Reproduction: sexual and asexual — AQA GCSE Biology Revision Notes

What you'll learn

This guide covers everything you need to know about sexual and asexual reproduction for AQA GCSE Biology. You'll learn how organisms reproduce, the differences between the two main types of reproduction, and why these processes are essential for life. Understanding these concepts is crucial as they underpin genetics, evolution, and variation topics in your exam.

Key terms and definitions

Sexual reproduction — the fusion of male and female gametes (sex cells) to produce offspring with genetic variation from both parents

Asexual reproduction — reproduction involving only one parent, producing genetically identical offspring (clones) with no fusion of gametes

Gamete — a specialised sex cell (sperm in males, egg/ovum in females) containing half the normal number of chromosomes

Meiosis — cell division that produces four non-identical gametes, each with half the number of chromosomes of the parent cell

Mitosis — cell division that produces two genetically identical daughter cells with the same number of chromosomes as the parent cell

Fertilisation — the fusion of male and female gametes to form a zygote, restoring the full chromosome number

Clone — a genetically identical organism or cell produced by asexual reproduction

Diploid — a cell containing two complete sets of chromosomes (one from each parent), represented as 2n

Core concepts

Sexual reproduction

Sexual reproduction requires two parents and involves the fusion of specialised sex cells called gametes. In humans and most animals, the male gamete is the sperm cell and the female gamete is the egg cell (ovum). In flowering plants, male gametes are found in pollen grains and female gametes are in ovules.

The key stages of sexual reproduction are:

Production of gametes through meiosis
Fertilisation — fusion of male and female gametes
Formation of a zygote containing genetic information from both parents
Development of the zygote into a new organism

Characteristics of sexual reproduction

Genetic variation: Offspring inherit a mixture of genetic information from both parents, making them genetically different from either parent and from each other (except identical twins). This variation occurs because:

Each gamete receives a random selection of chromosomes during meiosis
Fertilisation is random — any sperm can fuse with any egg
Genetic material is mixed from two parents

Chromosome numbers: Human body cells are diploid, containing 46 chromosomes (23 pairs). Gametes must be haploid (containing 23 chromosomes — half the normal number) so that when fertilisation occurs, the diploid number is restored in the zygote.

Advantages of sexual reproduction:

Produces variation in offspring
Variation allows species to adapt to changing environments through natural selection
Increases chance of survival if environment changes
Selective breeding can be used to develop organisms with desirable characteristics

Disadvantages of sexual reproduction:

Requires two parents (finding a mate takes time and energy)
Slower than asexual reproduction
Requires more energy to produce gametes and find mates

Asexual reproduction

Asexual reproduction involves only one parent and produces offspring that are genetically identical to the parent organism. These identical offspring are called clones. No gametes are involved and no fertilisation occurs.

Asexual reproduction involves mitosis — a type of cell division that produces two identical daughter cells with the same number of chromosomes as the parent cell. The process creates exact copies of genetic information.

Examples of asexual reproduction

In plants:

Runners/stolons — strawberry plants produce horizontal stems (runners) that grow along the ground and develop new plants
Bulbs — daffodils and onions produce small bulbs around the parent bulb
Tubers — potatoes develop underground stems (tubers) that can grow into new plants
Cuttings — sections of plant stem can be removed and grown into new plants (gardeners use this method)

In animals:

Budding — hydra produce small outgrowths (buds) that develop into new individuals
Binary fission — bacteria divide into two identical cells
Fragmentation — some starfish can regenerate from fragments

In fungi:

Spore production — fungi release spores that develop into new organisms genetically identical to the parent

Advantages of asexual reproduction

Only one parent needed — no time or energy spent finding a mate
Faster than sexual reproduction
Many identical offspring can be produced rapidly
Successful characteristics are guaranteed to pass to offspring
Useful in stable, favourable environments

Disadvantages of asexual reproduction

No genetic variation in offspring (except for rare mutations)
Population vulnerable to changes in environment — if one organism is susceptible to disease, all are
Cannot adapt to changing environments through natural selection
Overcrowding may occur if too many offspring produced in same area

Meiosis and gamete formation

Meiosis is essential for sexual reproduction because it produces gametes with half the normal number of chromosomes. This ensures that when fertilisation occurs, the offspring has the correct number of chromosomes.

Key features of meiosis

Location: Occurs in reproductive organs (testes and ovaries in animals; anthers and ovaries in flowering plants)

Process:

Cell duplicates its genetic information, forming two copies of each chromosome
Cell divides twice to form four gametes
Each gamete has a single set of chromosomes (haploid)
Gametes are genetically different from each other

Outcome: Four non-identical haploid gametes produced from one diploid parent cell

In humans:

Body cells contain 46 chromosomes (23 pairs)
Meiosis produces gametes with 23 chromosomes
Fertilisation restores 46 chromosomes in the zygote

Comparison with mitosis

Mitosis:

Produces two daughter cells
Daughter cells are genetically identical to parent cell
Maintains chromosome number (diploid to diploid)
Used for growth, repair, and asexual reproduction
Occurs throughout the body

Meiosis:

Produces four daughter cells
Daughter cells are genetically different from parent cell and each other
Halves chromosome number (diploid to haploid)
Only produces gametes for sexual reproduction
Only occurs in reproductive organs

Sexual and asexual reproduction in plants

Many plants can reproduce both sexually and asexually, giving them flexibility to adapt to different conditions.

Sexual reproduction in flowering plants

Structure: Flowers contain male parts (stamens with anthers and filaments) and female parts (carpels with stigma, style, and ovary)

Process:

Pollination — pollen grains transferred from anther to stigma (by wind, insects, or other animals)
Pollen tube grows down through the style to the ovary
Fertilisation — male nucleus from pollen fuses with female nucleus in ovule
Fertilised ovule develops into a seed
Ovary develops into a fruit

Advantages for plants:

Seeds can be dispersed away from parent plant, reducing competition
Seeds can remain dormant until conditions are favourable
Genetic variation produced

Asexual reproduction in plants

Plants use asexual reproduction when conditions are favourable and they want to colonise an area quickly. Methods include runners, bulbs, tubers, and cuttings (as described earlier).

Gardeners exploit asexual reproduction by:

Taking cuttings to produce many identical plants with desirable characteristics
Dividing bulbs or tubers to create new plants
Using tissue culture to mass-produce rare or valuable plants

Organisms using both strategies

Some organisms can reproduce both sexually and asexually depending on environmental conditions. This gives them the benefits of both methods.

Fungi:

Asexual reproduction through spores when conditions are favourable (rapid colonisation)
Sexual reproduction when conditions become unfavourable (produces variation for adaptation)

Parasites (e.g., Plasmodium causing malaria):

Asexual reproduction in human host (rapid multiplication)
Sexual reproduction in mosquito (produces variation)

Strawberry plants:

Asexual reproduction via runners during growing season (rapid spread)
Sexual reproduction via flowers and seeds (genetic variation, seed dispersal)

This flexibility allows organisms to take advantage of favourable conditions while maintaining the ability to adapt to changes.

Worked examples

Example 1: Comparing reproduction types (4 marks)

Question: A gardener has one rose bush with beautiful flowers. Describe one method the gardener could use to produce more identical rose bushes and explain why this method produces identical plants.

Answer: The gardener could take cuttings from the rose bush (1 mark). This is a form of asexual reproduction (1 mark) which involves mitosis (1 mark). Mitosis produces genetically identical cells, so the new plants are clones of the parent plant (1 mark).

Examiner note: The question asks for one method and an explanation. Make sure you name the specific method (cuttings), identify it as asexual reproduction, and explain the biological mechanism (mitosis creating identical genetic material).

Example 2: Meiosis and chromosome numbers (3 marks)

Question: A chicken body cell contains 78 chromosomes. How many chromosomes would be present in a chicken: a) Sperm cell b) Fertilised egg cell

Explain your answers.

Answer: a) Sperm cell would contain 39 chromosomes (1 mark) because meiosis halves the chromosome number (1 mark).

b) Fertilised egg cell would contain 78 chromosomes (1 mark) because fertilisation fuses the sperm (39) and egg (39), restoring the diploid number.

Examiner note: Always show your understanding of the process, not just the number. State that meiosis halves and fertilisation restores the full number.

Example 3: Advantages and disadvantages (6 marks)

Question: Aphids are insects that can reproduce both sexually and asexually. During summer, when food is plentiful, aphids reproduce asexually. In autumn, they reproduce sexually. Suggest why aphids use different reproduction methods in different seasons.

Answer: In summer, asexual reproduction allows rapid production of many offspring (1 mark) to take advantage of plentiful food resources (1 mark). Only one parent is needed, so reproduction is faster (1 mark). In autumn, conditions become less favourable (1 mark). Sexual reproduction produces variation in offspring (1 mark), which increases the chance that some offspring will survive changing conditions/winter (1 mark).

Examiner note: Link the advantages of each reproduction type to the specific environmental conditions described. Use biological terminology accurately.

Common mistakes and how to avoid them

Confusing mitosis and meiosis: Remember "mitosis = two identical" and "meiosis = four different gametes". Mitosis is for growth and repair; meiosis is only for making gametes. Make a table comparing them to avoid mixing them up.
Saying asexual reproduction produces "no variation": While offspring are genetically identical, mutations can still occur. It's more accurate to say asexual reproduction produces "very little" or "no genetic variation (except from mutations)".
Forgetting that some organisms use both methods: Don't assume organisms can only use one type of reproduction. Many plants, fungi, and some animals use both depending on conditions.
Incorrectly stating chromosome numbers after meiosis or fertilisation: Always remember: meiosis halves the number, fertilisation doubles it back to normal. If a body cell has 2n chromosomes, gametes have n, and the fertilised egg has 2n again.
Using vague terms instead of specific vocabulary: Don't say "splits" or "divides" without specifying mitosis or meiosis. Don't say "joins together" instead of fertilisation. Use precise biological terminology to gain marks.
Confusing clones with twins: Identical twins are from sexual reproduction (one zygote splits), but they're still clones of each other. Clones from asexual reproduction are genetically identical to the parent, not a sibling.

Exam technique for "Reproduction: sexual and asexual"

Command words matter: "Describe" requires you to state features or characteristics. "Explain" requires reasons or mechanisms (use "because" or "this causes"). "Compare" requires you to state similarities AND differences, not just one.
Use comparative language when asked to compare: Use words like "whereas", "however", "in contrast", "both", "similarly". For example: "Sexual reproduction produces variation, whereas asexual reproduction produces clones."
Link advantages to context: If a question gives you a scenario (like aphids in different seasons), always link your answer to that specific context. Generic advantages won't get full marks if the question asks you to "suggest why" or "explain in terms of".
Watch mark allocation: For 1-mark questions, one clear point is enough. For 4-6 mark questions, you need multiple developed points with explanations. Roughly plan for 1 mark = 1 distinct point or development.

Quick revision summary

Sexual reproduction involves two parents and fertilisation of gametes produced by meiosis, creating genetically varied offspring. Asexual reproduction requires one parent and uses mitosis to produce genetically identical clones. Sexual reproduction provides variation for adaptation but is slower; asexual reproduction is rapid and efficient but produces no variation. Many organisms use both methods depending on environmental conditions. Meiosis produces four non-identical haploid gametes; mitosis produces two identical diploid cells. Understanding chromosome numbers through meiosis and fertilisation is essential for exam success.