesis to rapidly increase population. In unfavourable conditions, they switch to sexual reproduction.
• Rotifers: Some species reproduce exclusively by parthenogenesis; males are unknown or rare.
• Lizards: Some species of whiptail lizards (e.g., Cnemidophorus uniparens) reproduce by parthenogenesis — all individuals are females.
• Turkeys: Occasional parthenogenetic development of eggs has been observed.

Artificial parthenogenesis:

• Eggs can be stimulated to develop without fertilisation using physical or chemical stimuli such as temperature shock, electric shock, needle pricking, or chemicals (e.g., calcium chloride).
• This is an experimental technique and does not occur naturally.

Significance:

• Allows rapid population growth when conditions are favourable (no need to find a mate).
• In honey bees, parthenogenesis determines sex — drones are haploid, workers and queen are diploid.
• Disadvantage: no genetic variation, making populations vulnerable to environmental changes.