IB Biology · Theme D Continuity and change · Organisms
D3.1 Reproduction
Asexual reproduction copies a proven genotype; sexual reproduction reshuffles alleles through meiosis and fertilisation. In humans, hormones run the menstrual cycle, IVF, puberty, pregnancy and birth. In flowering plants, pollination, fertilisation, cross-pollination and seed dispersal are separate steps.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D3.1.1 Asexual copies the parent; sexual mixes new combinations
Asexual reproduction has one parent, no meiosis, no gamete fusion: offspring are clones.
Advantage: a parent adapted to a stable environment passes on its whole genotype.
Sexual reproduction fuses gametes made by meiosis, giving new allele combinations.
Advantage: some varied offspring are likely to suit a changed environment.
Students often think sexual reproduction is always better. In fact variation only pays when the environment may change.
Students often think organisms produce the variations they need. In fact combinations arise at random; the environment then selects.
D3.1.2 Meiosis breaks up combinations; fertilisation makes new ones
Meiosis halves the chromosome number to make haploid gametes.
Independent assortment and crossing over break up the parent's allele combinations.
Fertilisation, the fusion of gametes, restores the diploid number.
Any male gamete may meet any female gamete, so new combinations arise.
Students often think meiosis makes new alleles. In fact it reshuffles the alleles the parent already has.
Students often think all variation comes from meiosis. In fact fertilisation adds combinations neither parent had.
D3.1.3 Why sperm are small and many, eggs large and few
The male gamete travels to the female gamete, so it is small, motile and cheap.
The female gamete stays put and stores food for the embryo: large and costly.
Males make huge numbers; human females release one egg per cycle.
So males typically compete for mates; females invest more in each offspring and choose.
Students often think the egg is larger because it carries more genes. In fact it carries the same set; the size is food reserves.
Students often think one sperm per egg means equal numbers. In fact cheap sperm are made by the hundreds of millions.
D3.1.4 The human reproductive systems, part by part
Testis makes sperm and testosterone; epididymis stores and matures sperm; sperm duct carries them.
Seminal vesicle adds fructose fluid; prostate adds alkaline fluid; the urethra carries semen or urine.
Ovary releases an egg from a follicle; the oviduct collects it and hosts fertilisation.
Uterus holds the embryo; its endometrium thickens and is shed; cervix and vagina lead outside.
Students often think sperm are stored in the seminal vesicles. In fact they are stored in the epididymis; the vesicles add fluid.
Students often think fertilisation happens in the uterus. In fact it happens in the oviduct; the embryo then moves to the uterus.
D3.1.5 The menstrual cycle and its hormones
FSH from the pituitary stimulates follicles to develop and secrete oestradiol.
Oestradiol rebuilds the endometrium; its high level triggers an LH surge by positive feedback.
The LH surge causes ovulation around day 14 and forms the corpus luteum.
The corpus luteum secretes progesterone, which maintains the endometrium and inhibits FSH and LH.
When the corpus luteum degenerates, progesterone falls and menstruation begins; low oestradiol and progesterone inhibit FSH and LH by negative feedback.
Students often think oestradiol only inhibits FSH and LH. In fact at high levels it stimulates them, producing the LH surge.
Students often think menstruation follows a hormone rise. In fact it follows the fall in progesterone when the corpus luteum degenerates.
D3.1.6 What happens at fertilisation in humans
The sperm's cell membrane fuses with the egg's cell membrane.
Only the sperm nucleus enters; its tail and mitochondria are destroyed.
Both nuclear membranes dissolve and all condensed chromosomes join one spindle.
This joint mitosis gives two diploid nuclei in the first two cells.
Students often think the whole sperm enters. In fact only the nucleus does, so all the zygote's mitochondria come from the egg.
Students often think the two nuclei fuse into one zygote nucleus first. In fact the chromosomes go straight into a joint mitosis.
D3.1.7 Hormones in IVF
In IVF, eggs are fertilised outside the body and embryos transferred to the uterus.
A drug first suspends the woman's own pituitary hormone secretion.
Injected FSH, with LH or hCG, then induces superovulation: many eggs mature at once.
Several eggs can be collected in one cycle, giving several embryos.
Students often think IVF hormones just boost the natural cycle. In fact the cycle is suspended, then superovulation is induced.
Students often think fertilisation still happens inside the body. In fact it happens in a dish; the embryos are then transferred.
D3.1.8 Sexual reproduction in flowering plants
Female gametes form in ovules in the ovary; male gametes in pollen grains from the anther.
Pollination moves pollen from anther to stigma; it is not fertilisation.
A pollen tube grows down the style and delivers the male gamete to the ovule.
Fertilisation gives a zygote, then an embryo in a seed; the ovary becomes the fruit.
Even a hermaphroditic plant reproduces sexually: its gametes come from meiosis and fuse at fertilisation.
Students often think pollen grains are the male gametes. In fact they carry the gamete nuclei and deliver them by a tube.
Students often think a self-fertilising plant reproduces asexually. In fact meiosis and fertilisation still happen, so it is sexual.
D3.1.9 Parts of an insect-pollinated flower
Stamen: the anther makes pollen; the filament holds it where insects brush past.
Carpel: the sticky stigma receives pollen; the style carries tubes; the ovary holds ovules.
Large coloured, scented petals attract insects; the nectary rewards them with sugar.
Green sepals enclose and protect the flower while in bud.
Students often mix up anther and stigma. In fact the anther produces pollen and the stigma receives it.
Students often think the ovary contains seeds. In fact it contains ovules, which become seeds only after fertilisation.
D3.1.10 How plants avoid pollinating themselves
Cross-pollination is pollen moving between flowers on different plants of one species.
Self-pollination stays within one plant and gives less varied offspring.
Anthers and stigma maturing at different times stops a flower using its own pollen.
Separate male and female flowers, or separate male and female plants, also promote crossing.
Animals and wind carry pollen from plant to plant.
Students often call pollen moving between flowers on one plant cross-pollination. In fact cross-pollination needs two different plants.
Students often think a flower with both parts must self-pollinate. In fact timing or self-incompatibility usually prevents it.
D3.1.11 Self-incompatibility keeps gametes from different plants
Self-pollination leads to inbreeding: less genetic diversity and less vigour.
Inbreeding raises homozygosity, so harmful recessive alleles are expressed more often.
Self-incompatibility is a genetic mechanism that rejects a plant's own pollen.
Typically the pollen tube fails to grow when pollen and carpel carry matching alleles.
Students often think inbreeding causes new mutations. In fact it exposes harmful recessive alleles already present.
Students often think self-incompatible plants block their own pollen physically. In fact the rejection is genetic, after pollination.
D3.1.12 Seeds are dispersed, then germinate on stored food
Seed dispersal moves seeds away from the parent by wind, water or animals.
It happens after fertilisation and is not the same as pollination.
In germination the seed absorbs water; the embryo makes gibberellin, which mobilises food reserves.
Amylase hydrolyses starch to maltose, then glucose, fuelling respiration and growth.
The radicle emerges first, then the shoot, until the leaves can photosynthesise.
Students often think pollinators also disperse seeds. In fact pollination is before fertilisation; dispersal is after.
Students often think a germinating seed feeds from soil or photosynthesis. In fact it lives on its own reserves until it has leaves.
D3.1.13 Puberty starts in the hypothalamus HL
Late in childhood the hypothalamus releases more GnRH.
GnRH makes the anterior pituitary secrete more LH and FSH.
These stimulate the testes to make testosterone, or the ovaries oestradiol and progesterone.
The rise in steroid sex hormones causes the changes of puberty.
Students often think the gonads start puberty on their own. In fact the trigger is increased GnRH from the hypothalamus.
Students often think GnRH acts on the gonads directly. In fact it acts on the pituitary, which releases LH and FSH.
D3.1.14 Making sperm and making eggs HL
Spermatogenesis: cells divide by mitosis, grow, complete both meiotic divisions equally, then differentiate.
Four equal sperm result from each meiosis; millions are made daily from puberty on.
Oogenesis begins before birth and pauses early in meiosis I.
From puberty, unequal divisions give one large egg and tiny polar bodies that degenerate.
The cell released at ovulation has finished only meiosis I; meiosis II completes if a sperm enters.
Students often think oogenesis gives four eggs per meiosis. In fact it gives one; the polar bodies get almost no cytoplasm.
Students often think ovaries make new eggs through life. In fact the supply is formed before birth and is finite.
D3.1.15 One sperm in, all others out HL
The acrosome reaction releases enzymes that digest a path through the zona pellucida.
The sperm can then fuse with the egg membrane.
The cortical reaction releases granules that harden the zona pellucida.
No further sperm can pass, so polyspermy is prevented.
Students often swap the two reactions. In fact the acrosome reaction gets a sperm in; the cortical reaction keeps others out.
Students often think only one sperm ever reaches the egg. In fact many reach it; the cortical reaction admits just one.
D3.1.16 From zygote to blastocyst to implantation HL
Repeated mitosis on the way down the oviduct gives a blastocyst in five to seven days.
It is a hollow ball: fluid cavity, inner cell mass, outer cell layer.
The inner mass forms the foetus; the outer layer attaches and helps form the placenta.
Implantation in the endometrium happens about seven days after fertilisation.
Students often think the zygote implants straight away. In fact it implants a week later, as a blastocyst.
Students often think the blastocyst is a solid ball of identical cells. In fact it is hollow, with distinct inner and outer cells.
D3.1.17 Pregnancy tests detect hCG with monoclonal antibodies HL
hCG is secreted by the embryo's outer cells after implantation, then by the placenta.
It keeps the corpus luteum alive, so progesterone continues and the endometrium stays.
Monoclonal antibodies come from one clone and bind one antigen.
A marked antibody binds hCG in urine; a fixed antibody captures the complex as a line.
Students often think hCG comes from the mother's ovary or pituitary. In fact only the embryo and placenta make it.
Students often think the test uses the mother's own antibodies. In fact the antibodies are made in a laboratory and built into the strip.
D3.1.18 The placenta supplies the foetus without mixing bloods HL
The placenta forms from foetal tissue and endometrium.
Oxygen, glucose, amino acids, lipids, water, vitamins, minerals and antibodies pass to the foetus.
Carbon dioxide and urea pass back; the two bloods never mix.
Placental villi give a huge surface area and a short diffusion distance.
Because the placenta provides for the foetus, it can stay in the uterus to a later stage than in mammals without one.
Students often think the mother's blood flows into the foetus. In fact a thin barrier separates the two circulations.
Students often think the placenta filters out everything harmful. In fact alcohol, nicotine, some drugs and some viruses cross it.
D3.1.19 Progesterone keeps pregnancy going; its fall lets birth begin HL
Progesterone maintains the endometrium and inhibits uterine contractions.
First the corpus luteum secretes it, kept alive by hCG; then the placenta takes over.
At term, progesterone falls, releasing the inhibition on contractions.
Oxytocin from the posterior pituitary drives contractions by positive feedback until delivery.
Each contraction stretches the cervix, which signals for more oxytocin and stronger contractions.
Students often think the placenta makes progesterone from implantation. In fact the corpus luteum does at first; the placenta takes over later.
Students often think birth is triggered by a rise in progesterone. In fact it is triggered by a fall.
D3.1.20 HRT and heart disease: correlation is not cause HL
HRT gives oestrogen, with or without progesterone, during and after the menopause.
Early epidemiological studies found HRT users had less coronary heart disease.
Later randomised controlled trials showed HRT slightly raises CHD risk.
The correlation was confounded: HRT users had higher socioeconomic status, which itself lowers CHD risk.
Students often think a strong correlation in a large study proves cause. In fact confounding variables can explain it.
Students often think a disproved correlation was a mistake. In fact the correlation was genuine; only the causal inference was wrong.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 What is the relative advantage of sexual reproduction over asexual reproduction?
Answer and reasoning
Its offspring carry new gene combinations, giving the variation needed for a changed environment. — Sexual reproduction produces offspring with new combinations of alleles, so among them some are likely to be adapted to an environment that has changed. This is its relative advantage; asexual reproduction's is the faithful copying of a genotype already adapted to the existing environment.
Its offspring gain the particular features that a changed environment comes to require. — A student who thinks variation is produced to meet a need picks this. Meiosis and fertilization generate new combinations blindly; the environment selects among them afterwards.
Its offspring are better adapted than the parent in every environment, so more of them survive. — A student who believes sexual reproduction is always superior picks this. In a stable environment a well-adapted parent does better to copy its genotype asexually than to break it up.
Its offspring carry new alleles produced by meiosis, which asexual offspring cannot receive. — A student who thinks meiosis makes new alleles picks this. New alleles arise only by mutation, in asexual lineages too; meiosis and fertilization produce new combinations of existing alleles.
2 In a fish species the female lays a few hundred large eggs in a nest and the male releases many millions of tiny sperm over them. Which statement explains the difference in size and number?
Answer and reasoning
Sperm must travel to the eggs, so they are small with few food reserves and can be produced cheaply in vast numbers. — The prime difference between the sexes is that the male gamete travels to the female gamete, so it is smaller with less food reserves than the egg. From this follow the differences in numbers of gametes.
Eggs are larger because each one carries far more of the genetic material than a sperm does. — A student who equates cell size with DNA content picks this. Sperm and egg each contribute one haploid set; the egg's extra bulk is cytoplasm with food reserves.
The numbers should be equal since one sperm fertilizes one egg; the surplus sperm are defective ones. — A student who reasons back from one-to-one fertilization picks this. Vast numbers of normal sperm are released because only a tiny fraction complete the journey to an egg.
Sperm are small because so many are made that only a little cytoplasm is available for each one. — A student who reverses the cause picks this. Sperm are small because they must travel; being small and cheap is what makes it possible to produce so many.
3 What is the role of follicle-stimulating hormone (FSH) in the ovarian cycle?
Answer and reasoning
It causes the mature follicle to burst and release its egg at ovulation. — A student who reads 'follicle-stimulating' as doing everything to the follicle picks this. Ovulation is caused by the surge of LH.
It stimulates the follicles to develop and to secrete oestradiol. — FSH from the pituitary stimulates a group of follicles to develop at the start of the cycle and stimulates the developing follicles to secrete oestradiol.
It is made in the corpus luteum to maintain the endometrium. — A student who has muddled which gland makes which hormone picks this. FSH is a pituitary hormone; the corpus luteum secretes progesterone.
Its rise at the end of the cycle is what triggers menstruation. — A student who looks for a rising hormone to explain menstruation picks this. FSH does rise as progesterone falls, but it is the fall in progesterone that causes the endometrium to break down.
4 IVF treatment begins with a drug that suspends the woman's own secretion of FSH and LH, followed by daily injections of FSH. Why is this done?
Answer and reasoning
Suspending her own cycle stops it interfering, so injected FSH can induce superovulation and many eggs be collected. — The normal secretion of hormones is suspended so the natural cycle does not interfere; artificial doses of FSH then stimulate many follicles to develop at once (superovulation), giving several eggs for collection.
The injected FSH strengthens her natural cycle so that one egg is released at a predictable time. — A student who thinks IVF works within the normal cycle picks this. The aim is many eggs, not one, and the natural cycle has been deliberately switched off.
The injections make fertilization possible in the oviduct once sperm are introduced into the body. — A student who confuses IVF with artificial insemination picks this. In IVF fertilization takes place outside the body, in the laboratory.
The FSH injections cause ovulation, so that the released eggs can be collected from the uterus. — A student who attributes ovulation to FSH picks this. FSH stimulates follicle development; eggs are collected from the follicles before ovulation occurs.
5 Which structure of an insect-pollinated flower is correctly paired with its function?
Answer and reasoning
Stigma: produces pollen grains and sheds them onto the bodies of visiting insects. — A student who has swapped stigma and anther picks this. The anther produces and releases pollen; the stigma receives it.
Ovary: holds the seeds until the fruit ripens and the seeds are dispersed. — A student who thinks the flower already contains seeds picks this. The ovary contains ovules, which become seeds only after fertilization.
Petal: brightly coloured to attract insects that carry the seeds away from the plant. — A student who merges pollination with seed dispersal picks this. Petals attract insects that transfer pollen; seed dispersal happens later, after fertilization.
Stigma: sticky surface on which pollen grains land and then germinate. — The stigma is the receptive tip of the carpel; its sticky surface traps pollen brushed off a visiting insect, and the pollen tube then grows down the style.
6 In a self-incompatible plant species, pollen from a flower lands on the stigma of the same flower and germinates, but no seed is set. Which statement about self-incompatibility explains this?
Answer and reasoning
It works by physically preventing the flower's own pollen from ever landing on its stigma. — A student who models it on separate flowers and timing picks this. Self-incompatibility is a genetic mechanism: matching alleles in pollen and carpel are recognized and the pollen tube is stopped.
It prevents inbreeding, which would otherwise cause harmful new mutations in the offspring. — A student who explains genetic defects by mutation picks this. Inbreeding makes harmful recessive alleles already present more likely to be homozygous; it creates no alleles.
It stops a plant's own pollen from fertilizing its eggs, so preventing inbreeding. — Self-pollination leads to inbreeding, which decreases genetic diversity and vigour. Genetic self-incompatibility mechanisms ensure the gametes fusing at fertilization are from different plants.
It is needed because self-fertilization would be asexual and so produce clones. — A student who thinks one parent means asexual picks this. Self-fertilization is still sexual reproduction; the problem is inbreeding, not cloning.
7 A child whose hypothalamus cannot release GnRH fails to enter puberty, although the pituitary gland and gonads are normal. Which explanation is correct? HL
Answer and reasoning
GnRH normally acts directly on the gonads, so without it they cannot make sex hormones; the pituitary gland plays no part in puberty. — A student who reads 'gonadotropin' as 'acts on gonads' picks this. GnRH acts on the pituitary, which releases the gonadotropins LH and FSH that act on the gonads.
Without GnRH the pituitary does not increase LH and FSH release, so the gonads are not stimulated to raise sex hormone production. — Puberty is triggered by increased GnRH release from the hypothalamus, which raises LH and FSH secretion by the pituitary; these stimulate the gonads to secrete the sex hormones that cause the changes of puberty.
The gonads should start puberty on their own, so the fault must in fact lie in the testes or the ovaries themselves. — A student who places the origin of puberty in the gonads picks this. Normal gonads stay inactive without LH and FSH, which in turn depend on GnRH.
Puberty requires the pituitary gland to secrete the steroid sex hormones, which it cannot do without GnRH. — A student who thinks the pituitary makes steroid hormones picks this. The pituitary secretes LH and FSH; testosterone, oestradiol and progesterone are made by the gonads.
8 In an experiment, eggs treated so that they could not carry out the cortical reaction were each entered by several sperm. Which explanation is correct? HL
Answer and reasoning
Without the cortical reaction the zona pellucida is not hardened, so further sperm can still digest through it and fuse. — The cortical reaction, triggered by the first sperm's fusion, alters the zona pellucida so that no more sperm can pass through. Blocking it removes the barrier to polyspermy.
The cortical reaction is what lets the first sperm in, so blocking it should have stopped fertilization altogether. — A student who has swapped the two reactions picks this. The acrosome reaction lets a sperm through the zona pellucida; the cortical reaction blocks the rest.
Sperm bore through the egg membrane with their enzymes, so hardening the zona could not have stopped them. — A student who thinks enzymes act on the egg membrane picks this. Enzymes digest only the zona pellucida; a hardened zona does stop sperm, which is why the treated eggs let more in.
Only one sperm normally reaches the egg, so the treatment must somehow have attracted extra sperm to it. — A student who pictures a single winning sperm picks this. Many sperm reach every egg; what normally stops all but one entering is the cortical reaction.
9 A home pregnancy test strip contains monoclonal antibodies. Which statement explains how the test indicates pregnancy? HL
Answer and reasoning
The antibodies bind progesterone, which is present in the urine at a high level only during a pregnancy. — A student who reaches for the pregnancy hormone picks this. Progesterone is also high in the luteal phase of every cycle; the test detects hCG, which only an embryo makes.
The antibodies bind hCG released into the blood by the mother's pituitary gland when she becomes pregnant. — A student who assigns hCG to a maternal gland picks this. hCG is secreted by the embryo and developing placenta, which is why its presence signals pregnancy.
The antibodies bind hCG in the urine, and only an implanted embryo or its placenta produces hCG. — Monoclonal antibodies specific to hCG bind it as urine flows along the strip and produce a coloured line; because only the embryo or placenta secretes hCG, its detection indicates pregnancy.
The woman's own antibodies against hCG appear in her urine and are captured on the test strip. — A student who thinks all antibodies come from the body picks this. The woman does not make antibodies to her own hormone; the strip's monoclonal antibodies are manufactured.
10 Which sequence correctly describes the hormonal changes that bring about childbirth? HL
Answer and reasoning
Progesterone falls, contractions begin, and oxytocin rises by positive feedback as the cervix is stretched. — A decrease in progesterone removes the inhibition of uterine contractions and allows oxytocin secretion to increase; stretching of the cervix stimulates more oxytocin release, a positive feedback loop that continues until birth.
Progesterone rises to a peak that triggers contractions, and oxytocin then falls until the baby is born. — A student who expects a rising hormone to trigger the event picks this. Progesterone keeps the uterus relaxed; it is its fall that allows childbirth to start.
Oxytocin is kept at a steady level by negative feedback so that the contractions stay regular. — A student who assumes all hormonal control is negative feedback picks this. Oxytocin secretion rises progressively by positive feedback during labour.
The pituitary gland stops secreting progesterone, allowing it to secrete oxytocin in its place. — A student who places progesterone in the pituitary picks this. Progesterone comes from the corpus luteum and then the placenta; the pituitary secretes oxytocin.
Read the ones marked not yet in Learn, then Verify.
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21 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 Aphids reproduce asexually through the summer, when conditions are stable and food is plentiful, and switch to sexual reproduction in autumn before conditions change. Which statement best explains this pattern?
Answer and reasoning
Sexual reproduction is used in autumn because varied offspring are invariably fitter; the summer clones are a poor substitute produced under time pressure. — A student who believes variation is always an advantage picks this. In stable summer conditions the clones are the better strategy: every offspring inherits the whole adapted genotype.
Summer clones copy an adapted genotype quickly; autumn sexual reproduction produces varied offspring for the changed conditions ahead. — Asexual reproduction suits an individual already adapted to the existing environment, giving many identical offspring fast. Sexual reproduction produces new gene combinations, the variation needed for adaptation to a changed environment.
Sexual reproduction is used in autumn so that the offspring gain the specific cold-tolerance features that the coming winter demands. — A student who thinks variation is directed towards what is needed picks this. New allele combinations are random; some may happen to suit winter, and selection then favours those.
Asexual reproduction in summer shows that aphids are simple organisms; the autumn sexual stage is their true form of reproduction. — A student who files asexual reproduction with bacteria and simple organisms picks this. Many eukaryotes, including these insects, use both modes; the choice depends on the environment, not on complexity.
2 In the sexual life cycle, what are the respective roles of meiosis and fusion of gametes in producing genetic variation?
Answer and reasoning
Meiosis produces new alleles by mutation, and fusion of gametes combines the alleles of the two parents. — A student who merges mutation with meiosis picks this. Meiosis does not make alleles; it separates and reshuffles the alleles the parent already has.
Meiosis is the only source of variation, and fusion of gametes merely restores the diploid number. — A student who treats fertilization as a mechanical ending picks this. Because any sperm may fuse with any egg, fusion of gametes is itself a source of new combinations.
Meiosis breaks up parental allele combinations, and fusion of gametes produces new combinations. — This is the guide's statement of the two roles: meiosis breaks up parental combinations of alleles, and fusion of gametes (fertilization) produces new combinations.
Meiosis halves the parental allele set but keeps it intact, and fusion of gametes restores it. — A student who pictures a gamete as half a copy of the parent picks this. Independent assortment and crossing over mean no gamete carries the parent's own combination intact.
3 Two full siblings have the same two parents yet differ in many of their alleles. Which explanation is correct?
Answer and reasoning
Each parent passed on the same half of its alleles both times, but the halves combined differently in each child. — A student who thinks a parent hands over a fixed half picks this. Meiosis gives every gamete a different selection of alleles, so the halves themselves differed.
New alleles arose by mutation during the meiosis that produced the gametes for each sibling. — A student who credits meiosis with making alleles picks this. Mutation is rare; the siblings differ because existing alleles were shuffled into different combinations.
Meiosis alone made the gametes differ; which sperm met which egg made no difference to the alleles. — A student who denies fertilization any role picks this. Which of the many varied sperm fused with which egg determined each sibling's combination, so fusion of gametes contributed too.
Each gamete carried a different selection of parental alleles, and different gametes fused. — Meiosis broke up each parent's allele combination differently in each gamete, and fertilization brought together a different pair of gametes for each child, producing two new combinations.
4 A student annotates a diagram of the male reproductive system. Which annotation is correct?
Answer and reasoning
Urethra: carries urine only, because semen leaves through a separate duct in the penis. — A student who knows the urethra only from the excretory system picks this. The sperm duct joins the urethra, which carries semen during ejaculation and urine at other times.
Epididymis: stores sperm while they mature after their production in the testis. — Sperm are produced in the testis and pass into the epididymis, the coiled tube on its surface, where they mature and are stored until ejaculation.
Seminal vesicle: stores the sperm until they are required for ejaculation. — A student misled by the name picks this. The seminal vesicles are glands that add alkaline, fructose-containing fluid to sperm; they do not store them.
Testis: produces the semen that carries sperm along the duct to the urethra. — A student who equates semen with sperm picks this. The testis produces sperm; the fluid of semen is added by the seminal vesicles and prostate gland.
5 A student annotates a diagram of the female reproductive system. Which annotation is correct?
Answer and reasoning
Uterus: the site of fertilization and, later, of implantation of the embryo. — A student who places every reproductive event in the uterus picks this. Fertilization occurs in the oviduct; only implantation occurs in the uterus.
Endometrium: the thick muscular wall of the uterus that contracts during childbirth. — A student who confuses the lining with the wall picks this. The endometrium is the inner lining that thickens each cycle and is shed at menstruation.
Oviduct: the site of fertilization; it moves the egg towards the uterus. — The oviduct collects the egg released at ovulation, its cilia move the egg towards the uterus, and it is where sperm normally meet and fertilize the egg.
Ovary: releases an egg into the oviduct on day 1 of each menstruation. — A student who ties the egg's release to the visible bleeding picks this. Ovulation happens around day 14, not at the start of menstruation.
6 Which statement correctly describes the feedback control operating in the days just before ovulation?
Answer and reasoning
High oestradiol inhibits LH secretion by negative feedback, as in the rest of the cycle. — A student who has only met negative feedback picks this. Before ovulation the high oestradiol from the mature follicle stimulates LH release, which is positive feedback.
Progesterone secreted by the pituitary stimulates oestradiol release by positive feedback. — A student who puts every cycle hormone in the pituitary picks this. Progesterone comes from the corpus luteum, which does not exist until after ovulation.
High oestradiol stimulates FSH secretion, and it is the FSH that then causes ovulation. — A student who attributes ovulation to FSH picks this. FSH does rise slightly, but the LH surge is what causes the follicle to burst.
High oestradiol stimulates LH secretion by positive feedback on the pituitary. — Rising oestradiol from the maturing follicle stimulates the pituitary to release more LH, which stimulates more oestradiol: positive feedback that produces the LH surge and ovulation.
7 Blood samples were taken from a woman on day 7 and day 21 of a regular 28-day cycle. Progesterone was very low on day 7 and high on day 21. What can be concluded about day 21?
Answer and reasoning
A corpus luteum has formed after ovulation and is secreting progesterone that maintains the endometrium. — High progesterone a week after mid-cycle shows that ovulation has occurred and the ruptured follicle has become a corpus luteum, whose progesterone keeps the thickened endometrium in place.
The pituitary gland has increased its progesterone output in preparation for menstruation. — A student who assigns progesterone to the pituitary picks this. Progesterone is secreted by the corpus luteum in the ovary, and it postpones menstruation rather than preparing for it.
Ovulation is about to occur, because the egg is released when menstruation begins on day 28. — A student who links egg release to menstruation picks this. Ovulation occurred around day 14; the high progesterone is evidence that it has already happened.
The rising progesterone will itself trigger the menstruation that is due on day 28. — A student who expects a rise to cause the event picks this. Menstruation follows the fall in progesterone when the corpus luteum degenerates.
8 Which event occurs during fertilization in humans?
Answer and reasoning
The whole sperm enters the egg, so the embryo inherits its mitochondria from both parents. — A student who pictures the sperm entering as a complete cell picks this. Only the nucleus enters; the tail and mitochondria are destroyed.
The sperm and egg nuclei fuse into a single diploid nucleus before any cell division occurs. — A student who learned 'the nuclei fuse' at GCSE picks this. The nuclear membranes dissolve and the chromosomes join a mitosis that directly produces two diploid nuclei.
The sperm nucleus enters the egg but the sperm's tail and mitochondria are destroyed. — After the sperm's cell membrane fuses with the egg cell membrane, the sperm nucleus enters the egg; the tail and mitochondria do not contribute to the zygote.
Enzymes from the sperm digest a hole in the egg membrane to let the nucleus in. — A student who extends the acrosome enzymes to the egg membrane picks this. Entry is by fusion of the two cell membranes; enzymes act only on the coat outside the membrane.
9 A mutation in mitochondrial DNA is passed from a mother to all her children but is never passed on by a father. Which feature of fertilization explains this?
Answer and reasoning
Sperm mitochondria enter the egg but are so outnumbered by the egg's own that their DNA is diluted away. — A student who believes the sperm's mitochondria get in picks this. They do not enter: they are destroyed with the tail, so no paternal mitochondrial DNA is present to be diluted.
Each parent hands over exactly half of its DNA, and the mitochondrial DNA happens to fall in the mother's half. — A student who pictures inheritance as each parent handing over a fixed half of its DNA picks this. Meiosis halves the nuclear DNA only; mitochondrial DNA is not part of that division, and the father passes on none because the sperm's mitochondria are destroyed at fertilization.
The egg is larger because it carries more genetic material, including all the mitochondria required. — A student who equates the egg's size with extra genetic material picks this. The egg's size is due to food reserves; maternal inheritance of mitochondria is due to the fate of the sperm's mitochondria.
The sperm's mitochondria are destroyed after its nucleus has entered the egg cytoplasm. — Only the sperm nucleus enters the egg; the sperm's tail and mitochondria are destroyed. All the zygote's mitochondria, and so all its mitochondrial DNA, come from the egg.
10 Where are the gametes of a flowering plant produced?
Answer and reasoning
The pollen grains themselves are the male gametes and the ovules are the female gametes. — A student who casts pollen as the plant's sperm picks this. Pollen grains and ovules are structures inside which the gametes are produced.
Male gametes are produced inside pollen grains and female gametes inside ovules. — Gametes are produced inside ovules (female, within the ovary) and inside pollen grains (male, within the anther); pollination and pollen tube growth bring them together.
Male gametes are produced in the stigma and female gametes in the anther. — A student who has swapped the two central structures picks this. The anther produces pollen (male); the stigma receives it and belongs to the female carpel.
Male gametes are produced inside pollen grains and female gametes in the seeds. — A student who backdates seeds to the flower picks this. The ovary contains ovules; an ovule becomes a seed only after its gamete has been fertilized.
11 Garden pea flowers pollinate and fertilize themselves before they open. Which statement about reproduction in the pea is correct?
Answer and reasoning
It is asexual, because only one parent plant takes part in the process. — A student who defines asexual reproduction by the number of parents picks this. The pea's gametes are made by meiosis and fuse, so its reproduction is sexual even though it self-fertilizes.
Fertilization is complete as soon as the pollen has landed on the flower's stigma. — A student who equates pollination with fertilization picks this. After pollination a pollen tube must grow down the style before a male gamete can fuse with the female gamete.
It is sexual, because meiosis makes the gametes and they fuse at fertilization. — Reproduction in flowering plants is sexual even if a plant is hermaphroditic: gametes formed by meiosis fuse to give a zygote. Whether the offspring differ from the parent depends on how heterozygous it is; what makes the process sexual is meiosis and fusion of gametes, not the number of parents.
The pollen grains, being the male gametes, fuse directly with the ovules in the ovary. — A student who treats pollen as the gamete picks this. The pollen grain grows a tube that delivers the male gametes it contains to the female gamete inside the ovule.
12 In one flower species the anthers shed their pollen and wither several days before the stigma of the same flower becomes receptive. How does this promote cross-pollination?
Answer and reasoning
The flower's own pollen is gone when its stigma is receptive, so only pollen brought from another plant can pollinate it. — Different maturation times for pollen and stigma are a method of promoting cross-pollination: a flower cannot pollinate itself if its pollen is shed before its stigma can receive pollen.
It does not; a flower that has both stamens and carpels will pollinate itself whatever the timing of the two. — A student who reasons from the closeness of the parts picks this. Timing is exactly what stops the nearby pollen mattering: it is no longer there when the stigma is ready.
It ensures the pollen reaches other flowers on the same plant, which is precisely what cross-pollination means. — A student who thinks any flower-to-flower transfer is cross-pollination picks this. Transfer between flowers on one plant is still self-pollination; cross-pollination needs a different plant.
It provides the physical barrier that self-incompatibility genes require to keep pollen off the stigma. — A student who thinks self-incompatibility is a physical block picks this. Self-incompatibility is a genetic recognition mechanism acting after pollen lands and is separate from timing.
13 Which of these is a method that promotes cross-pollination?
Answer and reasoning
Transfer of pollen between different flowers on the same plant. — A student who counts any movement between flowers as crossing picks this. Pollen from the same plant carries the same genotype, so this is self-pollination.
Male and female flowers borne on separate individual plants. — When a species has male and female flowers on different plants, pollen must be carried from one plant to another, so every pollination is a cross-pollination.
Seeds carried away from the parent plant by animals. — A student who confuses dispersal with pollination picks this. Seed dispersal happens after fertilization and does not move pollen at all.
The stigma physically rejecting its own flower's pollen. — A student who thinks self-incompatibility is a physical barrier picks this. Self-incompatibility is genetic: the plant's own pollen may land, but its tube fails to grow.
14 What is the difference between pollination and seed dispersal?
Answer and reasoning
They are the same process: the wind or insect that carries the pollen also carries the seeds away. — A student who sees the same agents in both lists picks this. Pollination happens before fertilization and moves pollen; dispersal happens after it and moves seeds.
Pollination is the fertilization of the flower; dispersal is the release of the fertilized pollen grains. — A student who equates pollination with fertilization picks this. Pollination merely delivers pollen to the stigma, and what is dispersed is seeds, not pollen.
Pollination delivers pollen to the seeds already in the ovary; dispersal releases those seeds. — A student who thinks the flower already contains seeds picks this. The ovary holds ovules, which become seeds only after fertilization.
Pollination moves pollen to a stigma before fertilization; dispersal moves seeds after it. — Pollination is the transfer of pollen from anther to stigma and precedes fertilization. Seed dispersal is the movement of the seeds formed after fertilization away from the parent plant.
15 A barley grain germinating in dark, moist soil grows a root and a shoot for several days. Where do the energy and materials for this growth come from?
Answer and reasoning
Starch stored in the grain is hydrolysed by amylase to soluble sugars, which are moved to the embryo and respired. — Germination involves the mobilization of food reserves: amylase, produced in response to gibberellin, hydrolyses starch to maltose, which is converted to glucose for cell respiration and growth of the embryo.
The emerging shoot photosynthesizes using the small amount of light that reaches it through the soil. — A student who links all plant growth to photosynthesis picks this. In darkness there is no photosynthesis; the seedling lives on its reserves until leaves reach the light.
The grain absorbs dissolved food from the surrounding soil along with the water that it takes up. — A student who treats soil as food picks this. The soil supplies water and oxygen; the organic material comes from the seed's own stores.
Water absorbed by the grain provides the mass, so the embryo grows by swelling rather than by respiring stores. — A student who equates growth with swelling picks this. New cells and cell walls are built from the mobilized reserves using ATP from respiration; dry mass falls as the stores are used.
16 Which comparison of spermatogenesis and oogenesis in humans is correct? HL
Answer and reasoning
Each meiosis yields four gametes in both processes, but three of every four eggs degenerate before birth. — A student who applies the 'four cells from meiosis' rule to eggs picks this. In oogenesis the cytoplasm divides unequally, so each meiosis gives one egg and small polar bodies.
Both processes continue to produce new gametes from puberty right through to old age. — A student who generalizes from sperm picks this. All egg-forming cells are made before birth; only spermatogenesis continues producing new gametes throughout adult life.
Each meiosis yields four sperm but only one egg, because oogenesis divides the cytoplasm unequally. — In spermatogenesis the two divisions of meiosis give four equal cells that differentiate into sperm; in oogenesis each division is unequal, producing one large egg with most of the cytoplasm and polar bodies that degenerate.
In both processes the two divisions of meiosis are completed inside the gonad before the gamete is released. — A student who assumes the ovulated cell is a finished gamete picks this. The second meiotic division of oogenesis is completed only after a sperm enters the egg.
17 A man produces about 100 million sperm per day. A woman is born with about a million egg-forming cells arrested in meiosis, and releases only about 400 eggs in her lifetime. Which conclusion do these figures best support? HL
Answer and reasoning
Eggs are produced continuously like sperm, but nearly all are broken down before they can be ovulated. — A student who assumes continuous production picks this. The figures show a fixed stock formed before birth; most of those cells degenerate, but none are newly made after birth.
Each of the million cells produced four eggs, most of which were not released from the ovary. — A student who applies the four-products rule picks this. Each cell that completes meiosis yields one egg plus polar bodies, so four million eggs were never present.
The sperm figure must be an overestimate, since fertilization needs only one sperm for each egg. — A student who expects gamete numbers to match picks this. Vast sperm numbers are the norm because so few survive the journey to the egg.
Egg-forming cells are all made before birth and are then used sparingly over a lifetime. — Oogenesis begins in the foetus with mitosis, growth and the start of meiosis; from puberty a few cells resume meiosis each cycle and one egg is usually released, whereas spermatogenesis produces millions of new sperm daily.
18 Which statement about the blastocyst and implantation is correct? HL
Answer and reasoning
The zygote itself implants in the endometrium immediately after fertilization, before it has divided into a ball of cells. — A student who runs fertilization straight into implantation picks this. The zygote divides for several days on its way to the uterus, and only the blastocyst implants.
The blastocyst is a hollow ball with an inner cell mass and implants in the endometrium about a week after fertilization. — By about day 5 to 7 the embryo is a blastocyst: a hollow ball with a fluid-filled cavity, an inner cell mass that forms the foetus and an outer layer that attaches to and sinks into the endometrium.
The blastocyst is a solid ball of identical cells, every one of which goes on to form part of the foetus. — A student who thinks all the early cells are equivalent picks this. The blastocyst is hollow and its outer layer contributes to the placenta, not the foetus.
The blastocyst forms and implants in the uterus, at the same site where fertilization took place. — A student who places fertilization in the uterus picks this. Fertilization occurs in the oviduct; the blastocyst forms during transit and implants in the endometrium.
19 A foetus in the uterus has fluid-filled, collapsed lungs, yet its blood is oxygenated. How does oxygen reach the blood of the foetus? HL
Answer and reasoning
Maternal blood carrying oxygen flows through the placenta into the blood vessels of the foetus. — A student who imagines a shared circulation picks this. Maternal and foetal blood stay separate; oxygen crosses from one to the other in the villi.
The placenta filters oxygen out of the mother's blood while blocking all harmful substances. — A student who sees the placenta as a selective filter picks this. Exchange is by diffusion and transport according to physical properties, and some harmful substances cross.
The foetus breathes it in from the surrounding amniotic fluid, using its own lungs. — A student who knows no route to oxygen other than breathing picks this. The foetal lungs are fluid-filled and unused; all gas exchange is across the placenta.
It diffuses from maternal blood into foetal capillaries in the placental villi. — Oxygen diffuses down its concentration gradient from maternal blood in the placenta into foetal blood in the capillaries of the villi, whose large surface area makes exchange rapid.
20 Early epidemiological studies found that women taking HRT had a lower incidence of coronary heart disease (CHD). Later randomized controlled trials found that HRT slightly increased CHD risk. Which conclusion is best supported? HL
Answer and reasoning
The early studies were correct because they involved far more women than the trials did, so their result should be preferred. — A student who judges studies by size picks this. However large, an observational study cannot remove differences between women who chose HRT and those who did not; random allocation can.
The correlation was not causal: women on HRT tended to have higher socioeconomic status, which itself lowers CHD risk. — The correlation between HRT and lower CHD was not a cause-and-effect relationship. HRT users had higher socioeconomic status, a factor with a causal relationship to lower CHD risk; the randomized trials showed HRT's own effect is a small increase.
HRT must lower CHD risk in most women, and the trials must have happened to sample an unusual group of women. — A student who treats a correlation as proof of cause picks this. The trials were designed to isolate HRT's effect, and they showed a small increase in risk.
The early correlation must have been a chance result: women on HRT did not really have lower CHD rates. — A student who thinks a non-causal correlation must be a false finding picks this. The correlation was real and repeatable; what was wrong was the inference that HRT caused the lower rate, when socioeconomic status explained it.
21 In a mammal, removing the ovaries (and with them the corpus luteum) early in pregnancy ended the pregnancy, but the same operation late in pregnancy did not. What does this show? HL
Answer and reasoning
The placenta maintained the pregnancy in both cases; the early loss was caused by the stress of surgery. — A student who thinks the placenta works from implantation picks this. Early on there is no functioning placenta; the corpus luteum is the only source of the progesterone needed.
Later in pregnancy the mother's pituitary gland takes over progesterone secretion, so the ovaries are no longer needed. — A student who places progesterone in the pituitary picks this. Progesterone is secreted by the corpus luteum and then by the placenta; the pituitary secretes FSH, LH and oxytocin, not progesterone.
Progesterone is secreted by the corpus luteum at first and later by the placenta once it has formed. — Continuity of pregnancy depends on progesterone, secreted initially by the corpus luteum and then by the placenta. Once the placenta has taken over, the ovaries are no longer needed.
Late in pregnancy the mother's pituitary secretes hCG, which replaced the missing progesterone. — A student who assigns hCG to a maternal gland picks this. hCG is made by the embryo and placenta, and it is placental progesterone, not hCG, that sustains late pregnancy.
That was your twenty minutes. Real practice on D3.1 is past-paper questions marked against the mark scheme.
What the exam asks of D3.1
Paper 1A asks you to match a hormone to its source or effect, label a flower or reproductive diagram, or classify a mechanism as self- or cross-pollination. Paper 1B gives hormone-level graphs across the cycle or HRT trial data and asks you to interpret them. Paper 2 uses *outline* for structures and *explain* for feedback: name the hormone, its source, its target, then the effect. HL questions use *distinguish* for spermatogenesis versus oogenesis and *evaluate* for epidemiological versus randomised evidence.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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