4.6.1 Reproduction

Why this matters

Why do most animals and plants bother with sex at all? Sexual reproduction is slow, needs two parents, and only half of your genes get passed on. Asexual reproduction is fast, needs only one parent, and passes on 100% of the parent's genome. The trade-off is variation. Sexual reproduction shuffles two parents' DNA into offspring that are all genetically unique — and in a changing environment (new diseases, new climate, new predators) that variation is what lets some individuals survive. Asexual reproduction is brilliant when conditions are stable and one good genotype can fill the habitat fast — bacteria splitting every twenty minutes, strawberries sending out runners across a damp meadow, a potato field sprouting clones from buried tubers. Many organisms hedge their bets and do both: strawberries flower AND send runners; the malaria parasite reproduces asexually inside humans but sexually inside the mosquito. The cellular machinery behind both is the cell-division pair you already met — mitosis (one parent cell → two identical diploid daughters) and meiosis (one parent cell → four genetically different haploid gametes).

How to learn this topic

Build on what you already know

• GCSE 4.1.2: mitosis produces two genetically identical diploid daughter cells; the cell cycle.
• GCSE 4.1.1: chromosomes are made of DNA; humans have 46 chromosomes (23 pairs).
• KS3: animals have male and female reproductive organs that make sperm and eggs; plants make pollen.

1. Define sexual vs asexual reproduction by counting parents and listing what is/is not shared.
2. Meiosis: one diploid cell → four genetically different haploid gametes; two divisions.
3. Why gametes need to be haploid — and why fertilisation restores the diploid number.
4. Compare mitosis (clones) and meiosis (varied gametes) side by side.
5. Sexual reproduction in plants: pollen + ovule → fertilisation → seed → varied offspring.
6. Asexual examples: binary fission in bacteria; runners in strawberries; tubers in potatoes.
7. Advantages and disadvantages of each method — and why some organisms use both.

Key terms

sexual reproduction

Reproduction involving two parents and the fusion of male and female gametes. Offspring inherit alleles from both parents and are genetically different from each other. (AQA wants 'fusion of gametes' AND 'two parents' AND 'mixing of genetic information' — three distinct marking points.)

asexual reproduction

Reproduction involving only one parent, with no fusion of gametes and no mixing of genetic information. Uses mitosis only; offspring are genetically identical clones of the parent. (Three required phrases: 'one parent', 'no gametes / no fertilisation', 'mitosis / identical offspring'.)

gamete

A sex cell — sperm or egg in animals, pollen or ovule in plants. Haploid (half the normal chromosome number) and produced by meiosis.

meiosis

A type of cell division that happens in reproductive organs. One diploid parent cell divides twice to produce four genetically different haploid gametes. (Examiner-ready triplet: 'meiosis produces gametes', 'gametes have half the chromosome number', 'gametes are genetically non-identical'.)

fertilisation

The fusion of a male gamete with a female gamete to form a zygote. Restores the full (diploid) chromosome number. (Always say 'restores the full chromosome number' or 'restores the diploid number' on meiosis questions.)

zygote

The single diploid cell formed when a sperm fertilises an egg. Divides repeatedly by mitosis to form an embryo.

diploid

Having two sets of chromosomes (the full number). In humans, 46 chromosomes — 23 pairs. Body cells are diploid. (AQA usually just asks for 'full chromosome number' — using 'diploid' is a bonus.)

haploid

Having one set of chromosomes (half the full number). In humans, 23 chromosomes. Gametes are haploid. (Often phrased as 'half the number of chromosomes' on the mark scheme — use that exact wording.)

clone

An organism that is genetically identical to its single parent. Produced by asexual reproduction (mitosis only). (Marking phrase: 'genetically identical offspring'. Don't say 'similar' or 'almost the same'.)

binary fission

Asexual reproduction in bacteria. The cell copies its DNA and divides into two genetically identical daughter cells. (Specifically named in the AQA spec as the bacterial example of asexual reproduction.)

runner

A horizontal stem produced by some plants (e.g. strawberry) that grows along the ground and develops new genetically identical daughter plants where it touches the soil. (AQA strawberry questions: 'runners produce clones / identical offspring'.)

tuber

A swollen underground stem used for food storage (e.g. potato). Has buds that can sprout into new plants — a form of asexual reproduction. (Often paired with strawberries as the second classic plant asexual example.)

Notes

Two ways to make new organisms

There are exactly two forms of reproduction in GCSE biology and AQA wants you to compare them carefully.

| Feature | Sexual reproduction | Asexual reproduction |

|---|---|---|

| Parents needed | Two | One |

| Gametes involved | Yes — male and female fuse | No gametes |

| Cell division used | Meiosis (gametes) + mitosis (growth of zygote) | Mitosis only |

| Genetic mixing | Offspring inherit alleles from BOTH parents | All DNA from the single parent |

| Offspring | Genetically different from each parent and from each other — variation | Genetically identical to the parent — clones |

Both use cell division, but the kind of division is what controls whether you get clones or variation.

Meiosis — making gametes

Meiosis is the special cell division that happens only in the reproductive organs (testes, ovaries, anthers, ovaries of plants). It makes gametes — sperm and egg cells in animals, pollen and ovules in plants.

The AQA sequence to memorise:

1. The cell copies its chromosomes (the same as in mitosis).
2. The cell divides twice in a row (this is what makes meiosis different from mitosis).
3. The result is four daughter cells.
4. Each daughter cell has half the number of chromosomes of the parent — they are haploid.
5. All four gametes are genetically different from each other because chromosomes are shuffled at each division.

In a human body cell there are 46 chromosomes (the diploid number). After meiosis each gamete has 23 chromosomes (the haploid number) — one from each pair.

Fertilisation — restoring the diploid number

At fertilisation, a sperm (23 chromosomes) fuses with an egg (23 chromosomes) to form a zygote with 46 chromosomes — half from each parent. This is why offspring inherit alleles from both parents and end up genetically unique.

The zygote then divides by mitosis to form an embryo, then a foetus, then a baby — all those cells are genetically identical to each other (and to the original zygote), but different from either parent because of how the two sets of 23 chromosomes combined.

Mitosis vs meiosis — the comparison examiners love

| | Mitosis | Meiosis |

|---|---|---|

| Divisions | 1 | 2 |

| Daughter cells | 2 | 4 |

| Chromosome number | Same as parent (diploid) | Half of parent (haploid) |

| Genetically | Identical to parent | All different |

| Where in the body | Almost everywhere — growth & repair | Only in reproductive organs |

| Used for | Growth, repair, asexual reproduction | Making gametes for sexual reproduction |

A single marking-phrase summary: meiosis produces non-identical gametes; mitosis produces identical copies.

Sexual reproduction in plants

Plants do exactly the same thing as animals but with different vocabulary.

• Pollen = male gamete (in the anther of the stamen).
• Ovule = female gamete (in the ovary of the carpel).
• Pollination = pollen transferred to the stigma (by wind, insects, etc.).
• Fertilisation = pollen nucleus fuses with ovule nucleus inside the ovary.
• The fertilised ovule develops into a seed; the ovary becomes the fruit.
• The seeds grow into new plants that are genetically varied because they have two parents.

Asexual reproduction — three examples

### 1. Binary fission (bacteria)

A bacterium copies its DNA, elongates, and splits into two identical daughter cells. Only mitosis-like division — no gametes, no fertilisation. In ideal conditions, bacteria can divide every 20 minutes, producing huge clonal colonies fast.

### 2. Strawberry runners

Strawberry plants send out long horizontal stems called runners. Where a runner touches the soil it grows roots and shoots, forming a new plant. Each new plant is genetically identical to the parent — a clone — because it grew from parent cells by mitosis only.

### 3. Potato tubers

The potatoes you eat are swollen underground stems (tubers) used to store food. Each tuber has buds that can sprout into a new plant, all genetically identical to the parent.

In all three cases: one parent, mitosis only, identical offspring.

Advantages and disadvantages

### Sexual reproduction

• Advantage: produces variation, so some offspring may survive new diseases or changes in the environment — the raw material for natural selection.
• Disadvantages: slower; needs two parents to find each other; only half the parent's DNA is passed on; uses energy on mate-finding, flowering, etc.

### Asexual reproduction

• Advantages: only one parent needed; faster; less energy spent finding a mate; useful characteristics are preserved exactly (farmers cloning a productive cow keep ALL the desirable alleles).
• Disadvantages: no variation, so if one individual is vulnerable to a disease, the whole population is; cannot adapt to a changing environment as easily.

Some organisms use both

The AQA spec singles out the malaria parasite Plasmodium: it reproduces asexually inside the human host (so it can multiply quickly during infection) and sexually inside the mosquito (creating varied offspring to spread to new hosts). Many plants — strawberries, daffodils, brambles — also do both: they make flowers AND clone themselves vegetatively, hedging their bets.

The principle: asexual when conditions are good (fast, low cost), sexual when conditions are changing or uncertain (variation gives survival options).

Genome and DNA (very brief — fuller story in 4.6.3)

The genome is the entire genetic material of an organism. Every cell carries the same genome, packaged into chromosomes made of DNA. Sexual reproduction shuffles the genome from two parents into varied offspring; asexual reproduction copies one parent's genome exactly. The Human Genome Project (sequencing the full human genome) is covered in section 4.6.3.

Exam tips

• Count parents first. ONE parent = asexual. TWO parents = sexual. This single fact unlocks half the marks on most questions in this topic.
• When asked about meiosis, hit four phrases: 'produces gametes', 'half the chromosome number', 'genetically different', 'fertilisation restores the full number'. Each is worth a mark.
• Mitosis vs meiosis: '1 division → 2 identical' versus '2 divisions → 4 different (half chromosomes)'. Memorise this contrast verbatim.
• On asexual offspring questions, the marking word is 'identical' (or 'clones'). Never write 'similar' or 'almost the same' — examiners cross those out.
• Farmer / breeder questions about clones: the advantage is that DESIRABLE CHARACTERISTICS are PRESERVED exactly. Drop that phrase into any 'advantage of asexual' answer.
• Strawberry-type questions: name BOTH the runner (asexual, clones) AND the flower (sexual, varied offspring). Plants doing both is a classic AQA setup.
• When asked WHY sexual reproduction is useful, the marking phrase is 'variation' or 'offspring genetically different — useful if the environment changes'.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in AQA GCSE Biology (8461)

• 4.6.2 Variation and evolution
• 4.6.3 The development of understanding of genetics and evolution
• 4.6.4 Classification of living organisms