Biology · 16 Reproduction

16.1 Asexual reproduction

Before you start

Reproduction is one of the characteristics of living organisms — making more of the same kind of organism.
Genetic material is carried on chromosomes inside the nucleus of a cell.
Mitosis is nuclear division that produces genetically identical cells (Topic 17).
Variation means differences between individuals of the same species.
A mutation is a random change in the genetic material.

Syllabus objectives covered (3)

Describe asexual reproduction as a process resulting in the production of genetically identical offspring from one parent
Identify examples of asexual reproduction in diagrams, images and information provided
Discuss the advantages and disadvantages of asexual reproduction: (a) to a population of a species in the wild (b) to crop production

Part 1 of 5

The definition, and the two things it insists on

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent. That is the marked sentence, so learn its exact shape.
It insists on two things, and both are worth a mark. One parent — no partner, no mate, no second organism contributing anything. And genetically identical offspring — every offspring carries the same genetic material as the parent.
The word genetically is doing real work. If you write only "identical offspring", you have made a claim about how they look, which is not quite true and is not what is being marked. Two genetically identical potato plants can end up different sizes if one gets more water. Their genes are the same; their appearance need not be.
Equally, if you write "the offspring are the same as the parent", the examiner cannot tell whether you mean the same species, the same appearance or the same genes. Write genetically identical.
Notice what the definition never mentions: gametes, and fertilisation. Their absence is the whole point. No gametes are made, and no nuclei fuse.
The word for a group of genetically identical individuals produced from one parent is a clone. If a question asks for that word, no other word will do.

Part 2 of 5

Why the offspring come out genetically identical

This is the part most answers skip, and it is often the second mark. The offspring are genetically identical because of how the cells were made.
Asexual reproduction uses mitosis — the nuclear division that produces genetically identical cells. Before mitosis, the chromosomes are copied exactly, so each new nucleus receives an exact copy of the parent's genetic material.
There is nothing else in the process to introduce a difference. No gametes are made, so there is no mixing of genetic material from two different individuals, and no fusion of nuclei.
So the chain is short and you should be able to write it in one line: one parent → mitosis → exact copies of the chromosomes → genetically identical offspring.
One honest qualification. Mutations still happen by chance, so a clone is not absolutely, permanently identical for ever. Mark schemes accept "genetically identical" and "no genetic variation", and if you want to be exact you can write "no variation apart from that caused by mutation". Never let this stop you writing the main point.

Part 3 of 5

Recognising asexual reproduction when you are shown a case

Questions on this objective hand you a picture or a paragraph about an unfamiliar organism and ask which type of reproduction it shows. You are not expected to have met the organism before — you are expected to apply a test.
The test is three questions. How many parents? If one, that points to asexual. Are gametes made and do two nuclei fuse? If yes, it is sexual, whatever else the case looks like. Are the offspring genetically identical to the parent? If yes, asexual.
The second question is the one that catches people out. A single plant that pollinates its own flowers still makes gametes, and the nuclei of two gametes still fuse. That is sexual reproduction happening within one plant — not asexual reproduction, even though only one plant is involved.
Now the cases you should recognise on sight, described as you would see them in an image. Binary fission in bacteria: one bacterial cell copies its genetic material and splits into two cells of equal size. Two organisms where there was one, no gametes anywhere.
Spore production in a fungus: an upright stalk ends in a rounded case (a sporangium) full of tiny spores, which are released and each grows into a new fungus. One parent fungus, no fusion of nuclei.
Runners: a strawberry plant sends a horizontal stem out across the soil surface, and where it touches down a small new plant with its own roots and leaves develops, still joined to the parent by that stem.
Tubers: a potato plant forms swollen underground stems, each with buds on it. Each tuber can grow into a whole new plant next season.
Bulbs: an onion or daffodil bulb divides to form new bulbs alongside it, each of which grows into a new plant.
Plantlets: some plants grow tiny complete plants — with leaves and roots — along the edges of their leaves or on the ends of shoots; these drop off and root themselves.
Look at the pattern in all six. A new individual grows directly out of part of one parent's body, or from one parent cell. No flowers, no pollen, no sperm, no egg, no fusion. That is what asexual reproduction looks like from the outside.
When you justify your identification, quote the evidence from the stem back at the examiner: "only one parent is involved and no gametes fuse, so the offspring are genetically identical". An unsupported "asexual" is worth one mark at most, and often none.

Part 4 of 5

In the wild: what a clone population gains, and what it risks

This objective asks you to discuss, which on the paper means you must be able to argue both sides. Get out of the habit of thinking asexual reproduction is simply the worse option — it is a trade-off, and the marks are split across both halves.
Advantage — no mate needed. Only one parent is required, so an organism can reproduce even when it is isolated or when individuals of its species are scarce. Nothing has to be found, attracted or pollinated.
Advantage — speed and numbers. Because no time or energy goes into finding a mate, producing gametes or fertilisation, a population can increase very rapidly when conditions are favourable.
Advantage — a proven set of genes. The parent survived and reproduced in this environment, so its genotype clearly works here. Every offspring gets exactly that genotype. In a stable, unchanging habitat that is a real advantage, not a consolation.
Disadvantage — no genetic variation. All the offspring are genetically identical, so the population contains almost no variation.
Now push that disadvantage one step further, because the step is where the mark is. If a new disease arrives, or a new pest, and one individual is susceptible to it, then all of them are susceptible, because they are genetically identical. None is resistant. The entire population can be wiped out.
Disadvantage — no raw material for adaptation. If the environment changes — the climate, the food supply, a new predator — there is no variation for natural selection to act on, so the population is much less able to adapt over time.
Disadvantage — competition close to home. The offspring grow right next to the parent and need exactly the same resources, so parent and offspring compete for the same light, water, mineral ions or food, and overcrowding can result.
A four-mark answer here is usually two advantages and two disadvantages, each with its consequence attached. "There is no variation" is half a point; "there is no variation, so if a disease can kill one plant it can kill them all" is the whole one.

Part 5 of 5

In a field: what a farmer gains, and what a farmer risks

Part (b) of this objective is the same biology pointed at a different question: what happens when a person deliberately chooses asexual reproduction to grow a crop?
Advantage — the good features are kept exactly. A grower picks a parent plant with desirable features — high yield, good flavour, resistance to a known disease — and every plant produced asexually from it has that same genotype. The desirable features are guaranteed, not gambled on.
Advantage — a uniform crop. Because the plants are genetically identical, they grow to a similar size and ripen at about the same time, so the whole field can be harvested in one operation and the produce is of predictable, consistent quality. That matters commercially — supermarkets buy uniformity.
Advantage — speed and certainty. Large numbers of plants can be produced quickly, and there is no dependence on pollination or on fertilisation happening, which might fail.
Disadvantage — one disease can take the lot. The plants are genetically identical, so they are all equally susceptible to the same disease or pest. A single pathogen arriving in the field can destroy the entire crop, whereas in a varied crop some plants would survive.
Disadvantage — nothing improves by itself. Since there is no genetic variation, no new or better varieties arise from the crop. Any improvement has to be introduced deliberately from elsewhere.
Disadvantage — the same weakness everywhere. If conditions change — a drought, an unusually cold season — every plant responds the same way, because they share the same genotype. There is no part of the crop that copes better.
Notice that the advantages and the disadvantages come from the same fact: the plants are genetically identical. Say that fact once, then take it in both directions. That structure reads as a discussion and picks up marks on both sides.

Worked examples

Example 1. A gardener describes a plant: "A horizontal stem grew sideways from the parent plant across the soil. Where it touched the ground, a small new plant with its own roots and leaves developed." Identify the type of reproduction shown and justify your answer.

Show the working

Run the three-question test rather than trying to recall the plant. How many parents does the description mention? One.
Are gametes made, and do two nuclei fuse? The description mentions no flowers, no pollen and no fertilisation — the new plant grew directly from a stem of the parent.
So the offspring received its genetic material from that one parent only, by mitosis, which makes it genetically identical to the parent.
Name the structure if you can — this is a runner — but do not stop at the name. The justification is what earns the marks.

Answer: Asexual reproduction, by means of a runner. Only one parent is involved, no gametes are produced and no nuclei fuse, so the new plant is genetically identical to the parent.

Example 2. A single-celled organism copies its genetic material and then divides into two cells of equal size. A student writes: "This is asexual reproduction because there is only one parent." Explain what the student has left out.

Show the working

The student's statement is true, and it would score the first mark. The question is asking what the second mark would have been.
"One parent" on its own does not settle it, because a single plant pollinating its own flowers also involves one parent and that is sexual reproduction.
What is missing is the outcome: the genetic material was copied exactly before the cell divided, so the two new cells are genetically identical to the parent cell.
It is also worth stating the negative explicitly — no gametes were produced and no nuclei fused — since that is what rules out sexual reproduction.

Answer: The student has left out the outcome. Because the genetic material was copied exactly and no gametes were produced and no nuclei fused, the two offspring cells are genetically identical to the parent. 'One parent' alone does not prove reproduction is asexual, since self-pollination also involves one parent but is sexual.

Example 3. A farmer grows an entire field of banana plants, all produced from side shoots taken from one parent plant that gave a high yield. Suggest one advantage and explain one risk of growing the crop in this way.

Show the working

Start by stating the fact both halves of the answer will hang on: all the plants are genetically identical to that one high-yielding parent.
Take it in the helpful direction first. Every plant carries the parent's high-yield genotype, so the farmer knows in advance what the yield and the quality of the fruit will be, and the crop will be uniform and ready to harvest together.
Now take the same fact in the harmful direction. Genetically identical also means identically susceptible.
Add the consequence, because the consequence is the mark: if a disease or pest that can infect one plant reaches the field, no plant in it is resistant, so the whole crop could be destroyed.

Answer: Advantage: all the plants are genetically identical to the high-yielding parent, so every plant has the desirable high-yield genotype and the crop is uniform and ripens together for a single harvest. Risk: because they are genetically identical they are all equally susceptible to the same disease or pest, so a single pathogen could destroy the whole crop, as none of the plants would be resistant.

Where people go wrong

Mistake: Treating asexual reproduction as simply the worse option — "there is no variation, so it is bad" — and writing only disadvantages when the question asks you to discuss it.

Instead: It is a trade-off. In a stable environment, with no mate available, producing many offspring quickly that all carry a genotype already proven to work there is a genuine advantage.

The objective asks for advantages and disadvantages, and the marks are split between them. An answer that gives four disadvantages and no advantages cannot score more than half.

Mistake: Reading "genetically identical" as "identical in every way", so that any difference in size or appearance seems to disprove asexual reproduction.

Instead: Genetically identical means the same genetic material. Features can still differ, because the environment affects how an organism grows.

Two potato plants from the same parent will differ in height if one gets more light or water. Their genes are the same; their appearance is genes plus environment. Dropping the word 'genetically' is what causes the confusion.

Mistake: Calling self-pollination in a single plant asexual reproduction, because only one plant is involved.

Instead: It is sexual reproduction. Gametes are produced and the nuclei of two gametes fuse, so the offspring are genetically different from each other.

The number of parents is only one of the three tests. The decisive test is whether gametes are made and whether nuclei fuse. If they do, it is sexual, however few individuals are involved.

Mistake: Assuming asexual reproduction only happens in bacteria and other 'simple' organisms.

Instead: Many flowering plants reproduce asexually — runners, tubers, bulbs and plantlets are all examples — and growers use it deliberately for crops.

A question showing a strawberry runner or a potato tuber is testing exactly this. Expecting only microorganisms means you fail to recognise the commonest examples on the paper.

Mistake: Answering "one parent" and then, later in the same answer, mentioning fertilisation or gametes.

Instead: Decide which process it is, then keep every statement consistent with that decision.

Examiners do not choose between contradictory statements in the same part. A correct point sitting next to a contradicting one cannot score, so a hedged answer scores less than a committed one.

Check yourself

Describe asexual reproduction.
Answer
A process resulting in the production of genetically identical offspring from one parent.
Why are the offspring of asexual reproduction genetically identical to the parent?
Answer
They are produced by mitosis, in which the chromosomes are copied exactly, and there is only one parent, with no gametes produced and no fusion of nuclei, so there is nothing to introduce genetic differences.
Give three questions you can ask about an unfamiliar organism to decide whether it is reproducing asexually.
Answer
How many parents are involved? Are gametes produced and do two nuclei fuse? Are the offspring genetically identical to the parent? One parent, no fusion of nuclei and genetically identical offspring together mean asexual reproduction.
A daffodil bulb divides to produce new bulbs beside it, each of which grows into a new plant. Identify the type of reproduction and give one piece of evidence from the description.
Answer
Asexual reproduction. The new plants grow directly from one parent bulb, with no gametes and no fertilisation, so they are genetically identical to the parent.
One rose plant is pollinated with pollen from its own flowers and produces seeds. Is this asexual reproduction? Explain.
Answer
No. Gametes are produced and the nuclei of two gametes fuse at fertilisation, so it is sexual reproduction, even though only one plant is involved and the offspring will be genetically different.
Give two advantages of asexual reproduction to a population of a species living in a stable habitat in the wild.
Answer
Only one parent is needed, so no mate has to be found; and the population can increase rapidly, with every offspring carrying a genotype already proven to be successful in that habitat.
Explain why a wild population that reproduces only asexually is vulnerable to a new disease.
Answer
All the individuals are genetically identical, so there is no genetic variation. If one individual is susceptible to the disease then all of them are, since none carries an allele for resistance, so the whole population could be killed.
State one advantage and one disadvantage to crop production of growing plants that have all been produced asexually from one parent.
Answer
Advantage: all the plants have the parent's desirable features, such as high yield, so the crop is uniform and ripens together for a single harvest. Disadvantage: they are all equally susceptible to the same disease or pest, so one pathogen could destroy the entire crop.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student is given information about how three organisms reproduce. Organism A: an upright stalk produces a rounded case containing many spores. The spores are released and each one can grow into a new organism. Organism B: pollen from the flower of one plant is carried to the flower of a second plant, and the nuclei of two gametes fuse. Organism C: an underground stem becomes swollen and forms a tuber with buds on it. Each tuber grows into a new plant the following season.

(a) Identify the two organisms that reproduce asexually. 2 marks No calculator

Mark scheme

B1 A
B1 C

(b) Explain how the information given shows that organism B does not reproduce asexually. 2 marks No calculator

Mark scheme

B1 two parent plants are involved / gametes are produced
B1 the nuclei of two gametes fuse, so the offspring are genetically different, not genetically identical, ORA

(c) Explain why the new plants grown from the tubers of organism C are genetically identical to the parent plant. 2 marks No calculator

Mark scheme

B1 only one parent is involved / no gametes are produced and no nuclei fuse
B1 the cells are produced by mitosis / the chromosomes are copied exactly, so each offspring receives the same genetic material as the parent

Question 2. A small floating water plant in a lake reproduces only asexually. In one summer, a single plant produced more than a thousand new plants, which covered most of the lake surface.

(a) Suggest two advantages of asexual reproduction to this population of water plants. 2 marks No calculator

Mark scheme

B1 only one parent is needed, so no mate / no pollination or fertilisation is required
B1 the population can increase very rapidly when conditions are favourable (accept: all offspring inherit a genotype already successful in this habitat)

(b) A fungal disease that kills this species of water plant reaches the lake. Explain why the whole population is at risk. 3 marks No calculator

Mark scheme

B1 the plants are all genetically identical / are clones of one parent
B1 so there is no genetic variation in the population
B1 none of the plants is resistant to the disease, so if one is susceptible all are and all could be killed, owtte

(c) Suggest one disadvantage, other than disease, of the offspring growing close to the parent plant. 1 mark No calculator

Mark scheme

B1 parent and offspring compete for the same resources, such as light or mineral ions / overcrowding occurs

Question 3. A farmer produces a whole field of banana plants from side shoots taken from one parent plant chosen for its high yield.

(a) Explain two advantages to the farmer of producing the crop in this way. 4 marks No calculator

Mark scheme

B1 all the plants are genetically identical to the chosen parent
B1 so every plant has the desirable feature, here the high yield, and the yield is predictable
B1 the plants grow and ripen at the same rate, giving a uniform crop
B1 so the whole field can be harvested in one operation / large numbers can be produced quickly without relying on pollination

(b) A neighbouring farmer grows bananas grown from seed, which vary genetically. Evaluate the risk to each farmer if a new banana disease spreads through the area. 3 marks No calculator

Mark scheme

B1 the first farmer's plants are genetically identical, so all are equally susceptible and the whole crop could be destroyed
B1 the neighbour's plants vary genetically, so some may be resistant and survive
B1 the risk of losing the entire crop is therefore greater for the first farmer, although the neighbour's crop is less uniform and its yield less predictable, ORA

How this is examined

Why a clone population is wiped out by a new disease, when a varied one is not: a three- or four-mark Explain, in a stem about a wild population or a plantation that has been hit by a pathogen The chain, not the headline: all the individuals are genetically identical → there is no genetic variation in the population → none of them is resistant to the disease / if one is susceptible all are → the whole population or crop can be destroyed. 'There is no variation' alone is one mark; the consequence attached to it is where the rest sit.
The advantages a grower actually buys by propagating a crop asexually: a two- to four-mark Suggest or Explain about a farmer growing a field from cuttings, tubers or side shoots of one plant Each advantage tied back to genetic identity: the plants are genetically identical to a parent with desirable features, so the desirable feature (high yield, flavour, disease resistance) is guaranteed in every plant; the crop is uniform and ripens at the same time, so it can be harvested in one operation; large numbers can be produced quickly without depending on pollination or fertilisation. Naming the desirable feature from the stem scores better than a generic 'they are all good plants'.
Identifying asexual reproduction from an unfamiliar case and justifying it: a two-mark Identify or Explain following a short paragraph or a picture of a runner, tuber, bulb, spore case or dividing cell The word 'asexual' plus the evidence: only one parent is involved and no gametes are produced or nuclei fused, so the offspring are genetically identical. A bare 'asexual' with no justification usually takes only one of the two marks.
Stating what asexual reproduction is, in the syllabus's exact words: a one- or two-mark Describe or Define, normally the opening part of a reproduction question 'Genetically identical offspring' and 'from one parent' — both phrases, in those words. 'Identical offspring' without 'genetically' is not safe, and 'the offspring are the same as the parent' is too vague to credit.
The advantage of asexual reproduction to a wild population, which candidates forget exists: a two-mark Suggest, often as the first half of a discuss-style question whose second half is about the risks Only one parent is needed, so no mate has to be found and reproduction can happen even when the species is scarce or isolated; and the population can increase rapidly, with every offspring inheriting a genotype already successful in that habitat. Writing only disadvantages here scores nothing, however good they are.
Self-pollination is still sexual reproduction: a one- or two-mark Explain, in a stem designed to look like asexual reproduction because only one plant is involved Gametes are produced and the nuclei of two gametes fuse at fertilisation, so it is sexual reproduction and the offspring are genetically different. The number of parents is not the deciding test — the fusion of gamete nuclei is.

Words to know

Asexual reproduction: A process resulting in the production of genetically identical offspring from one parent.
Clone: A group of genetically identical individuals produced from one parent by asexual reproduction.
Mitosis: Nuclear division giving rise to genetically identical cells. It is the division used in asexual reproduction.
Runner: A horizontal stem growing out from a parent plant along the ground, from which a new, genetically identical plant develops where it touches the soil.
Tuber: A swollen underground stem, such as a potato, bearing buds that can each grow into a new plant.
Spore: A single reproductive cell, released in large numbers by fungi and some other organisms, that grows into a new individual without any fusion of nuclei.
Genetic variation: Differences in the genetic material between individuals of the same species. Asexual reproduction produces offspring with no genetic variation between them.

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