D3.1 Reproduction

Asexual reproduction uses one parent and usually produces genetically similar offspring; sexual reproduction combines genetic material from two gametes.

Asexual copying is efficient but preserves little new variation. Sexual reproduction uses meiosis and fusion, so offspring inherit a new combination of alleles.

Classify a case by counting parents and asking whether gamete fusion occurs.

A strawberry runner forms a new plant from one parent without gamete fusion; a seed formed after pollen and ovule fusion is sexual.

‘Different-looking’ offspring are not enough to prove sexual reproduction; identify the reproductive mechanism.

Meiosis halves chromosome number in gametes, and fusion of two gametes restores the diploid number in the zygote.

The alternation prevents chromosome number doubling every generation. Independent assortment and crossing over also create combinations before fusion adds another random combination.

Track chromosome number through meiosis; then track the fusion event and the first embryo cell.

A diploid human cell with 46 chromosomes produces gametes with 23; fusion returns the zygote to 46.

Meiosis does not simply make ‘smaller’ cells; its defining outcome is reduced chromosome number plus variation.

In anisogamous species, the male produces smaller motile gametes and the female produces larger nutrient-rich gametes.

The distinction is based on gamete type, not on every secondary trait or an individual’s identity. Different species organise reproductive roles around these gametes in different ways.

Identify the gametes first, then infer the biological sex category used in the syllabus model.

In humans, sperm are small and motile while ova are large and non-motile, so the model labels their producers male and female.

Gamete definitions do not justify assumptions about behaviour, gender, or all reproductive biology.

Human reproductive anatomy positions gamete production, transport, fertilization and implantation in linked organs.

Testes produce sperm; ovaries release oocytes; oviducts are the usual site of fertilization; the uterus supports implantation and development. Structure follows each function.

Follow one gamete’s route; name the organ; link each structure to its function.

Sperm travel from a testis through ducts to the oviduct, where an oocyte may be fertilized before the embryo reaches the uterus.

The pathway is not a guarantee of conception; timing, transport and hormonal conditions can interrupt it.

The ovarian cycle matures and releases an oocyte, while the uterine cycle builds and then maintains or sheds the lining according to hormone levels.

FSH supports follicle development; an LH surge triggers ovulation; oestrogen rebuilds the endometrium; progesterone stabilises it after ovulation. Hormone changes coordinate the two cycles.

Place the hormone event on the cycle timeline before predicting ovulation or menstruation.

A sharp LH rise around the middle of a cycle triggers release of the mature oocyte; falling progesterone later can lead to menstruation.

Cycle length varies and ovulation is not proven by a calendar date alone.

Human fertilization is the fusion of sperm and oocyte nuclei to form a diploid zygote.

Sperm activation, recognition of the oocyte surface, membrane fusion and nuclear fusion link gamete contact to a new genome. The zygote then begins cleavage divisions.

Distinguish sperm entry; nuclear fusion; and the later embryo divisions when ordering events.

One sperm nucleus carrying 23 chromosomes fuses with the oocyte nucleus carrying 23, producing a zygote with 46.

Fertilization is not the same as implantation; those events occur at different sites and times.

In vitro fertilization combines oocytes and sperm outside the body, then transfers a selected embryo to the uterus.

Hormonal stimulation can mature several follicles; collected oocytes are fertilized in vitro and embryos are cultured before transfer. Each step adds opportunities and risks.

Trace the procedure in order: stimulation; retrieval; fertilization; culture; transfer; test.

A clinic transfers one embryo after culture; a later pregnancy test detects whether implantation and early development occurred.

IVF can assist fertilization but cannot guarantee implantation, healthy development, or a live birth.

Flowering plants reproduce sexually when pollen delivers male nuclei to an ovule and fertilization produces a zygote and seed.

Anthers make pollen and ovaries contain ovules. Pollination moves pollen to a stigma; a pollen tube then carries nuclei to the ovule, linking flower structure to seed formation.

Separate pollination (transfer) from fertilization (nuclear fusion) when describing the process.

A bee carries pollen from one flower to a stigma; the pollen tube grows to an ovule where a male nucleus fuses with the egg cell.

A flower receiving pollen has not necessarily been fertilized; compatibility and tube growth still matter.

Insect-pollinated flowers have features that attract insects and place pollen where it can be transferred to another stigma.

Colour, scent, nectar and landing structures increase visits; sticky or rough pollen adheres to the insect. The arrangement improves transfer while limiting waste.

Explain each feature by its effect on insect behaviour; pollen attachment; and later transfer.

A scented flower with nectar guides attracts a bee, and anthers brush pollen onto its body as it feeds.

Not every colourful flower is insect-pollinated; link a feature to a plausible pollinator and transfer mechanism.

Cross-pollination is promoted when a flower’s pollen reaches a different plant, reducing self-fertilization and increasing genetic mixing.

Different maturation times, separated anthers and stigmas, self-incompatibility, or separate male and female flowers make self-transfer less likely. Each is a barrier or timing mechanism.

Identify whether the adaptation changes timing; position; compatibility; or plant sex.

If anthers release pollen before the stigma becomes receptive, pollen from another plant is more likely to fertilize the ovules.

Cross-pollination raises outcrossing probability but does not ensure it in every visit or population.

Self-incompatibility prevents fertilization when pollen and stigma carry an incompatible recognition combination.

Molecular recognition at the stigma can stop pollen germination or tube growth. The plant therefore accepts compatible pollen while reducing self-fertilization.

Predict the result by comparing pollen genotype; stigma recognition; tube growth; and compatibility, not pollen quantity alone.

A pollen grain from the same incompatibility class fails to grow a tube, while pollen from another plant reaches the ovule.

Self-incompatibility is not the same as physical separation or pollen sterility; it is a recognition response.

Seed dispersal separates offspring from the parent, and germination begins when water, oxygen and a suitable temperature allow metabolism and growth.

Dispersal reduces crowding and competition. During germination, water activates enzymes, oxygen supports respiration, and the embryo uses stored food until photosynthesis begins.

Check each condition before deciding whether a seed can germinate.

A bean seed kept dry does not germinate; after water and warmth are supplied, respiration rises and the radicle emerges.

A seed can be viable but remain dormant; failure to germinate does not prove it is dead.

Puberty is the coordinated maturation of reproductive function driven by hypothalamic–pituitary–gonadal signalling.

GnRH stimulates pituitary gonadotropins, which act on ovaries or testes. Sex steroids then drive gamete maturation and secondary sexual characteristics through regulatory loops.

Trace the signal chain from brain; to pituitary; to gonad before assigning a hormone effect.

A rise in gonadotropins stimulates testicular testosterone production, supporting sperm production, reproductive maturation, and secondary traits.

Puberty timing varies; one visible trait alone does not identify the whole endocrine sequence.

Gametogenesis uses meiosis and cell specialization to produce haploid gametes with distinct structures and functions.

Spermatogenesis yields many small motile sperm; oogenesis retains cytoplasm in one large ovum and produces polar bodies. Both processes reduce chromosome number before fertilization.

Compare output number; cell size; motility; and timing rather than treating sperm and ova as identical products.

One primary oocyte produces one large ovum plus polar bodies, whereas a primary spermatocyte can produce four sperm cells.

The simplified pathway does not imply every individual has identical rates or fertility.

Preventing polyspermy ensures that a zygote receives one sperm genome and the correct chromosome number.

Sperm–oocyte fusion triggers cortical-granule changes in the oocyte surface. The altered layer prevents additional sperm binding or fusion.

Order the event: first fusion; cortical response; block to later sperm.

After one sperm fuses with an oocyte, the surrounding membrane changes so a second sperm cannot enter and create an abnormal chromosome number.

The block is a safeguard, not a mechanism that chooses the ‘best’ sperm or guarantees embryo survival.

After early cleavage, the embryo forms a blastocyst whose outer cells attach to and invade the uterine lining during implantation.

The inner cell mass forms the embryo; the outer trophoblast helps attachment and later placenta formation. Implantation connects embryonic development to maternal support.

Distinguish cleavage; blastocyst formation; implantation; and later organ development in sequence.

A blastocyst reaches the uterus and its trophoblast attaches to the endometrium, allowing implantation to begin.

Fertilization in the oviduct is not implantation; implantation requires later attachment to the uterine lining.

Pregnancy tests detect human chorionic gonadotropin (hCG), which is produced after implantation and enters maternal blood or urine.

Developing trophoblast tissue releases hCG; its presence supports the corpus luteum so progesterone remains high. The test therefore detects a hormone signal, not the embryo directly.

Interpret a test by asking when implantation and detectable hCG could occur.

A urine test becomes positive after hCG rises above its detection threshold, while a test taken too soon after fertilization may be negative.

A negative early test does not rule out pregnancy; timing and test sensitivity matter.

The placenta is an exchange organ that supplies oxygen and nutrients, removes wastes, and secretes hormones while maternal and fetal blood normally remain separate.

Large surface area, thin barriers and fetal–maternal concentration gradients support diffusion and active transport. Placental hormones help maintain pregnancy and prepare the body for birth.

For each substance, identify direction, transport mechanism, and whether the two blood supplies mix.

Oxygen diffuses from maternal blood to fetal blood while carbon dioxide diffuses back, across a thin placental barrier.

The placenta is selective, not a complete filter; some drugs and pathogens can cross.

Reproductive hormones act in regulatory loops that coordinate gamete maturation, ovulation, uterine preparation and pregnancy maintenance.

GnRH, FSH, LH, oestrogen and progesterone change one another’s targets and regulation. A temporary positive-regulation event can trigger ovulation, while negative regulation stabilizes most of the cycle.

Draw the direction of each regulatory link before predicting what happens when one hormone rises or falls.

High oestrogen shortly before ovulation switches to positive regulation on the pituitary, producing an LH surge.

Do not label every reproductive regulatory loop positive; most control is negative outside the ovulatory switch.

Hormone replacement therapy supplies hormones to reduce symptoms caused by declining endogenous hormone production, but its benefits and risks depend on formulation and patient context.

Exogenous oestrogen, sometimes with a progestogen, changes target-tissue signalling and can relieve vasomotor symptoms. Adding a progestogen protects an intact endometrium from persistent stimulation.

Evaluate HRT by matching hormone, tissue, symptom, duration and relevant risk—not by treating all users as equivalent.

A person with an intact uterus may receive combined therapy so oestrogen relief is paired with progestogen protection of the endometrium.

HRT is not contraception or a universal anti-ageing treatment; clinical decisions require individualized medical advice.