ReproductionOxford AQA IGCSE Biology: Revision notes
Section 1
What is sexual reproduction?
Sexual reproduction involves the fusion of two gametes (sex cells) to produce genetically varied offspring. This process requires two parents and occurs through the combination of male and female gametes, such as sperm and egg cells in animals, or pollen and ovule in plants.
Key characteristics:
- Gametes are haploid (containing half the chromosome number)
- Fusion of gametes produces a diploid zygote (containing full chromosome number)
- Meiosis produces gametes with genetic variation
- Fertilisation restores the full chromosome number
- Each offspring is genetically unique (except identical twins)
This genetic variation is created through the mixing of alleles from two different parents, resulting in independent assortment and crossing over during meiosis.
Examiners expect you to state that sexual reproduction produces 'genetically varied' or 'different' offspring, not just 'different'—the genetic basis matters. Always link gamete fusion to genetic variation.
Section 2
What is asexual reproduction?
Asexual reproduction produces genetically identical offspring (clones) from a single parent. No gamete fusion occurs, and all offspring are exact genetic copies of the parent because no meiosis or genetic recombination takes place.
Key characteristics:
- Requires only one parent
- Offspring are clones (genetically identical to parent)
- No fertilisation or gamete fusion
- Mitosis produces identical daughter cells
- Faster and more efficient than sexual reproduction
- All offspring are identical to the parent and to each other
Asexual reproduction is common in microorganisms, plants, and some animals. The lack of genetic diversity means all individuals are suited to the same environmental conditions, but populations cannot adapt if conditions change.
Students often write that asexual reproduction involves 'one cell' dividing. The key point is 'one parent'—a parent organism can have many cells, but they are all genetically identical and produce clones.
Think of asexual reproduction like photocopying: the copy is identical to the original with no variation, whereas sexual reproduction is like mixing two recipes—you get something unique but unpredictable.
Section 3
What are the advantages and disadvantages of sexual reproduction?
| Advantage | Disadvantage |
|---|---|
| Genetic variation creates diversity in populations, allowing adaptation to environmental changes | Slower process—requires finding a mate, courtship, and mating take time and energy |
| Selection acts on variation, allowing evolution and survival of fittest in changing environments | Energy-costly—producing gametes, courtship behaviour, and mating require significant resources |
| Beneficial mutations can combine in advantageous ways | Two parents required—less efficient in sparse populations; some organisms may not find mates |
| Harmful recessive alleles masked by beneficial dominant alleles in offspring | Produces fewer offspring compared to asexual reproduction |
| Population has greater chance of survival if environment changes |
Why genetic variation matters: When the environment changes (disease, climate, food scarcity), a genetically diverse population has individuals with different traits that may survive. Genetically identical offspring cannot adapt to new threats.
Examiners want to see why advantages matter: link genetic variation to 'adaptation' or 'survival in changing environments', and link disadvantages to 'energy cost' or 'time'. Don't just list—explain the consequence.
If a new disease kills rabbits without a specific allele, sexually reproducing rabbits are more likely to have diverse immunity genes, so some survive and breed. Asexually reproduced clones all lack immunity and all die—population crashes.
Section 4
What are the advantages and disadvantages of asexual reproduction?
| Advantage | Disadvantage |
|---|---|
| Fast and efficient—no need to find a mate; rapid population growth | No genetic variation—all offspring genetically identical to parent and each other |
| Requires less energy—no courtship, mating, or gamete production | Cannot adapt to environmental changes; if environment changes, all individuals are equally affected |
| Single parent sufficient—useful in isolated environments or sparse populations | Susceptible to disease—if one individual is affected, entire clone population likely susceptible |
| Produces more offspring from one parent than sexual reproduction | Evolution cannot occur because there is no variation for selection to act upon |
| All offspring are pre-adapted to parent's successful environment | No beneficial mutations can be selected; harmful mutations spread through entire population |
Why lack of variation matters: A genetically uniform population is vulnerable. If a pathogen evolves to infect one individual, it can infect all clones. If the environment changes, no individual has traits suited to survival.
Link disadvantages to real consequences: 'no genetic variation' should lead to 'cannot adapt' or 'all die if environment changes'. Examiners mark for reasoning, not just listing problems.
A farmer grows thousands of genetically identical potato plants (asexual reproduction). A new pest appears and destroys all plants because they all lack the same defence gene. A diverse crop would have some resistant plants surviving.
Section 5
What are the main examples of asexual reproduction in nature?
1. Binary fission in bacteria
- Bacterial cell divides into two identical daughter cells
- Plasmids (circular DNA) copied and passed to daughters
- Process is rapid—can occur every 20 minutes under ideal conditions
- Example: E. coli, Salmonella
2. Budding in yeast
- Small outgrowth or bud forms on parent cell
- Bud grows, receives a copy of nucleus and cytoplasm
- Bud detaches as new, genetically identical yeast cell
- Faster than binary fission; common in Saccharomyces cerevisiae
3. Runners (stolons) in strawberry plants
- Parent plant produces horizontal stems called runners or stolons
- Runners touch soil and develop roots at nodes
- New plantlet grows, still connected to parent
- Eventually detaches as independent plant
- Genetically identical to parent; allows rapid colonisation of area
4. Bulbs (and tubers) in plants
- Underground storage organs containing a bud surrounded by fleshy leaves
- Bulb divides and produces daughter bulbs that grow into new plants
- Examples: onions, tulips, daffodils, lily bulbs
- Tubers (potatoes) can also produce new plants from buds or "eyes"
- Clones grow from single parent bulb
All these examples share the same principle: one parent produces genetically identical offspring through mitosis, not meiosis and fertilisation.
Examiners test recognition of these four examples specifically. For each, state: (1) what structure/process is involved, (2) how many parents, (3) that offspring are clones. Know the difference between runners (strawberry) and bulbs (tulip).
A strawberry plant produces a runner that roots in soil 20 cm away. This new plant is a clone with identical genes to the parent. If the parent is susceptible to a disease, the clone is too—but the parent plant persists if the clone dies.
Must Know
- Sexual reproduction involves fusion of two gametes producing genetically varied/different offspring from two parents; asexual reproduction produces genetically identical clones from one parent
- Genetic variation in sexual reproduction allows adaptation to environmental change; lack of variation in asexual reproduction means population cannot adapt if conditions change
- Advantages of sexual reproduction: genetic variation, adaptation, evolution possible | Disadvantages: slower, more energy-costly, requires two parents
- Advantages of asexual reproduction: fast, efficient, single parent sufficient | Disadvantages: no variation, cannot adapt, disease susceptibility, no evolution
- Four key examples of asexual reproduction: (1) binary fission—bacteria divide into two clones; (2) budding—yeast grow buds that detach as clones; (3) runners/stolons—strawberry plants produce horizontal stems that root and form clones; (4) bulbs—tulips and onions produce underground bulbs that divide to form clones
- All asexual reproduction occurs through mitosis (identical cell division), never meiosis; all involve one parent producing genetically identical offspring
That's the notes covered.
Carry on to the next subtopic.