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Ploidy questions look like memory tests: is the nucellus n or 2n, what about the antipodals, the perisperm, the seed coat? Students who memorise a list tend to slip on the odd ones out, such as perisperm or nucellar embryos. This post gives you one rule that answers all of them, works it on onion with real chromosome numbers, then extends it to a genotype question that list-learners usually get wrong.
Which part of the syllabus is this?
The NEET (UG) 2026 syllabus from the NMC lists development of male and female gametophytes, double fertilisation, development of endosperm and embryo, and apomixis and polyembryony under sexual reproduction in flowering plants. The CBSE Biology curriculum for 2026-27 lists the same chapter for Class 12. The CBSE 2026-27 sample paper (Question 23) shows a fertilised embryo sac and asks for the ploidy of the zygote and the primary endosperm nucleus; the marking scheme gives diploid and triploid. Check the syllabus for your own exam year.
What is the one rule for ploidy?
Ask where the cell came from, and how.
Is it tissue of the parent plant? Then it is diploid (2n), like any body cell of the plant.
Did it come from a meiosis product by mitosis? Then it is haploid (n), because mitosis copies whatever it starts with.
Is it the result of a fusion? Add the sets: n + n = 2n, n + n + n = 3n.
That is all. The NCERT chapter on sexual reproduction in flowering plants even poses the question this way: "What will be the ploidy of the cells of the nucellus, MMC, the functional megaspore and female gametophyte?"
How does the rule sort the ovule and seed?
Follow NCERT's account step by step.
Nucellus (2n). The mass of cells inside the integuments is parent tissue.
Megaspore mother cell (2n). It sits in the nucellus and undergoes meiosis.
Megaspores (n). Meiosis gives four; in most flowering plants one is functional.
Embryo sac cells (n). The functional megaspore's nucleus divides by mitosis three times, giving 8 nuclei in 7 cells. So the egg, both synergids, all three antipodals and each polar nucleus are n.
Central cell (n + n). It holds two separate haploid polar nuclei until fertilisation.
Zygote and embryo (2n). Egg (n) plus one male gamete (n).
Primary endosperm nucleus and endosperm (3n). Two polar nuclei plus the other male gamete.
Seed coat (2n). NCERT says integuments harden into seed coats, so they stay parent tissue.
Pericarp (2n). The wall of the ovary becomes the fruit wall.
Perisperm (2n). In black pepper and beet, NCERT says the persistent nucellus is the perisperm. It is nucellus, so it is 2n, even though it stores food like endosperm does.
Worked example: onion, with real numbers
NCERT's Class 11 chapter on the cell cycle says onion root tip cells have 16 chromosomes. So for onion, 2n = 16 and n = 8.
Nucellus, integuments, seed coat, pericarp: 16
Megaspore, egg, synergid, antipodal, each polar nucleus: 8
Zygote and embryo: 8 + 8 = 16
Primary endosperm nucleus and endosperm: 8 + 8 + 8 = 24
Pollen vegetative cell, generative cell and each male gamete: 8
Check: the endosperm should have 1.5 times the chromosomes of the embryo. 24 / 16 = 1.5, so the numbers agree.
Worked example: whose genes are in which part?
Here is a step beyond ploidy. A plant of genotype aa (seed parent) is pollinated by a plant of genotype AA. What is the genotype of each part of the seed and fruit?
The egg and both polar nuclei come from the same megaspore of the aa parent, so each carries a.
Each male gamete from the AA parent carries A.
Embryo: a + A = Aa.
Endosperm: a + a + A = aaA, with two maternal sets for every paternal one.
Seed coat, pericarp, nucellus: parent tissue, so aa, with no paternal contribution.
So the seed coat and fruit wall carry only the mother plant's genes, while the embryo carries both parents' genes. Gymnosperms work differently. The NCERT Plant Kingdom chapter says their multicellular female gametophyte develops from a megaspore, so it is haploid, and NCERT calls double fertilisation "an event unique to flowering plants". No triple fusion means no triploid tissue.
Which mistakes cost marks?
Calling perisperm 3n because it is a food store. It is nucellus.
Calling the central cell 2n before fertilisation. It has two separate n nuclei.
Calling the seed coat 3n or n. It is parent tissue: 2n.
Assuming every embryo is a zygote product. Apomictic embryos from nucellus cells, which NCERT describes in Citrus and mango, are 2n and genetically like the mother.
Mixing the gymnosperm and angiosperm answers. Only flowering plants have triple fusion and a 3n endosperm.
If you want to see the 2, 4 and 8-nucleate stages before you count them, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson. For the full story from flower to seed, read our post on sexual reproduction in flowering plants, and for why plants avoid self-pollen, see outbreeding devices.
A 15-minute practice routine
Draw an ovule from memory and label nucellus, integuments, MMC and embryo sac.
Write n, 2n or 3n beside every label, and one word of origin: "parent", "meiosis" or "fusion".
Repeat with a plant of 2n = 14, writing the chromosome number for each part.
Do one genotype cross like the aa × AA example, swapping which parent is the seed parent.
Tonight, take three ploidy questions you got wrong and answer each by naming the cell's origin first.
