Key Idea: Topic D3.1 is about how living things reproduce — and the master question running through every part of it is: are the offspring genetically identical, or are they varied? Asexual reproduction uses one parent and mitosis, so the offspring are identical clones. Sexual reproduction uses two parents and gametes, so the offspring are genetically varied. From there the topic builds up the details: cloning methods, the human reproductive system and fertilisation, the menstrual cycle and its four hormones, and reproduction in flowering plants. This is a regular on Paper 1 (quick recall/graph MCQs) and Paper 2 (outline and explain questions).
🧬 Sexual vs asexual reproduction (4.7.1)
Sexual reproduction needs two parents: each makes gametes (sex cells) by meiosis, and two gametes fuse at fertilisation. Asexual reproduction needs one parent and uses mitosis only — no gametes, no fertilisation. The difference shows up in the offspring: sexual reproduction makes genetically varied offspring, while asexual reproduction makes genetically identical clones.
| 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 the parents | Genetically identical to the parent — clones |
| Genetic variation | Lots of variation is produced | No new variation (unless a mutation occurs) |
Key Idea: 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 make new combinations of alleles, so every offspring is genetically unique. Asexual offspring vary only if a mutation happens by chance.
Sexual = Separate parents → shuffled genes → variation. Asexual = A single parent → copied genes → identical clones.
🌱 Cloning & asexual methods (4.7.2)
A clone is genetically identical to its parent. Clones are made by asexual reproduction using mitosis, so the parent's DNA is copied unchanged. Some organisms clone naturally (their normal way of reproducing); humans also clone organisms artificially, on purpose. Either way, the offspring are genetically identical to the parent.
| Method | Natural or artificial? | Example |
|---|---|---|
| Budding | Natural | Yeast (and Hydra) — a bud pinches off as an identical daughter |
| Runners / tubers / bulbs | Natural | Strawberry runners, potato tubers — new plants from the parent |
| Stem cuttings (vegetative propagation) | Artificial | Cut a stem, grow roots, raise an identical new plant |
| Tissue culture | Artificial | Many identical plantlets grown from a few cells on nutrient medium |
| Somatic-cell nuclear transfer (SCNT) | Artificial | Dolly the sheep — the first cloned adult mammal |
Cloning an adult animal — SCNT (Dolly the sheep)
- Take a body (somatic) cell from the adult to be cloned and remove its nucleus (it holds the full set of genes).
- Take a donor egg cell and remove its own nucleus, leaving an empty egg.
- Insert the body-cell nucleus into the empty egg, so it now carries the donor's genes.
- Stimulate the egg to divide (e.g. with an electric shock) so it forms an early embryo.
- Implant the embryo into a surrogate mother, where it grows into a genetic clone of the nucleus donor.
Natural or artificial, every cloning method copies the parent's DNA by mitosis — so the answer to 'how similar are the clones?' is always genetically identical, with no variation.
♀️♂️ Human anatomy & fertilisation (4.7.3)
The male gamete is the sperm, made in the testes; the female gamete is the egg (ovum), made in the ovaries. Both are made by meiosis, so each is haploid (n) — it carries half the chromosome number. At fertilisation a sperm nucleus fuses with an egg nucleus to form a single diploid (2n) zygote, which grows into a new organism. Fertilisation usually happens in the oviduct (fallopian tube).
| Gamete | Where it is made | Built for its job |
|---|---|---|
| Sperm (male gamete) | Testes | Tiny, made in huge numbers, swims with a tail (flagellum), almost no food store |
| Egg / ovum (female gamete) | Ovaries | Large (the biggest human cell), few released, cannot swim, big food store |
If gametes were diploid, fertilisation would double the chromosome number every generation. Making them haploid by meiosis means that when two fuse, the diploid number is restored — kept constant: n + n → 2n.
🩸 The menstrual cycle & its hormones (4.7.4)
The menstrual cycle is the roughly 28-day sequence that prepares the body for a possible pregnancy. It is run by four hormones: FSH and LH from the pituitary gland, and oestrogen and progesterone from the ovary. Read the cycle as a chain of cause and effect: FSH grows a follicle → the follicle makes oestrogen → high oestrogen triggers the LH surge → the LH surge causes ovulation (~day 14) → the empty follicle becomes the corpus luteum, which makes progesterone to maintain the lining.
The four hormones across the ~28-day cycle: FSH grows a follicle, oestrogen rises and triggers the sharp LH surge at ovulation (~day 14), then progesterone from the corpus luteum maintains the lining.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Hormone | Where it is made | Its main role in the cycle |
|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Stimulates a follicle in the ovary to grow and mature |
| Oestrogen | Growing follicle (ovary) | Repairs and thickens the uterus lining; high oestrogen triggers the LH surge |
| LH (luteinising hormone) | Pituitary gland | A sharp surge (~day 14) causes ovulation |
| Progesterone | Corpus luteum (ovary) | Maintains the thick lining; inhibits FSH and LH |
Key Idea: Negative feedback (most of the cycle): oestrogen and progesterone inhibit FSH and LH, so no extra follicle ripens. Positive feedback (just before ovulation): a high level of oestrogen stimulates a big release of LH — the LH surge that causes ovulation. The same hormone (oestrogen) acts by negative feedback when low and positive feedback when high — that switch is the clever part of the cycle.
FSH = Follicle grows. LH = Launches the egg (ovulation). Oestrogen = builds the lining. Progesterone = protects/maintains the lining.
🌸 Reproduction in flowering plants (4.7.5)
A flower is the plant's reproductive organ. The male part is the stamen (an anther, which makes pollen, on a filament). The female part is the carpel (a sticky stigma, a style, and an ovary holding ovules). Pollination moves pollen from an anther to a stigma; then a pollen tube grows down the style and the male gamete fuses with the egg in the ovule — that fusion is fertilisation. Afterwards the ovule becomes a seed and the ovary becomes a fruit.
| Feature | Pollination | Fertilisation |
|---|---|---|
| What happens | Pollen is transferred from an anther to a stigma | A male gamete fuses with the egg inside the ovule |
| Comes first? | First — it only delivers the pollen | Second — once the pollen tube reaches the ovule |
| Gametes fuse? | No — the gametes have not met yet | Yes — this is the actual joining of the gametes |
| Result | Pollen is now on the stigma | A zygote forms; ovule → seed, ovary → fruit |
Many plants avoid self-pollinating to keep up genetic variation: Self-incompatibility alleles — the plant rejects its own pollen, so only pollen from a different plant can fertilise it. Separating the sexes in time or space — anthers and stigmas mature at different times, or the male and female parts are on separate flowers or plants.
Pollination = Pollen Put on stigma (delivery only). Fertilisation = gametes Fuse (the real joining, inside the ovule). Pollination is the postman; fertilisation is the gametes actually meeting.
✍️ Worked examples
IB-style question — predict the genomes of cuttings
A gardener grows five new rose bushes from cuttings taken from one parent bush. Predict, with a reason, how similar the genomes of the five new bushes will be. [2]
Model answer:
Make the prediction. The five new bushes will be genetically identical — to the parent and to one another (clones).
Give the reason. Growing a plant from a cutting is asexual reproduction: one parent, no gametes, no fertilisation, so the genome is simply copied by mitosis with no mixing of alleles. (Mark 1: identical / clones. Mark 2: asexual / one parent / genome copied — a bare prediction with no reason scores only one mark.)
Genetically identical clones — produced asexually from one parent, so the genome is copied by mitosis with no mixing of gametes.
IB-style question — roles of progesterone
Outline the roles of progesterone in the menstrual cycle. [4]
Model answer:
Source. Progesterone is secreted by the corpus luteum (the empty follicle after ovulation).
Effect on the uterus. It maintains and thickens the uterus lining, ready to receive a fertilised egg.
Feedback role. It inhibits FSH and LH (negative feedback), so no new follicle develops.
When it falls. If there is no pregnancy, progesterone falls, the lining breaks down (menstruation), and FSH can rise to begin a new cycle. (1 mark per distinct point, up to 4.)
Made by the corpus luteum; maintains/thickens the uterus lining; inhibits FSH and LH (negative feedback); and when it falls (no pregnancy) the lining is shed and a new cycle begins.
IB-style question — define pollination
Define the term pollination. [1]
Model answer:
Name what moves and from where to where: pollination is about pollen moving from an anther to a stigma.
State it precisely: pollination is the transfer of pollen from an anther to a stigma. (Do not describe gametes fusing — that is fertilisation and scores zero.)
Pollination is the transfer of pollen from an anther to a stigma.
✅ Quick self-check
Tap each card to check yourself.
How does sexual reproduction generate genetic variation? Two parents contribute different alleles, meiosis shuffles them into many different gametes, and random fertilisation joins any two gametes — making new allele combinations in every offspring.
Why are the products of asexual reproduction genetically identical? One parent and mitosis only copy the genes exactly; with no gametes and no fertilisation the alleles are never mixed, so the offspring are clones.
Outline how an adult animal is cloned (SCNT). Take the nucleus from a body cell, put it into a donor egg whose nucleus has been removed, stimulate it to form an embryo, and implant it into a surrogate mother — the clone matches the nucleus donor.
Why must gametes be haploid? So fertilisation (n + n → 2n) restores the diploid number instead of doubling the chromosome count every generation.
What causes ovulation, and roughly when? A sharp surge of LH (triggered by high oestrogen via positive feedback) causes ovulation around day 14.
What is the difference between pollination and fertilisation? Pollination is the transfer of pollen from anther to stigma (delivery only); fertilisation is the later fusion of the male gamete with the egg inside the ovule.
Exam Tips
- The master idea of the whole topic: asexual (one parent, mitosis) → identical clones; sexual (two parents, gametes) → genetic variation.
- 'Predict, with a reason' needs BOTH the prediction (identical clones) AND the reason (asexual / one parent / genome copied) — a bare prediction scores only one mark.
- Growing a plant from a cutting, runner, tuber or bud is asexual → the offspring are genetically identical to the parent.
- For SCNT, give it as a sequence of steps: get the body-cell nucleus → put it in an enucleated egg → grow and implant the embryo.
- Sort the four cycle hormones by source: FSH and LH are pituitary; oestrogen and progesterone are ovarian. Exams love 'name one of each'.
- On a hormone graph, the tall sharp LH spike around day 14 marks ovulation — find the spike, find ovulation.
- Keep pollination (pollen → stigma) and fertilisation (gametes fuse in the ovule) strictly separate — mixing them is the classic lost mark.
- Cross-pollination is encouraged by self-incompatibility alleles or by separating the sexes in time or space — it keeps up genetic variation.