Biology Error Book

A running error book for biology. Every question I have answered wrongly is filed here under its chapter, with the reasoning that repairs it and my original mistake kept one click away — options can be clicked, key answers checked, and any card cleared and tried again.

Asexual reproduction and sexual reproduction in plants

01Multiple choicePollinationGermination

Which of the following descriptions about pollen grains is correct?

AA zygote is formed when a pollen grain fuses with an egg cell.
The pollen grain never fuses with the egg. It is a whole organism — a highly reduced male gametophyte — and it germinates into a pollen tube that delivers two male nuclei to the embryo sac. One of those nuclei, not the grain, fuses with the egg.
BPollen grains are the male gametes of a plant.
Same distinction, one step earlier. The grain contains the male gametes; it is not one. Structurally it is a generative cell (which divides to give the two male nuclei) plus a tube cell, wrapped in an exine and an intine. Calling it a gamete conflates gametophyte with gamete.
CThe sugar on the stigma stimulates the pollen grains to germinate.
DPollen grains can germinate only when they land on the stigma of another flower.
Germination requires a compatible stigma, not a different flower. Self-pollination onto the same flower’s stigma works perfectly well in peas, wheat and tomatoes. The converse also fails: incompatible pollen may refuse to germinate even on another flower of the same species — self-incompatibility systems reject it at the stigma surface, or arrest the tube part-way down the style.

I answeredACorrect answerC

Solution

The mature stigma secretes a sugary, sticky fluid that does two jobs at once — it traps pollen on contact, and it stimulates germination. The sugar supplies both an osmotic and a nutritive stimulus: the grain takes up water and nutrients, the tube cell is triggered, and it grows out through a germ pore as the pollen tube.

The error in option A is the one worth naming precisely, because it is a category error rather than a factual slip. Three things get collapsed into one:

Structure	What it is	What it does
Pollen grain	Male gametophyte (multicellular)	Germinates into a pollen tube
Generative cell	Cell inside the grain	Divides into two male nuclei
Male nucleus	The gamete	Fuses with the egg, or with the polar nuclei

The tube carries two male nuclei down to the embryo sac, and both fuse — double fertilisation. One joins the egg to give the 2n zygote; the other joins the two polar nuclei to give the 3n endosperm nucleus, which becomes the seed’s food store.

The test that separates A and B from C: ask how many cells the thing has. A gamete is one nucleus doing one fusion. A gametophyte is a generation of the life cycle — small enough here to fit inside a grain of dust, but a generation nonetheless.

02Multiple choicePollenGametophyteDouble fertilisation

Pollen grains are not male gametes because of what?

AThey are diploid.
Factually false, and it inverts the actual chain. A microspore mother cell in the anther divides by meiosis, so each microspore — and the pollen grain that develops from it — is haploid (n). Ploidy cannot be the disqualifier anyway: gametes are haploid too, so “haploid” is the one property the grain and a true gamete share.
BThey are larger than the female gametes.
Size is not part of the definition of a gamete. Anisogamy usually runs the other way — the egg cell is the large, food-loaded partner and the male nucleus is little more than a packet of DNA — but a gamete would remain a gamete at any size. The comparison is also being drawn between the wrong pair of objects: a whole gametophyte against a single cell.
CThey contain some nuclei that do not fuse with the female gametes.
DEach pollen grain contains two nuclei.
True of a bicellular grain at shedding — tube nucleus plus generative nucleus — but not universal: in grasses and many other families the generative cell divides before release, so the grain is shed tricellular, with three nuclei. A raw nucleus count is a description, not a criterion. What matters is not how many nuclei are present but what they do, which is why C survives and D does not.

I answeredACorrect answerC

Solution

A gamete is a single haploid cell whose function is to fuse. The pollen grain fails that test not on ploidy, not on size, but on division of labour: it carries nuclei that are never destined to fuse with anything.

Inside the mature grain:

Nucleus	Origin	Fate
Tube (vegetative) nucleus	Vegetative cell	Directs growth of the pollen tube down the style, then degenerates. Never fuses.
Male nucleus 1	Generative cell, after mitosis	Fuses with the egg cell -> 2n zygote
Male nucleus 2	Generative cell, after mitosis	Fuses with the two polar nuclei -> 3n endosperm nucleus

The tube nucleus is the decisive one. It is haploid, it is inside the grain, and it takes no part in fertilisation at all — it is a pilot, not a passenger. A structure containing a nucleus with a purely somatic job is by definition more than a gamete: it is a male gametophyte, a whole multicellular generation of the plant life cycle, reduced until it fits inside a dust-sized wall of exine and intine.

Your remark is exactly the answer. The vegetative nucleus never fuses, and that single fact is what demotes the grain from gamete to gametophyte. Option A tempts because “diploid” sounds like a disqualification — but haploidy is what the grain and the gamete have in common, not what separates them.

03Multiple choiceMeiosisMitosisPollen developmentPloidy

The diagram below shows the development of pollen grains, all derived from repeated divisions of a single cell (key: n = haploid, 2n = diploid):

Figure 1. Pollen grain development through Division P, Q and R

What are the types of division for Division P, Q and R?

AP: meiosis; Q: mitosis; R: mitosis
BP: mitosis; Q: meiosis; R: meiosis
P is charged with mitosis here, but P takes the ploidy from 2n down to n. A chromosome-number halving can only be meiosis — mitosis by definition leaves ploidy unchanged, so it can never turn a 2n cell into n products. Q and R are then charged with meiosis, but both go n → n; there is no diploid starting point left to halve; a haploid nucleus dividing into haploid products is, by the same ploidy test, mitosis.
CP: mitosis; Q: mitosis; R: meiosis
The same ploidy error opens this option — P is 2n → n, so it cannot be mitosis. R is the trap: the generative nucleus splitting into two sperm nuclei looks like a “reduction” because it produces two products from one, but ploidy stays at n → n throughout. Meiosis is defined by halving chromosome number, not by cell count; an already-haploid nucleus has nothing left to halve, so this is the second (mitotic) pollen division, not meiosis.
DP: meiosis; Q: meiosis; R: meiosis
This over-applies meiosis to every division in the sequence. Meiosis is a one-time event, confined to the diploid microsporocyte — once a lineage is haploid, it can only propagate itself by mitosis, since there is no lower ploidy to reduce to. Q and R both occur entirely within the haploid (n) generation.

I answeredCCorrect answerA

Solution

Track ploidy through the diagram rather than counting resulting cells — that single check resolves all three divisions.

Division P — the pollen mother cell (microsporocyte, 2n) undergoes meiosis I and II to yield a tetrad of four haploid microspores (n). A 2n → n change is only possible through meiosis; mitosis cannot alter chromosome number.

Division Q — each haploid microspore now divides once more, but it is already at n and stays at n: this is the first pollen mitosis, an asymmetric division producing a bicellular grain with a large vegetative (tube) nucleus and a small generative nucleus, both haploid.

Division R — the generative nucleus undergoes the second pollen mitosis, splitting into two sperm nuclei, still n → n. The vegetative nucleus takes no further part and does not divide again.

So P is meiosis, and Q and R are both mitosis — option A. The general rule this question is testing: meiosis happens exactly once per generation, at the sporophyte-to-gametophyte transition where ploidy halves. Every division after that, for as long as the lineage stays haploid, is mitosis — regardless of how many cells or nuclei come out of it.

When a division question gives you cell or nucleus counts, resist the urge to pattern-match “splits into multiple products = meiosis.” The only test that never fails is whether ploidy is halved. Two products from one cell is consistent with either process; a ploidy drop is unique to meiosis.

04Multiple choiceFruit and seed formationGenotypeDouble fertilisation

The diagram below shows the cross section of a peach fruit and the parts inside the fruit.

Figure 2. Cross section of a peach: P is the fleshy part, X is the stone, cut open to show its outer coat Q and the part R inside it

Of parts P, Q and R, which parts have the same genotype?

AP and Q are the same
BQ and R are the same
This pairs Q with R because both sit inside structure X and the label “outer coat” invites reading Q as a seed coat wrapped around the seed. Physical enclosure is not developmental origin. Q is the endocarp — the innermost of the three pericarp layers, lignified into a stone — and it is ovary-wall tissue, while R is the product of fertilisation. Note that the pairing fails even under the misreading: a true seed coat (testa) develops from the integuments of the ovule, which are also maternal tissue, so Q would still not match R.
CP and R are the same
P is mesocarp, grown from the ovary wall; R is the embryo, grown from a fertilised egg. Half of R’s alleles arrived in a pollen nucleus from a different plant, so the two genotypes coincide only in the degenerate case where the tree self-pollinated and was homozygous at every locus in question. A special case is not an answer to a general question.
DP, Q and R are the same
The reasoning behind this is “all three parts grew on one tree, so all three carry that tree’s genes.” It holds for P and Q, which are maternal sporophyte tissue produced by ordinary mitosis, and breaks at R. The seed is not part of the mother — it is the next generation, a new individual assembled at fertilisation. This is why a single tree can bear seeds of many different genotypes when different pollen donors reach different flowers, while every peach’s flesh on that same tree stays genetically identical.

I answeredDCorrect answerA

Solution

Sort the three labels by which structure they developed from, not by where they sit in the finished fruit. Everything derived from the ovary wall is maternal; everything derived from the fertilised ovule is not.

After fertilisation the ovary wall thickens into the pericarp, which in a drupe such as peach differentiates into three layers:

Layer	In the peach	Develops from	Genotype
Exocarp	The fuzzy skin	Ovary wall	Same as parent tree
Mesocarp — P	The juicy flesh you eat	Ovary wall	Same as parent tree
Endocarp — Q	The hard stone, i.e. the “outer coat” of X	Ovary wall	Same as parent tree
Seed — R	The kernel inside the stone	Fertilised ovule	New combination, generally different

P and Q are therefore mitotic descendants of the same maternal cells and are genetically indistinguishable from each other and from the parent plant. R contains an embryo formed when a male nucleus from the pollen tube fused with the egg cell, so its genotype is a fresh assortment of maternal and paternal alleles.

The trap this question is built on is a naming collision. In ordinary speech the stone is “the peach seed” — you plant it, and something grows. Botanically the stone is fruit: a woody endocarp. The actual seed is the kernel it protects, and the seed’s own coat, the testa, is the thin brown papery skin lying just inside the shell, against the kernel. Reading Q as that testa is the natural mistake, and it is worth noticing that the answer survives it — the testa forms from the integuments of the ovule, which are maternal tissue too, so Q lands on the same side as P under either reading.

Your remark identifies the boundary correctly: the hard shell belongs to the fruit, not to the seed. What option D was really assuming is that “grown on the parent plant” implies “genetically the parent plant.” Fertilisation is the line where that stops being true — cross the line and you are looking at an offspring, however small and however deeply buried it is inside its mother’s tissue.

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Biology Error Book

Asexual reproduction and sexual reproduction in plants