The big idea: Living things make new individuals in one of two ways.
Sexual reproduction needs two parents. Each parent makes special sex cells called gametes, and two gametes fuse to start the new organism.
Asexual reproduction needs only one parent. There are no gametes and no fertilisation — the parent simply makes copies of itself.
The key difference is in the offspring: sexual reproduction makes offspring that are genetically different, while asexual reproduction makes genetically identical clones.
- Sexual reproduction
- Reproduction that involves two parents and the fusion (joining) of two gametes to form the new organism.
- Asexual reproduction
- Reproduction that involves only one parent and no fusion of gametes — the offspring are genetically identical to the parent.
- Gamete
- A sex cell (such as a sperm or an egg). Gametes fuse together at fertilisation.
- Fertilisation
- The fusion of two gametes (for example a sperm and an egg) to form a single new cell.
- Clone
- An organism that is genetically identical to its parent (or to another organism).
One parent or two?: The quickest way to tell the two apart:
Sexual = two parents + gametes + fertilisation → variation.
Asexual = one parent + no gametes → identical clones.
The two methods produce different offspring because they handle genes differently.
In sexual reproduction, two parents each pass on a different set of alleles, and those alleles get shuffled before they join up. In asexual reproduction, the single parent's genes are simply copied, so nothing gets mixed.
- Gene
- A length of DNA that codes for a particular feature (for example flower colour).
- Allele
- One of the different versions of a gene (for example a red-flower allele or a white-flower allele).
- Genetic variation
- Differences in the genes (alleles) between individuals of the same species.
- Meiosis
- A type of cell division that makes gametes and shuffles the parent's alleles into many different combinations.
- Mitosis
- A type of cell division that makes genetically identical copies of a cell.
Sexual reproduction → genetic variation: Sexual reproduction generates variation in three ways:
Two parents each contribute a different set of alleles.
Meiosis shuffles each parent's alleles into many different gametes.
Random fertilisation means any gamete can fuse with any other.
Together these create new combinations of alleles, so every offspring is genetically unique.
| Source of variation in sexual reproduction | What happens | Why it makes offspring different |
|---|---|---|
| Two different parents | Each parent carries a different set of alleles | Offspring inherit a brand-new mix, half from each parent |
| Meiosis | Shuffles the parent's alleles into many different gametes | No two gametes are guaranteed to carry the same combination |
| Random fertilisation | Any one gamete can fuse with any other | A huge number of different offspring combinations is possible |
Asexual reproduction → identical clones: Asexual reproduction has one parent and uses mitosis, which copies the genes exactly.
There is no meiosis, no gametes and no fertilisation, so the alleles are never mixed.
Every offspring therefore has the same genome as the parent — they are clones. (The only way new variation appears is if a mutation happens by chance.)
Sexual reproduction
- Two parents
- Gametes fuse at fertilisation
- Meiosis + random fertilisation shuffle the alleles
- Offspring are genetically varied (each one unique)
Asexual reproduction
- One parent
- No gametes, no fertilisation
- Mitosis copies the genes exactly
- Offspring are genetically identical clones
A memory hook: Sexual = Separate parents → shuffled genes → variation.
Asexual = A single parent → copied genes → identical clones.
Variation is useful: it gives a species the raw material for natural selection to act on if the environment changes.
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How this is tested: A favourite Paper 2 format gives you a scenario — for example several new plants grown from the branches of one parent tree — and asks you to predict, with a reason, how similar their genomes will be.
The thinking the examiner wants: growing a plant from a branch is asexual (one parent, no gametes), so the new plants are genetically identical to the parent and to each other.
Related questions ask you to compare sexual and asexual reproduction, or to explain how sexual reproduction generates genetic variation.
IB-style question — predict how similar the genomes are
A gardener grows five new apple trees by planting cuttings taken from the branches of a single parent tree. Predict, with a reason, how similar the genomes of the five new trees will be. [2]
How to score both marks
- Make the prediction. The five new trees will be genetically identical — to the parent tree and to one another.
- Give the reason. Growing a tree from a cutting (a branch) is asexual reproduction: there is one parent, no gametes and no fertilisation, so the parent's genome is simply copied by mitosis. With no mixing of alleles, the offspring are clones. (Mark 1: genetically identical / clones. Mark 2: asexual reproduction from one parent / no gametes, so the genome is copied unchanged.)
Final answer
Genetically identical (clones) — they are produced asexually from one parent, so the genome is copied by mitosis with no mixing of gametes.
✓ Why this scores full marks: It does both things the command term 'Predict, with a reason' asks for:
the prediction (genetically identical / clones) and the reason (asexual, one parent, genome copied — no gametes to shuffle the alleles).
Just writing 'they are the same' with no reason would score only one mark.
| Feature | Sexual reproduction | Asexual reproduction |
|---|---|---|
| Number of parents | Two parents | One parent |
| Cells involved | Gametes (sex cells) fuse at fertilisation | Ordinary body cells — no gametes, no fertilisation |
| Cell division used | Meiosis makes the gametes, then fertilisation | Mitosis only |
| Offspring | Genetically different from each other and from the parents | Genetically identical to the parent — clones |
| Genetic variation | Lots of variation is produced | No new variation (unless a mutation occurs) |