Ploidy of gametes and zygote in different organisms, parthenogenesis, and why external fertilisation needs so many eggs
Nisha Verma12th Sample
28 Aug 2026 · 1 min read
Boards are done in my head and NEET mode is on, so I'm redoing Reproduction in Organisms with proper diagrams. The sexual reproduction events section has three things I can't line up.
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NCERT says gametes are always haploid and the zygote is diploid. Fine. But organisms differ: some algae and fungi are haploid, some (humans, flowering plants) are diploid. So if an alga is haploid, where is the diploid zygote in its life and what happens to it? Does the organism itself have the same ploidy as the gametes? If so, how does it produce gametes, by mitosis or meiosis?
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Parthenogenesis: NCERT gives honeybees as an example where drones develop from unfertilised eggs. Are drones haploid? Then how do they make sperm if meiosis can't halve a haploid number?
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External fertilisation in water vs internal fertilisation on land. I know examples (frogs, fish vs. reptiles, birds, mammals) but I need the reason each pattern produces what it does, in terms of number of eggs and survival of the young.
Also what happens after fertilisation: oviparous vs viviparous, and in plants what do ovule and ovary turn into? Short version is fine.
7 answers & replies
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[+][–]Rohan Sharma11thSample· 1mo ago
Biology guy here (kind of lol). External fertilisation animals lay few eggs because water protects them and carries them to the sperm? I think that's what makes it efficient. Internal fertilisation is the one that needs lots of eggs since the egg might not meet the sperm inside the body.
[+][–]Kavya NaikDropperSample· 1mo ago
Rohan, other way round actually. In water, fertilisation is external: gametes are released into the surrounding water (fish, frogs, many algae). The sperm and eggs can be washed away, and the zygotes and young face predators and unfavourable conditions. Because the chance of any one egg surviving is low, the animal produces a very large number of gametes (and offspring).
In internal fertilisation (reptiles, birds, mammals) sperm and egg meet inside the female's body, so there's a high chance of fertilisation, and the embryo is protected in the body or in a shell. They produce fewer eggs, but often give parental care. So: external = many, internal = fewer.
[+][–]Nisha Verma12thSample· 1mo agoAuthor
oh wow, I had it reversed in my head too. "Low odds, so many eggs" is the logic. Thank you all, writing zoya's haploid/diploid split on a sticky note.
[+][–]Rohan Sharma11thSample· 1mo ago
lol okay I clearly shouldn't comment on bio. sorry nisha didi. kavya's explanation is the correct one 🙏
[+][–]Zoya Khan12thSample· 1mo ago
Ploidy, in an organized way:
- Haploid organisms (many algae, fungi, the dominant stage of bryophytes): the organism is haploid, so gametes are formed by mitosis (no need for reduction). After fusion the diploid zygote undergoes meiosis and the haploid body is restored.
- Diploid organisms (most animals, seed plants): gametes are formed by meiosis from specialised meiocytes (gamete mother cells). The zygote is diploid and develops by mitosis into the new individual.
So gametes are haploid in both, but the way they are produced depends on whether the body is haploid or diploid.
[+][–]Tanvi Deshmukh12thSample· 1mo ago
Parthenogenesis: development of a new individual from an unfertilised egg. NCERT lists some rotifers, honeybees, some lizards and some birds (like turkey) as examples. In honeybees, the queen and workers come from fertilised eggs (diploid) while the drones come from unfertilised eggs, so they are haploid. Since a drone is haploid, sperm are produced by mitosis, not meiosis, same logic as zoya's haploid organisms. That's why a drone has only one set of chromosomes but no problem in making sperm.
[+][–]Rahul DubeyDropperSample· 1mo ago
After fertilisation, two directions:
Animals. Oviparous = lays eggs (reptiles, birds): the egg is covered by a hard calcareous shell and hatches outside, in an environment the mother has chosen. Viviparous = embryo develops inside the female's body (most mammals, including humans), gets nutrition and protection, and is born as a young one with a better chance of survival.
Flowering plants. Zygote → embryo (inside the ovule). Ovule → seed. Ovary → fruit. Ovary wall → pericarp (fruit wall). Remember the "zygote becomes embryo, ovule becomes seed, ovary becomes fruit" ladder, it's the most repeated line in this chapter.
