Sexual and asexual reproduction, and meiosis
Reproduction makes new individuals. There are two fundamentally different strategies, and most of the rest of B6 (variation, evolution, genetic engineering) only makes sense once you understand the difference.
Sexual reproduction
Sexual reproduction involves the fusion of male and female gametes (sperm + egg in animals, pollen + ovule in plants). Each parent contributes half the genetic material, so the offspring contain a mixture of two parents' genes — they are genetically unique.
Key points:
- Gametes are made by meiosis and are haploid (n = 23 chromosomes in humans).
- At fertilisation two haploid gametes fuse to give a diploid zygote (2n = 46 chromosomes).
- Two parents contribute, so offspring show genetic variation.
Asexual reproduction
Asexual reproduction involves only one parent and no gametes. New cells are made by mitosis, so offspring are genetically identical to the parent — they are clones.
Examples: bacterial binary fission, strawberry runners, potato tubers, Hydra budding, identical twins (after the embryo splits).
Meiosis — making gametes (the four-stage GCSE story)
Meiosis only happens in the reproductive organs (testes and ovaries in humans).
- The cell copies its DNA so each chromosome is doubled.
- The cell divides twice (in mitosis it divides only once).
- Four genetically different, haploid gametes are produced from one parent cell.
- Genetic variation comes from (HT) independent assortment and (HT) crossing over of chromosomes.
A parent cell with 46 chromosomes → four gametes with 23 chromosomes each.
Fertilisation and growth
- Two haploid gametes fuse → diploid zygote (46 chromosomes).
- The zygote divides by mitosis to form an embryo, then a fetus.
- Cells differentiate into specialised cell types as the body develops.
Comparison table — sexual vs asexual
| Feature | Sexual | Asexual |
|---|---|---|
| Number of parents | 2 | 1 |
| Gametes | Yes (meiosis) | No |
| Offspring | Genetically different (variation) | Genetically identical (clones) |
| Speed/energy | Slower; more energy needed to find mate | Fast; very low energy |
| Advantage | Variation → adapt to change / natural selection | Rapid colonisation / no mate needed |
| Disadvantage | Slower; needs a mate | No variation → all vulnerable to same threats |
Organisms that can do both
Many organisms switch strategies depending on conditions:
- Malarial parasite Plasmodium — sexual in the mosquito, asexual in the human host.
- Strawberry plants — sexual via flowers/seeds; asexual via runners.
- Fungi — usually asexual (spores from mitosis); switch to sexual when stressed.
- Daphnia (water fleas) — asexual when conditions are good; sexual when conditions worsen.
The reason: asexual reproduction is fast when the environment is stable, but sexual reproduction creates variation so that when conditions change at least some offspring may survive.
⚠Common mistakes
- Saying meiosis happens in all body cells. It only happens in reproductive organs.
- Confusing mitosis (body cells, identical, 2 from 1) with meiosis (gametes, different, 4 from 1).
- "Asexual reproduction means identical, so it's worse." It's faster and uses less energy — it's a different trade-off, not "worse".
- Forgetting fertilisation produces diploid cells. The embryo divides by mitosis, not meiosis.
Links
Builds on B1.2 (mitosis and the cell cycle). Leads to B6.4 (inheritance using Punnett squares), B6.5 (variation), B6.6 (natural selection) and B6.8 (cloning, which is artificial asexual reproduction).
AI-generated · claude-opus-4-7 · v3-deep-biology