Sexual and Asexual Reproduction

Understanding the difference between sexual and asexual reproduction is fundamental to the AQA GCSE Combined Science Trilogy specification (8464). This lesson covers how organisms reproduce, the advantages and disadvantages of each type, and how meiosis produces gametes with genetic variation.

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Two Types of Reproduction

All living organisms reproduce to pass on their genes to the next generation. There are two fundamentally different strategies:

Feature	Sexual Reproduction	Asexual Reproduction
Number of parents	Two	One
Gametes involved?	Yes (egg and sperm)	No
Type of cell division	Meiosis (to make gametes)	Mitosis
Genetic variation in offspring	Yes — offspring are genetically different from parents	No — offspring are genetically identical (clones)
Speed of reproduction	Slower	Faster

Exam Tip: A very common exam question asks you to compare sexual and asexual reproduction. Make sure you can list at least three differences using the correct terminology — gametes, meiosis, mitosis, variation, clones.

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Sexual Reproduction

Sexual reproduction involves the fusion of two specialised sex cells called gametes. This fusion is called fertilisation and produces a cell called a zygote, which then divides by mitosis to form a new organism.

Gametes

Organism	Male Gamete	Female Gamete
Animals	Sperm cell	Egg cell (ovum)
Flowering plants	Pollen grain (contains male nucleus)	Ovule (contains egg cell)

Each gamete contains half the normal number of chromosomes. In humans, body cells have 46 chromosomes (23 pairs), so each gamete has 23 chromosomes. When two gametes fuse at fertilisation, the full number of 46 is restored.

$\text{Gamete (23)} + \text{Gamete (23)} \xrightarrow{\text{fertilisation}} \text{Zygote (46)}$Gamete (23)+Gamete (23)fertilisation​Zygote (46)

Meiosis

Meiosis is the type of cell division that produces gametes. It is sometimes called reduction division because the chromosome number is halved.

Key features of meiosis:

1. The cell copies its DNA so there are two copies of each chromosome
2. The cell divides twice to produce four daughter cells
3. Each daughter cell has half the number of chromosomes of the parent cell
4. Each daughter cell is genetically different from the others

graph TD
    A["Parent cell<br/>(46 chromosomes)<br/>DNA replicates"] --> B["First division<br/>Chromosome pairs<br/>separate"]
    B --> C["Cell 1<br/>(23 chromosomes)"]
    B --> D["Cell 2<br/>(23 chromosomes)"]
    C --> E["Gamete 1"]
    C --> F["Gamete 2"]
    D --> G["Gamete 3"]
    D --> H["Gamete 4"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#c8e6c9,stroke:#2e7d32
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#fff9c4,stroke:#f9a825
    style E fill:#ffccbc,stroke:#d84315
    style F fill:#ffccbc,stroke:#d84315
    style G fill:#ffccbc,stroke:#d84315
    style H fill:#ffccbc,stroke:#d84315

Exam Tip: Do not confuse mitosis and meiosis. Mitosis produces two identical daughter cells (for growth and repair). Meiosis produces four genetically different daughter cells (gametes) with half the chromosome number.

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Why Does Sexual Reproduction Produce Variation?

Sexual reproduction produces variation in offspring for two reasons:

1. Meiosis — during meiosis, chromosomes are shuffled randomly, so each gamete receives a different combination of alleles
2. Fertilisation — the random fusion of any male gamete with any female gamete creates a unique combination of genetic information

This genetic variation is important because it means that some offspring may have characteristics that help them survive changes in the environment. This is the basis for natural selection.

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Asexual Reproduction

Asexual reproduction involves only one parent and no fusion of gametes. The offspring are produced by mitosis and are genetically identical to the parent — they are clones.

Examples of Asexual Reproduction

Organism	Method
Bacteria	Binary fission — the cell divides into two identical cells
Strawberry plants	Runners (stolons) — horizontal stems grow along the ground and produce new plants
Daffodils	Bulb division — the bulb splits to form new bulbs
Hydra	Budding — a new organism grows from the parent and breaks off
Fungi	Spore production — genetically identical spores grow into new organisms

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Advantages and Disadvantages

	Sexual Reproduction	Asexual Reproduction
Advantages	Produces genetic variation, increasing the chance of survival if conditions change; natural selection can occur	Faster; only one parent needed; successful characteristics are always passed on; large populations can be produced quickly
Disadvantages	Slower; requires finding a mate; not all offspring may be well-suited to the environment	No genetic variation; if the environment changes or a new disease appears, all individuals are equally vulnerable

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Organisms That Use Both Strategies

Some organisms can reproduce both sexually and asexually, switching between the two depending on conditions:

Organism	When Asexual	When Sexual
Strawberry plants	Produce runners in favourable conditions to colonise an area quickly	Produce flowers and seeds for genetic variation
Fungi	Produce spores by mitosis in stable conditions	Produce spores by meiosis when conditions are stressful
Malarial parasite	Reproduces asexually inside human red blood cells	Reproduces sexually inside the mosquito

Exam Tip: If asked why an organism uses asexual reproduction in good conditions and sexual reproduction in bad conditions, the key point is: asexual reproduction is fast and efficient for colonising a stable environment, but sexual reproduction produces variation that increases the chance of some offspring surviving a change.

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Common Mistakes

Mistake	Correction
Saying meiosis produces two cells	Meiosis produces four cells
Saying gametes have 46 chromosomes	Gametes have 23 chromosomes (half the normal number)
Saying clones have "no DNA"	Clones have the same DNA as the parent — they are not lacking DNA
Confusing mitosis and meiosis	Mitosis = growth/repair, 2 identical cells; Meiosis = gametes, 4 different cells

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Summary

• Sexual reproduction involves two parents and the fusion of gametes (fertilisation). Offspring are genetically different from parents.
• Asexual reproduction involves one parent and mitosis. Offspring are genetically identical clones.
• Meiosis is the type of cell division that produces gametes. It halves the chromosome number and produces genetic variation.
• In humans, body cells have 46 chromosomes and gametes have 23 chromosomes.
• Sexual reproduction produces variation through meiosis and random fertilisation; this is important for natural selection and adaptation.
• Some organisms can switch between sexual and asexual reproduction depending on environmental conditions.

Exam Tip: AQA (8464) commonly asks you to explain why sexual reproduction is important for evolution. The answer must include the ideas of genetic variation, natural selection and adaptation — without variation, evolution cannot occur.

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Worked Example: Chromosome Counting in Meiosis vs Mitosis

A type of onion has 16 chromosomes in each body cell. Calculate the number of chromosomes in:

1. A cell about to undergo mitosis after DNA replication
2. A pollen grain (gamete) produced by meiosis
3. A cell produced by fertilisation between two onion gametes

Working:

• After DNA replication, each chromosome has two chromatids but the chromosome number is still 16 (replication copies DNA, it does not make new chromosomes).
• A gamete has half the diploid number: $\frac{16}{2} = 8 \text{ chromosomes}$216​=8 chromosomes
• A zygote receives 8 chromosomes from each gamete: $8 + 8 = 16 \text{ chromosomes}$8+8=16 chromosomes

Common Mistake Callout: Students often write that DNA replication doubles the chromosome number. It does not. Replication doubles the amount of DNA (each chromosome gains a second chromatid) but the number of chromosomes is unchanged until the cell divides.

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Comparison Table: Mitosis vs Meiosis at a Glance

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Making gametes for sexual reproduction
Daughter cells produced	2	4
Chromosome number in daughters	Same as parent (diploid)	Half of parent (haploid)
Genetically identical?	Yes (to parent and each other)	No (all four are different)
Number of divisions	One	Two
DNA replication before division?	Yes	Yes (only once, before first division)
Occurs in	All body cells	Only in reproductive organs (testes, ovaries)

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Diagram: Life Cycle Summary

graph LR
    A["Diploid body cell<br/>(46 chromosomes)"] -->|Meiosis| B["Haploid gamete<br/>(23 chromosomes)"]
    C["Haploid gamete<br/>(23 chromosomes)"] -->|Fertilisation| D["Diploid zygote<br/>(46 chromosomes)"]
    B --> D
    D -->|Mitosis, repeated| E["Diploid embryo,<br/>fetus, adult"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#fff9c4,stroke:#f9a825
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#ffccbc,stroke:#d84315

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Why Strawberry Growers Use Both Methods: an Applied Example

A commercial strawberry grower wants large numbers of plants that all produce the same sweet fruit, but also wants to be ready if a new fungal disease appears.

• Asexual runners (stolons) give many clones of a proven plant. Offspring fruit tastes identical every season, guaranteeing product consistency.
• Sexual reproduction (flowers, pollination, seeds) generates variation. Breeders can then select offspring with disease resistance when a new pathogen emerges.

This is a real-world illustration of why the two strategies exist side by side in many species — and a typical AQA application question.

Answering at different grade levels

AQA uses response bands based on how precisely key terminology is used and how coherently ideas are linked.

• Grade 3 answer (basic recall): "Sexual reproduction needs two parents and asexual only one. Sexual makes variation." Correct but vague; fails to define the key terms or explain why variation matters.
• Grade 5 answer (clear explanation): "Sexual reproduction involves the fusion of male and female gametes at fertilisation to form a zygote. Meiosis halves the chromosome number so gametes are haploid. This produces genetic variation. Asexual reproduction uses mitosis, has one parent and produces clones that are genetically identical."
• Grade 7+ answer (analytical and linked): "In sexual reproduction, meiosis generates haploid gametes; independent assortment and random fertilisation create new allele combinations, so offspring show genetic variation. This variation is the raw material of natural selection, allowing populations to adapt when selection pressures change. Asexual reproduction, by mitosis, yields clones, which is efficient in stable environments but leaves populations vulnerable because there is no allelic variation on which selection can act."
• Grade 8–9 answer: Adds an evaluative dimension: "An organism like the strawberry uses asexual reproduction via runners to exploit favourable local conditions quickly, while still using sexual reproduction to maintain biotic resilience against pathogens — demonstrating how life-cycle strategy itself is shaped by natural selection."

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Extended Application: Why Bananas Need Sexual Reproduction Too

The commercial Cavendish banana is propagated almost entirely by asexual reproduction (offshoots from the parent plant). This gives identical, seedless fruit — a huge commercial advantage — but it also means every Cavendish banana in the world is a genetic clone of every other.

A single new fungal disease, Tropical Race 4 (TR4) of Fusarium wilt, is currently spreading through Cavendish plantations. Because there is no genetic variation between plants, there are no resistant individuals and the fungus spreads unchecked. The entire Cavendish variety may need to be replaced — exactly as its predecessor, the Gros Michel, was wiped out in the 1950s.

This illustrates the central AQA idea: asexual reproduction is fast and efficient but fragile in the face of a new biotic threat. Sexual reproduction, with its variation, is evolution's insurance policy.

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Quick Retrieval Questions

1. State two differences between sexual and asexual reproduction.
2. How many chromosomes are in a human gamete, and why?
3. Define the term zygote.
4. Give one advantage and one disadvantage of asexual reproduction.
5. Name two organisms that can switch between sexual and asexual reproduction.
6. Explain why meiosis is called "reduction division".
7. Why does sexual reproduction produce variation?
8. Explain why bacteria growing by binary fission can adapt to a new antibiotic despite being clones.
9. Suggest one reason a farmer might choose asexual propagation of apple trees rather than growing from seed.

AQA alignment: This content is aligned with AQA GCSE Combined Science: Trilogy (8464) specification section 4.6 Inheritance, variation and evolution — specifically 4.6.1.1 Sexual and asexual reproduction and 4.6.1.2 Meiosis. Assessed on Biology Paper 2.