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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.
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.
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.
| 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.
$$\text{Gamete (23)} + \text{Gamete (23)} \xrightarrow{\text{fertilisation}} \text{Zygote (46)}$$
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:
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.
Sexual reproduction produces variation in offspring for two reasons:
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.
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.
| 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 |
| 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 |
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.
| 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 |
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.