Reproduction: Asexual and Sexual Strategies
Reproduction opens the KSEAB II PUC course. Questions ask you to separate asexual from sexual reproduction, identify vegetative propagules, trace gametogenesis, and apply chromosome-number logic to zygote, embryo and endosperm.
Core rules
- Asexual reproduction uses mitosis only, so offspring are clones. Learn the structure–organism pairs: zoospores (Chlamydomonas), conidia (Penicillium), buds (Hydra), gemmules (sponges), fragmentation (Spirogyra), and plant propagules - rhizome (ginger), tuber (potato), offset (Pistia), bulb (onion), sucker (banana), leaf buds (Bryophyllum).
- Sexual reproduction requires meiosis to make haploid gametes, then syngamy to restore the diploid zygote. Meiosis is the only step that halves chromosome number; fertilisation doubles it again.
- Oviparous animals lay eggs; viviparous animals give birth. Seasonal breeders show oestrus; primates show the menstrual cycle.
- Parthenocarpy is fruit formation without fertilisation; parthenogenesis is an embryo from an unfertilised egg.
Worked example
A plant has 2n = 24. Find the chromosome number in meiocyte, megaspore, egg, zygote and endosperm. The meiocyte is a somatic diploid cell, so 24. Meiosis halves it: megaspore = 12. The egg forms by mitosis from the megaspore, so it stays 12. Zygote = egg (12) + male gamete (12) = 24. Endosperm = two polar nuclei (12 + 12) + one male gamete (12) = 36.
Traps
Do not halve chromosome number at mitosis. Endosperm is triploid only in angiosperms; gymnosperm endosperm is haploid. Vegetative propagules are not seeds and do not involve meiosis.