Sexual and Asexual Reproduction

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.1 — Differences between sexual and asexual reproduction
• 3.2 — Fertilisation as the fusion of gametes to produce a zygote that develops into an embryo
• 3.7 — Asexual reproduction in plants by natural methods (runners) and artificial methods (cuttings)

Specification section 3(a) Reproduction — also taught in Flowers, Pollination and Fertilisation, Seeds and Germination, The Human Reproductive Systems, The Menstrual Cycle.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A strawberry plant can make a new strawberry plant in two entirely different ways, and it does both in the same summer. It flowers, is pollinated, and sets seed — and each seed grows into a plant that is not quite like either parent. It also sends a long stem sideways across the soil, which roots where it touches down and becomes a new plant that is an exact copy of the one that made it. Same plant, same season, two strategies. The interesting question is not how each works but why anything would bother with both.

Sexual reproduction — where the difference comes from

1. Two special sex cells, called GAMETES, are made — one by each parent. In animals these are the sperm and the egg cell; in flowering plants, the pollen nucleus and the ovule.
2. A gamete carries only HALF the normal number of chromosomes, so that when two combine the total comes back to normal rather than doubling every generation.
3. FERTILISATION is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.
4. The single cell this produces is a ZYGOTE, and it now has the full number of chromosomes — half from each parent.
5. The zygote divides repeatedly to form an EMBRYO, which develops into the new organism.
6. Because its chromosomes came from two different individuals, the offspring is genetically different from both parents and from every other offspring. That is where variation comes from, and it is the whole point.

Asexual reproduction skips all of that. There are no gametes, no fusion and only ONE parent. The new individual is produced by ordinary cell division, so it receives an exact copy of the parent's genetic material and is genetically identical to it. A group of genetically identical organisms produced this way is called a CLONE — a word that describes the relationship between them, not a laboratory technique.

Think of it like copying a recipe against writing one with someone else

Photocopy a recipe and you get exactly what you had, instantly, as many times as you like — and if there was a mistake in it, every copy has the same mistake. Sit down with somebody else and combine their recipe with yours, and what comes out is slower to produce, takes two people, and might be worse than either. But it might also be better, and it will certainly be different. Now suppose the kitchen changes — new oven, different ingredients. The photocopies are all equally unsuited to it. Among the combined versions, some will happen to suit the new kitchen better. Neither approach is superior; they answer different questions, which is exactly why the strawberry does both.

The trade-off, stated as it will be examined

1. ASEXUAL: only one parent needed, so no mate has to be found and no pollinator has to arrive. It is fast, and every offspring inherits a set of characteristics that has already been shown to work in this exact place.
2. ASEXUAL, the cost: every offspring is identical, so if the environment changes — a new disease, a drought, a colder winter — a weakness that affects one affects all of them, and the whole population can be destroyed together. If none of them carries RESISTANCE to a new disease, then none of them does, and there is no exception anywhere in the population.
3. SEXUAL: offspring vary, so a change in the environment is likely to find some of them able to survive it. Where a new disease appears, a few individuals may happen to carry resistance to it already — and it has to be already, because variation is not produced in response to a need. Variation is also what natural selection needs to work on, so it is the route by which a species can change over time.
4. SEXUAL, the cost: it needs two parents, it is slower, and it requires gametes to meet at all — which for a plant means a pollinator or the wind, and for an animal means finding a mate.

Plants do asexual reproduction naturally in several ways, and the specification asks for RUNNERS. A strawberry sends out a horizontal stem, called a runner or stolon, which grows away across the surface of the soil. Where a node on that runner touches the ground it grows roots downwards and a shoot upwards, and once that new plant is established the runner connecting them withers away. The result is a genetically identical daughter plant a little distance from the parent, already supplied by the parent while it established itself.

The artificial version: taking a cutting

1. Cut a healthy shoot from the parent plant, just below a node, using a clean blade.
2. Remove the lower leaves so that the buried part does not rot, and to reduce water loss by transpiration while the cutting has no roots.
3. Dip the cut end in rooting powder, which contains plant hormones that promote root growth.
4. Plant it in damp compost, and cover it with a clear plastic bag or a propagator to keep the air around it humid, so it loses less water before it can take any up.
5. Roots form at the cut end, and the cutting becomes an independent plant — genetically identical to the one it was cut from.

Notice what a grower gets out of that, because this is what application questions ask. A cutting is quick, needs no pollinator, and — the point that matters commercially — produces a plant with exactly the characteristics of the parent. If you have one apple tree whose fruit is outstanding, growing its seeds will not give you outstanding apples, because each seed is a genetic mixture. Cuttings and grafts will. Every Bramley apple tree in the world descends by cutting from a single tree grown from a pip in Nottinghamshire around 1809, which is also why every one of them is vulnerable to the same diseases.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Sexual reproduction

Reproduction involving the fusion of the nuclei of two gametes, one from each of two parents, producing offspring that are genetically different from the parents and from each other.

Learn this definition · Asexual reproduction

Reproduction involving only one parent and no fusion of gametes, producing offspring that are genetically identical to the parent.

Learn this definition · Fertilisation

The fusion of the nucleus of a male gamete with the nucleus of a female gamete to produce a zygote.

The four words in order — the sequence a question can ask you to place

Gamete

A sex cell, containing half the normal number of chromosomes. In animals, the sperm and the egg cell; in flowering plants, the pollen nucleus and the ovule.

Fertilisation

The fusion of a male gamete nucleus with a female gamete nucleus.

Zygote

The single cell produced by fertilisation, with the full number of chromosomes — half from each parent.

Embryo

The ball of cells formed when the zygote divides repeatedly, which develops into the new organism.

Clone

A group of genetically identical organisms produced from one parent by asexual reproduction.

Resistance

The ability of some individuals to survive a disease or condition that kills others. It is inherited, and must already be present in the population — it is not produced in response to the threat.

Statement 3.1 — the comparison, written the way marks are given

Taking a cutting — the method, with the reason for each step

1. Cut a healthy shoot from the parent plant just below a node, with a clean blade — a clean cut reduces the risk of infection.
2. Remove the lower leaves, so the buried part does not rot and less water is lost by transpiration.
3. Dip the cut end in rooting powder, which contains plant hormones that promote root growth.
4. Plant in damp compost and cover with a clear plastic bag or propagator, keeping the air humid so water loss is reduced while the cutting has no roots.
5. Roots grow from the cut end; the new plant is genetically identical to the parent.

Runners — the natural method, in four lines

• A horizontal stem, called a runner or stolon, grows outwards from the parent plant across the surface of the soil
• Where a node on the runner touches the soil, roots grow downwards and a shoot grows upwards
• The parent supplies the new plant until it is established, after which the connecting runner withers
• The daughter plant is genetically identical to the parent — strawberry and spider plant are the standard examples

Why a grower chooses one method or the other

A. ASEXUAL (cuttings, runners): every offspring has exactly the parent's characteristics, so a good variety is reproduced unchanged — and it is fast and needs no pollinator.

B. SEXUAL (seed): offspring vary, so a valued variety is NOT reproduced faithfully — but the variation is what allows new varieties to arise and what protects a population against a new disease.

Not this: Asexual reproduction means an organism reproduces without a partner, so the offspring are its children in the ordinary way.

This: There is no fusion of gametes at all, so no genetic mixing occurs. The offspring are genetically identical to the single parent — copies, not combinations.

Model answer [3 marks]

Explain why the offspring of sexual reproduction are genetically different from their parents. [3]

Each gamete contains half the normal number of chromosomes, and the two gametes come from two different parents. At fertilisation the nuclei of a male and a female gamete fuse to form a zygote, so the offspring receives half of its chromosomes from each parent. The combination of chromosomes it receives is different from that of either parent and from that of any other offspring, which produces variation.

Model answer [4 marks]

A grower has one apple tree that produces exceptional fruit. Explain why they take cuttings from it rather than planting its seeds. [4]

A cutting is a form of asexual reproduction, so the new plants are genetically identical to the parent tree and will produce fruit with exactly the same characteristics. Seeds are produced by sexual reproduction, so each one contains a different combination of chromosomes from two parents and would grow into a tree whose fruit is unpredictable and probably not as good. Cuttings are also faster and do not depend on pollination.

Model answer [3 marks]

Explain one disadvantage to a population of reproducing only asexually. [3]

All the offspring are genetically identical, so there is no variation between them. If the environment changes — for example a new disease appears — then any weakness present in one individual is present in all of them. None is likely to survive where the others cannot, so the whole population may be destroyed.

Mark-losing trap. Fertilisation is the fusion of two gamete NUCLEI. 'The sperm meets the egg' is not the definition and rarely scores.

Mark-losing trap. Asexual offspring are genetically IDENTICAL to the parent — not 'similar', not 'like'. The word identical is the mark.

Mark-losing trap. Say two PARENTS, not two gametes, when asked for the difference in number. The gametes are the next mark, not the same one.

Mark-losing trap. Variation is the advantage of sexual reproduction. Then say what variation is FOR — surviving a change in the environment.

Mark-losing trap. A zygote is one cell; an embryo is what it becomes after dividing. Do not use the words interchangeably.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Define [1 mark] — What is fertilisation?
2. Grade 7 · Explain [2 marks] — A gardener takes twenty cuttings from one lavender bush. What can be said about the genetic make-up of the twenty new plants, and why?
3. Grade 8 · Explain [5 marks] — Explain how sexual reproduction produces offspring that are genetically different from both parents. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — A strawberry plant produces 6 runners in its first year, and every plant produced by a runner itself produces 6 runners the following year. Assuming every plant survives and none reproduces sexually, calculate the total number of plants present at the end of the third year, counting the original plant.
5. 9+ · Analyse [5 marks] — A commercial banana variety is grown worldwide entirely from cuttings, because it produces no fertile seeds. A fungal disease appears that this variety cannot resist, and within a few years plantations on several continents are destroyed. A wild banana population in the same region, which reproduces from seed, survives with heavy losses. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who worked out what the offspring inherit, and the one who explained why variation is worth the trouble. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Gregor Mendel — the one who worked out what the offspring inherit

This page claims that offspring of sexual reproduction receive half their inheritance from each parent, and that the combination is what makes each one different. Mendel is where that claim comes from. By crossing pea plants and counting tens of thousands of offspring, he showed that inherited characteristics come in discrete units, that each parent contributes one of each pair, and that these separate and recombine in predictable ratios. Nobody paid attention for thirty-four years, and he died an abbot rather than a scientist.

• “Why did you count so many plants instead of describing a few?”
• “What did you conclude each parent passes on to its offspring?”
• “Why do offspring of the same two parents differ from each other?”
• “What would you expect if a plant reproduced without a partner at all?”
• “Did it trouble you that nobody read your paper?”

Charles Darwin — the one who explained why variation is worth the trouble

The costs of sexual reproduction listed on this page are real — two parents, slower, dependent on a pollinator arriving — and Darwin supplies the reason they are worth paying. His argument is that where offspring vary, some will by chance be better suited to conditions as they change, and those are the ones that survive to reproduce. Without variation there is nothing for that process to act on, which is precisely why a purely asexual population is defenceless against a new disease. He also spent years on the reproductive arrangements of orchids and primroses, largely to work out what all that machinery was avoiding.

• “Why is it worth the cost of needing two parents?”
• “What happens to a population in which every individual is identical?”
• “Why did you spend so long studying how flowers avoid fertilising themselves?”
• “Is variation useful in itself, or only when conditions change?”

Carry on through the course

• Previous lesson: Gas Exchange in Plants, Day and Night
• Next lesson: Flowers, Pollination and Fertilisation
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• The Placenta and the Amniotic Fluid — An exchange surface with a constraint no other one has: two bloodstreams that must trade materials without ever mixing.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification