One parent or two: asexual and sexual reproduction, and the life cycles that use both
By the end of this lesson you will be able to
- Define asexual and sexual reproduction and compare them in terms of parents, gametes, type of cell division and variation in the offspring.
- Describe six forms of asexual reproduction with a named example of each, and calculate the growth of a population that reproduces by binary fission.
- Explain why gametes must be haploid and how meiosis and random fertilisation produce variation.
- Evaluate the advantages and disadvantages of each strategy, including the “cost of males”, and suggest when each is favoured.
- Describe the life cycles of a human and a flowering plant, and of an aphid and a coral, which use both asexual and sexual reproduction.
You met this before
- Reproduction is one of the seven life processes: every organism alive today exists because its ancestors made copies of themselves before they died (Year 1, Unit 4).
- Body cells divide by mitosis: the chromosomes are copied and one full set goes to each of two genetically identical daughter cells — used for growth, repair and replacement (Year 2, Unit 3).
- Human body cells have 46 chromosomes in 23 pairs; sperm and egg cells have 23, one of each pair, so that the cell formed at fertilisation has 46 again (Year 3, Unit 1).
- A sperm cell is small and mobile (head with the nucleus, mid-piece full of mitochondria, tail); an egg cell is large and still, with a food store in its cytoplasm (Year 3, Unit 1).
- Variation is the differences between individuals of the same species; some is caused by genes, some by the environment, much by both (Year 3, Unit 4).
A cell with 46 chromosomes divides by mitosis. State how many chromosomes each daughter cell has, and whether the two daughter cells are genetically identical.
Reveal
Each daughter cell has 46 chromosomes, and the two are genetically identical to each other and to the parent cell: every chromosome was copied before division, and one copy of each went to each side. That single fact — mitosis copies, it does not mix — is the whole reason asexual offspring are clones.
Revise it in full: Year 2 · Cell functions · Lesson 2 — Cells dividing: copy, split, and why they stay small · Year 3 · Body systems · Lesson 5 — Movement, reproduction and the whole body · Year 1 · Living things · Lesson 1 — The seven life processes
The puzzle
In 2006 a Komodo dragon called Flora, at Chester Zoo in England, laid a clutch of eggs. She had never shared an enclosure with a male. Several of the eggs hatched in early 2007 into healthy young dragons — all of them male.
Genetic tests showed that every one of them carried only their mother’s DNA. A female reptile had produced offspring with no father at all.
The same thing happens every summer, billions of times, on the leaves of the cabbages and roses in your own garden: aphids give birth to daughters that were never fertilised, and those daughters are born already carrying daughters of their own.
If making offspring alone is possible — and faster — why does almost every animal and plant on Earth bother with sex?
A field of sweet potatoes grown from cuttings of one plant, and a field of maize grown from seed, are both hit by a new fungal disease. Predict which field is more likely to lose every plant, and give a reason.
Compare your answer
The sweet potato field. Cuttings are pieces of one plant, so every plant in the field is genetically identical — a clone. If the fungus can infect one, it can infect them all.
The maize plants grew from seeds made by sexual reproduction, so they differ genetically; some may carry alleles that give partial resistance, and they survive to produce the next generation.
Hold on to this: variation is the insurance that sexual reproduction buys, and this lesson is about its price.
The science — structured notes
1.1 Two ways to make a new organism
Asexual reproduction is the production of offspring from one parent, with no gametes and no fertilisation. The new cells are made by mitosis, so the offspring are genetically identical to the parent and to each other. A group of genetically identical organisms is called a clone.
Sexual reproduction is the production of offspring by the fusion of two gametes (sex cells), usually from two parents. Each gamete carries half the normal number of chromosomes; fusion restores the full number in a single new cell, the zygote. The offspring are genetically different from each parent and from each other.
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Number of parents | One | Usually two (some plants fertilise themselves) |
| Gametes and fertilisation | None | Two haploid gametes fuse to form a diploid zygote |
| Cell division used | Mitosis only | Meiosis to make gametes; mitosis to grow the zygote into an organism |
| Offspring | Genetically identical — clones (except for rare mutations) | Genetically different — variation |
| Speed and numbers | Fast; every individual can reproduce | Slower; needs a mate, pollinator or water to bring gametes together |
| Energy cost | Low | High: finding mates, flowers, nectar, courtship, gametes that are never used |
| Examples | Bacteria, yeast, potato tubers, strawberry runners, aphids in summer | Humans, flowering plants from seed, most animals, corals at spawning |
“Offspring produced by asexual reproduction show no variation at all.”
Reveal answer
False, but nearly true. Asexual offspring are genetically identical except for rare mutations — copying errors in DNA — which arise in any cell division. They can also differ because of their environment: two cuttings of one sweet potato, one in rich soil and one in sand, grow to very different sizes. What asexual reproduction does not produce is new combinations of alleles.
1.2 Six forms of asexual reproduction
Asexual reproduction has evolved many times, in many forms. All of them rely on mitosis, and all of them give clones. The eAssessment has used the time-lapse of a yeast cell budding and asked which characteristic of life it shows: the only answer accepted was reproduction.
| Form | What happens | Examples |
|---|---|---|
| Binary fission | The cell copies its DNA, grows and splits into two equal daughter cells | Bacteria (every 20 minutes for E. coli in ideal conditions), Amoeba, Paramecium |
| Budding | An outgrowth (bud) forms on the parent, grows and separates, or stays attached to build a colony | Yeast, Hydra, coral polyps |
| Fragmentation | The body breaks into pieces and each piece regrows the missing parts | Flatworms, sea stars (an arm with part of the central disc), many corals, filamentous algae |
| Spore formation | Huge numbers of tiny cells, made by mitosis, are released and each can grow into a new organism | Bread mould (Rhizopus), Penicillium and other moulds (ferns, mosses and mushrooms also release spores, but theirs are made by meiosis as part of a sexual life cycle) |
| Vegetative propagation | A part of a plant — stem, root or leaf — grows into a new, independent plant | Runners (strawberry), tubers (potato), bulbs (onion, garlic), rhizomes (ginger), plantlets on leaf edges (Kalanchoe), suckers (banana) |
| Parthenogenesis | An egg develops into an offspring without being fertilised | Aphids in summer, some stick insects, the all-female New Mexico whiptail lizard, occasionally Komodo dragons and sharks in zoos |
Parthenogenesis is the odd one out because it uses an egg. In aphids the egg is made by mitosis, so the daughters are clones of their mother.
In Komodo dragons a different route is used: the egg’s nucleus fuses with another product of the same meiosis, so the young carry only their mother’s DNA but are not exact copies of her. In Komodo dragons females are ZW and males ZZ; the eggs that develop end up ZZ, which is why Flora’s young were all male.
A single E. coli cell divides by binary fission every 20 minutes in warm broth. Calculate the number of cells after 6 hours, assuming none die, and give your answer in standard form.
Number of divisions (generations) = time ÷ time per division
Number of generations = 360 min ÷ 20 min
Number of generations = 18
Number of cells = 218
Number of cells = 262 144
Number of cells ≈ 2.6 × 10⁵
Each generation doubles the number, so the growth is exponential: the last 20 minutes alone added 131 072 cells. In a real flask the food runs out and waste builds up long before this can continue for a day.
A potato plant grown from one tuber produces 12 new tubers. Each is planted the following season and produces 12 more. Calculate the number of tubers after three seasons, and state how many different genotypes are among them.
Reveal answer
After season 1: 12 tubers
After season 2: 12 × 12 = 144 tubers
After season 3: 144 × 12 = 1 728 tubers
There is one genotype: every tuber is a piece of the original plant’s stem, grown by mitosis, so all 1 728 are a single clone (barring rare mutations). That is why farmers can rely on a variety’s taste and yield — and why one disease can take the whole crop.
1.3 Sexual reproduction: halving and then joining
If each parent simply passed on all 46 of its chromosomes, the offspring would have 92, and the number would double every generation. Sexual reproduction solves this with a special cell division, meiosis, used only to make gametes. Meiosis halves the chromosome number: a diploid cell (2n, with two sets — 46 in humans) gives rise to haploid gametes (n, one set — 23 in humans).
At fertilisation two haploid gametes fuse; their nuclei join and the diploid number is restored in the zygote. The zygote then divides by mitosis, again and again, to build every cell of the new body. You will study meiosis step by step in Unit 9 (Genetics), Lesson 2; here you need what it does.
| Organism | Body cells (2n) | Gametes (n) |
|---|---|---|
| Human | 46 | 23 |
| Maize | 20 | 10 |
| Fruit fly (Drosophila) | 8 | 4 |
| Dog | 78 | 39 |
Sexual reproduction produces variation in two ways. First, meiosis makes gametes that are all genetically different: each receives a random one of every pair of chromosomes, and pieces of chromosome are swapped between the members of a pair. A human can make more than 8 million (223) combinations of whole chromosomes alone. Second, fertilisation is random: which of the millions of sperm fuses with which egg is a matter of chance.
Explain how fertilisation leads to genetic variation in a population. [4]
Reveal answer
Every individual has a different combination of alleles (apart from identical twins) [1]. Meiosis makes every gamete genetically unique, because each receives a random member of each chromosome pair [1].
At fertilisation the genetic material of two different parents is combined in the zygote [1], and which sperm fuses with which egg is random — every gamete has an equal chance — so each offspring is unique and different from both parents [1].
This is the markscheme of November 2021 Biology, Question 2(c), almost point for point.
1.4 Costs and benefits: why bother with sex?
Asexual reproduction is faster, cheaper and needs no partner. A single organism can colonise a new place alone, and an organism that is well suited to a stable environment passes on exactly the combination that works. Sexual reproduction is slow and expensive — and in most animals half the offspring are males, which cannot themselves produce offspring.
Its benefit is variation. When the environment changes — a drought, a new disease, a new predator — some individuals are likely to carry combinations of alleles that survive. Variation is the raw material of natural selection, so sexually reproducing species can adapt.
Imagine two populations of lizards, each starting with 10 females. In population A every female reproduces asexually; in population B females mate with males and half of all offspring are male. Every female lays 4 eggs that all survive to breed once and then die. Calculate the number of females in each population after four generations, and the ratio.
Population A: every offspring is female, so females multiply by 4 each generation.
Females after 4 generations = 10 × 44
Females after 4 generations = 10 × 256 = 2 560
Population B: half of the 4 eggs are female, so females multiply by 2 each generation.
Females after 4 generations = 10 × 24
Females after 4 generations = 10 × 16 = 160
Ratio = 2 560 ÷ 160 = 16
After only four generations the asexual population has 16 times as many females. This “two-fold cost of males” means sex must bring a large benefit to have survived in almost every species — the benefit is variation.
Many organisms can do both. Suggest when an organism that can reproduce either way should switch to sexual reproduction, and explain why.
Reveal answer
When conditions become harsh or unpredictable — the start of winter or a dry season, crowding, or a food supply running out. While conditions are good, the fastest way to fill the available space is asexual reproduction of a form that already works.
When conditions are about to change, offspring that vary are more likely to include some that survive the new conditions, and sexual reproduction often produces a tough resting stage (an egg, a seed, a spore) that survives the bad season.
Aphids switch in autumn; the water flea Daphnia switches when ponds crowd or begin to dry — the examples in Section 1.6.
1.5 Life cycles: the human and the flowering plant
A life cycle is the sequence of stages an organism passes through from one generation to the next. In a life cycle with sexual reproduction two events must happen once per generation: meiosis, which halves the chromosome number to make gametes, and fertilisation, which doubles it again. Everything else is growth by mitosis.
In a human, meiosis happens in the testes and ovaries; the gametes are sperm and egg cells; fertilisation in the oviduct forms the zygote; mitosis builds an embryo, fetus, baby, child and adult; at puberty the reproductive organs mature and the cycle can repeat (Lessons 4 and 5).
In a flowering plant, meiosis happens in the flower — in the anthers, which make pollen grains carrying the male gametes, and in the ovules, which each hold an egg cell. Pollination carries pollen to the stigma; fertilisation in the ovule forms a zygote; the ovule becomes a seed containing an embryo; the seed is dispersed, germinates, and grows into a new plant that flowers (Lessons 2 and 3).
Bean plants are annuals: they germinate, flower and disperse seeds in one year. A life-cycle diagram has three empty boxes: between “sprout” and “adult plant”, between “adult plant” and “flowering plant”, and between “bean pod” and “bean”. Place the labels flowering, growth and seed dispersal.
Reveal answer
Sprout → adult plant: growth. Adult plant → flowering plant: flowering. Bean pod → bean: seed dispersal — the pod (the fruit) splits and releases the seeds.
This was November 2022 Integrated Sciences, Question 3(a), Select [2]: one label correct for 1 mark, all three for 2.
1.6 Life cycles that use both: the aphid and the coral
Aphids (greenfly and blackfly) are small sap-sucking insects. In temperate countries many species follow a yearly cycle. In spring a wingless female, the stem mother, hatches from an egg that survived the winter. She reproduces by parthenogenesis: her eggs develop inside her without fertilisation, and she gives birth to live daughters that are clones of herself.
Each daughter is born with embryos already developing inside her, so a new generation can start giving birth within about a week. Through spring and summer many generations of females follow; when a plant becomes crowded some daughters develop wings and fly to new plants.
In autumn, as days shorten and temperatures fall, the females produce males and sexual females. These mate, and the sexual females lay fertilised eggs on the stems of the host plant. The eggs, protected by a tough shell, survive the winter — and because they came from sexual reproduction, next spring’s stem mothers are genetically varied.
In warm climates, where there is no winter to survive, many aphid populations never make males and reproduce asexually all year.
Reef-building corals are colonies of small animals called polyps, each a tube of tissue with a ring of tentacles, sitting in a cup of limestone it has made. A colony grows by budding: polyps divide or bud new polyps, all genetically identical, so a coral head the size of a car can be one clone. A branch broken off by a storm can settle and grow — fragmentation.
Many corals also reproduce sexually in a mass spawning. On the Great Barrier Reef it happens on a few nights after a full moon in late spring (October to December): colonies release bundles of eggs and sperm that float to the surface, break apart, and fertilise gametes from other colonies.
Each zygote grows into a tiny swimming larva, the planula, which drifts for days to weeks, then settles on rock and becomes a single polyp that starts a new colony by budding. Sexual reproduction gives a coral variation and a way to reach new reefs; budding lets one successful genotype cover a reef.
A student writes: “In summer, aphids reproduce sexually very fast because they mate all the time. The daughters are all different, which is why the population grows. In autumn they switch to asexual reproduction and lay eggs that are all clones, to survive the winter.” Find three errors.
Reveal answer
(1) In summer aphids reproduce asexually, by parthenogenesis — there are no males, and no mating. (2) The summer daughters are clones, genetically identical to their mother; the population grows fast because every individual is a female that reproduces and each is born already carrying embryos, not because they differ.
(3) In autumn they switch the other way: males and sexual females appear, mate, and lay fertilised eggs that are genetically varied. The eggs survive the winter because of their tough shell; their variation prepares the population for whatever next year brings.
1.7 Cloning by people: from cuttings to Dolly
Farmers have used asexual reproduction for thousands of years to keep a good variety exactly as it is. Cuttings: a piece of stem with a few buds is planted and grows roots (cassava, sweet potato, sugar cane, roses). Grafting: a shoot of a good variety is joined onto the rooted stem of a hardy one, so every mango or citrus tree of a named variety is a clone.
Tissue culture (micropropagation): tiny pieces of a plant, often from the growing tip, are placed on sterile agar with nutrients and plant hormones. Each grows into a mass of cells that forms shoots and roots, so thousands of disease-free, identical plantlets can come from one parent in months. Bananas, orchids and potatoes are produced this way.
Animals are harder to clone. In 1996 at the Roslin Institute in Scotland, the nucleus of a cell from the udder of a six-year-old ewe was put into an egg cell whose own nucleus had been removed. The egg was stimulated to divide, and the embryo was placed in a surrogate mother. The lamb, Dolly, born on 5 July 1996, was a clone of the ewe that gave the udder cell.
To make Dolly the team fused 277 egg cells with udder-cell nuclei. 29 of these developed into early embryos that were transferred to surrogate mothers, and one lamb was born. Calculate the percentage of fused eggs that became embryos and the percentage of transferred embryos that became a lamb, and explain why Dolly was a clone of the udder-cell donor and not of the egg donor or the surrogate.
Reveal answer
Embryos from fused eggs = 29 ÷ 277 × 100
Embryos from fused eggs = 10.5 %
Lambs from transferred embryos = 1 ÷ 29 × 100
Lambs from transferred embryos = 3.4 %
Overall = 1 ÷ 277 × 100
Overall = 0.36 %
Almost all the genes of a cell are in its nucleus; the nucleus came from the udder cell, so Dolly’s chromosomes were those of the udder-cell donor. The egg donor supplied the cytoplasm (and its mitochondria, with their own small DNA ring), and the surrogate supplied only the uterus in which the embryo grew — no genes at all.
Dolly was a white-faced Finn Dorset like the nucleus donor; her surrogate was a black-faced Scottish Blackface ewe.
Try it — binary fission and temperature: colonies on agar
Bacteria reproduce only asexually, by binary fission. Spread a drop of diluted yoghurt (live Lactobacillus) on nutrient agar and incubate it: every cell that divides again and again at the spot where it landed builds a visible colony of clones. The number of colonies after 24 hours shows how well the cells survived and divided at the temperature you chose.
Mean counts: 5 °C → 0; 15 °C → 31; 25 °C → 97; 35 °C → 118; 45 °C → 106; 55 °C → 64; 65 °C → 0. Describe the pattern with values, state whether it is linear, and explain the fall above 45 °C.
Reveal answer
The count rises from 0 at 5 °C to a peak of 118 at 35 °C, then falls to 106 at 45 °C, 64 at 55 °C and 0 at 65 °C. It is not linear: equal 10 °C steps give rises of 31, 66 and 21, then falls of 12, 42 and 64 — the curve is a hill with its optimum between 35 and 45 °C.
Above 45 °C, heat breaks the weak bonds holding the enzymes and membrane proteins in shape; the active sites change (denaturation), DNA copying and respiration stop, and cells die before they can divide enough times to build a visible colony. A colony is a clone: its size and number measure asexual reproduction directly.
Common mistakes to avoid
- “Asexual reproduction uses meiosis” — it uses mitosis only; meiosis is used only to make gametes.
- “Gametes contain half the genes” — they contain one set of chromosomes (haploid), with one allele of every gene.
- “Sexual reproduction needs two parents” — it needs two gametes; a self-pollinated pea is still reproducing sexually.
- Answering “reproduction” for “state two reasons cells divide” without saying which kind — the November 2021 markscheme gave one mark for “reproduction” alone; name sexual (making gametes) or asexual.
- Calling asexual offspring “the same” without “genetically” — they can differ in size or appearance because of the environment.
- Thinking parthenogenesis is rare or odd — aphids, many plants (dandelions set seed without fertilisation), some fish and reptiles use it.
- Confusing a clone with a copy that has the same age — Dolly was born as a lamb; only her genes came from a six-year-old ewe.
Study smart: how this is examined
Study routine. Today: cover the comparison table in Section 1.1 and rewrite it from memory; then draw both life-cycle rings of Section 1.5 and colour the haploid stages.
In two days: without notes, give one example of each of the six forms of asexual reproduction and do the potato and bacteria calculations again with new numbers. In a week: explain the aphid life cycle aloud in under a minute, run the colony lab as a full investigation, and take the quiz.
How we found out — virgin births and the meaning of sex
In 1740 the young Swiss naturalist Charles Bonnet, then 20, took a newborn aphid, shut it alone in a glass jar on a twig and watched it for weeks. It had no possible mate, yet it gave birth to 95 young. He had shown that an animal can reproduce without fertilisation, and the word parthenogenesis (“virgin birth”) was later coined for it.
Four years later his cousin Abraham Trembley reported that the freshwater Hydra reproduces by budding and that a Hydra cut into pieces regrows a whole animal from each piece — evidence that asexual reproduction and regeneration are the same kind of process.
What happens in sexual reproduction was only seen a century later. In 1876 Oskar Hertwig, studying the transparent eggs of sea urchins, watched a single sperm enter an egg and the sperm nucleus fuse with the egg nucleus. Fertilisation was the union of two nuclei, and one sperm was enough.
In 1883 Edouard van Beneden, working on the roundworm Ascaris, which has very few chromosomes, showed that the egg and sperm nuclei each carry half the chromosome number of body cells — the halving that meiosis achieves.
Why sex exists at all was argued by August Weismann in the 1880s: he proposed that its value lies in producing variation for natural selection to act on. Evolutionary biologists are still testing the details.
One modern idea, the “Red Queen” hypothesis named by Leigh Van Valen in 1973, holds that sexual reproduction lets hosts keep changing their genes faster than their parasites can adapt. Experiments with snails in New Zealand lakes, and with worms infected by bacteria, have found more sexual reproduction where parasites are common.
Bonnet isolated the aphid “from the moment of its birth”. Explain why this detail was essential to his conclusion, and suggest what a sceptic in 1740 could still have objected.
Reveal answer
If the aphid had been in contact with any other aphid, even briefly, a sceptic could say it had been fertilised then and stored sperm. Isolating a newborn removed every chance of mating, so young born later could only have come from an unfertilised egg — the control that made the conclusion valid.
A sceptic could still object that one aphid is a sample of one, that a tiny male could have been hidden on the twig, or that fertilisation in the mother’s generation could carry over. Bonnet answered this by repeating the isolation over several generations, and every generation still produced young.
Where it is used now
- The banana that cannot have sex. Almost all bananas sold for export are one variety, Cavendish, which is seedless and is propagated only from suckers and tissue culture — a single clone grown worldwide. A soil fungus, Fusarium wilt Tropical Race 4, has spread from Asia to Australia, the Middle East, Mozambique (reported 2013) and Colombia (2019), and every Cavendish plant is equally susceptible.
- Cassava and sweet potato in southern Africa. Both are grown from stem or vine cuttings, so a good variety stays uniform; but viruses spread in the cuttings themselves. Programmes in East and southern Africa multiply clean, virus-tested planting material by tissue culture and distribute it to farmers.
- Aphids on the farm. Cabbage aphids and green peach aphids are among the main pests of vegetables across southern Africa; in the warm climate they reproduce asexually for most or all of the year, so a few aphids arriving on a field can become thousands within weeks.
- Coral reefs. Mass spawning was first described on the Great Barrier Reef in the early 1980s. Reef restoration now uses both routes: fragments of heat-tolerant colonies are grown in underwater nurseries and replanted (asexual), and gametes collected at spawning are fertilised in tanks and the larvae released onto damaged reefs (sexual).
- Seed potatoes. Potatoes are planted as tubers — clones — and must be kept free of disease; certified “seed potatoes” come from tested stock that is multiplied in clean conditions for this reason.
Science now — and the scientist you need to be
What is happening in 2026
Cloning has become a conservation tool. In the United States a black-footed ferret named Elizabeth Ann, born in December 2020, was cloned from skin cells frozen in 1988 from a ferret with no living descendants, bringing lost genetic variation back into a species that descends from only seven founders. A Przewalski’s horse was cloned from cells frozen in 1980 for the same reason.
On reefs, scientists freeze coral sperm and larvae in liquid nitrogen so that genetic variation can be stored against marine heatwaves, and several projects breed corals from heat-tolerant parents to find offspring that survive warmer water. Sexual reproduction is being used deliberately to produce the variation that selection needs.
In agriculture, plant breeders are trying to give crops apomixis — the ability some wild plants have to set seed without fertilisation — so that a high-yielding hybrid could be kept true from its own seed. Experimental rice lines that do this have been reported since 2019; whether they will ever reach farmers is not yet known.
Tissue-culture laboratories increasingly use cameras and machine-learning software to sort plantlets and spot contamination early; the decisions about which clones to multiply are still made by breeders.
The mindshift
What you need to be able to do
Define both kinds of reproduction precisely; give a named example of each form of asexual reproduction; draw and annotate a life cycle with haploid and diploid stages; explain variation in four linked points; and weigh the strategies against each other for an organism or a crop in a changing environment.
See an example
Sweet potato — vine cuttings (vegetative propagation): uniform crop, but viruses pass on in the cuttings. Garlic — cloves of a bulb: each clove is a clone; one soil fungus can take every plant. Banana — suckers: no seeds needed, but no variation for resistance. Tomato — seed (sexual): varied plants, some possibly more resistant, but less uniform fruit.
An assistant asked about wheat called it “asexual because it self-pollinates” — wrong: self-pollination is still sexual reproduction, because gametes made by meiosis fuse.
Key terms
| Asexual reproduction | Production of genetically identical offspring from one parent, by mitosis, with no gametes. |
| Sexual reproduction | Production of offspring by the fusion of two haploid gametes to form a diploid zygote; offspring vary. |
| Clone | A group of genetically identical organisms (or cells) descended from one parent. |
| Gamete | A haploid sex cell (sperm, egg cell; the male gamete in a pollen grain and the egg cell in an ovule). |
| Haploid / diploid | One set of chromosomes (n) / two sets (2n). Human n = 23, 2n = 46. |
| Meiosis | Cell division that halves the chromosome number and produces genetically different haploid gametes (Unit 9). |
| Fertilisation | Fusion of the nuclei of two gametes to form a zygote. |
| Zygote | The single diploid cell formed at fertilisation. |
| Binary fission / budding | Splitting of a cell into two equal cells / growth of an outgrowth that becomes a new individual. |
| Vegetative propagation | Asexual reproduction of a plant from a stem, root or leaf (runners, tubers, bulbs, rhizomes, suckers). |
| Parthenogenesis | Development of an egg into an offspring without fertilisation. |
| Life cycle | The sequence of stages from one generation to the next. |
| Tissue culture | Growing new plants from small pieces of tissue on sterile nutrient agar with plant hormones. |
Think like a scientist
Criterion A A farmer near Maun grows two crops. Sweet potatoes are planted as vine cuttings taken from last year’s best plant. Maize is grown from bought seed produced by cross-pollination between two parent lines.
In year three a virus appears that is carried in sweet potato vines. The farmer also keeps a patch of Kalanchoe, a succulent whose leaf edges carry rows of tiny plantlets that drop off and root.
(a) State the type of reproduction used to produce each of the three: the sweet potato plants, the maize plants, the Kalanchoe plantlets. [2]
(b) Explain why every sweet potato plant in the field is likely to be infected once the virus arrives, while the maize plants differ in how badly a disease affects them. [3]
(c) A Kalanchoe leaf carries 36 plantlets. Each plantlet grows into a plant with 20 leaves, each with 36 plantlets. Calculate the number of plantlets produced by the second generation, and give your answer in standard form. [2]
(d) Maize body cells have 20 chromosomes. State the number of chromosomes in a maize pollen grain’s male gamete, in the zygote, and in a cell of the embryo in the seed. [2]
(e) Outline the life cycle of the maize plant from seed to seed, naming where meiosis and fertilisation happen. [3]
(f) Suggest one advantage to the farmer of propagating sweet potatoes asexually, and one way the farmer could reduce the risk from the virus. [2]
Model answer
(a) Sweet potato: asexual (vegetative propagation from cuttings). Maize: sexual (from seed made by fertilisation). Kalanchoe: asexual (vegetative propagation by plantlets). [All three 2; two correct 1.]
(b) The cuttings came from one plant by mitosis, so every plant is a genetically identical clone [1]; if the virus can infect one plant it can infect all, because none carries an allele that gives resistance [1]. The maize seeds came from sexual reproduction, so the plants differ genetically; some may carry alleles that reduce infection, so the effect varies and some survive [1].
(c) Plantlets on the first leaf = 36; each becomes a plant with 20 leaves.
Second-generation plantlets = 36 × 20 × 36
Second-generation plantlets = 25 920
Second-generation plantlets ≈ 2.6 × 10⁴ [2; 1 for 36 × 20 × 36 shown]
(d) Male gamete: 10 (haploid). Zygote: 20 (diploid). Embryo cell: 20 [2 for all three; 1 for two].
(e) The seed germinates and grows by mitosis into an adult plant with flowers [1]. Meiosis in the anthers makes pollen carrying male gametes, and in the ovules makes egg cells [1]. Wind carries pollen to the silks (stigmas) of another plant (pollination); fertilisation in the ovule forms a zygote that becomes the embryo of a new seed (kernel), which is harvested or dispersed [1].
(f) Advantage: the plants are identical to the best parent — same yield, taste and harvest time — and cuttings are free and quick [1]. Reducing risk: take cuttings only from plants tested free of virus, or buy clean tissue-culture planting material; or plant several different varieties so that one disease cannot take the whole field [1].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
State two differences between asexual and sexual reproduction.
Reveal answer
Any two: asexual has one parent, sexual usually two [1]; asexual involves no gametes, sexual involves fusion of gametes [1]; asexual uses only mitosis, sexual uses meiosis to make gametes [1]; asexual offspring are genetically identical, sexual offspring vary [1]. (Maximum 2; each must state both sides.)
State two reasons why cells must divide in organisms.
Reveal answer
Growth, or repair and replacement of damaged or worn-out cells [1]; reproduction — asexual reproduction, or making gametes for sexual reproduction [1]. (“Reproduction” alone with no qualifier, alongside growth, scores 1 in total, as in November 2021.)
Outline why there are fewer chromosomes in sex cells than in body cells.
Reveal answer
Two sex cells (sperm and egg) fuse at fertilisation [1], so each must carry half the number (23) for the zygote — the offspring — to have the full 46 [1].
A yeast cell buds every 90 minutes. Calculate the number of cells that one yeast cell can give rise to in 12 hours if every cell buds and none die, and explain why the real number in a sealed flask of sugar solution would be smaller.
Reveal answer
Generations = time ÷ time per generation
Generations = 720 min ÷ 90 min
Generations = 8
Cells = 2⁸
Cells = 256 [2]
In a sealed flask the sugar is used up and waste products — ethanol and carbon dioxide from fermentation — build up; oxygen runs out, so budding slows and cells die [1].
Describe the life cycle of a coral, identifying the asexual and the sexual stages.
Reveal answer
A colony grows by budding: polyps bud new genetically identical polyps (asexual); broken fragments can grow into new colonies (fragmentation, asexual) [1]. At mass spawning colonies release eggs and sperm into the water, and gametes from different colonies fuse — fertilisation (sexual) [1]. The zygote develops into a swimming planula larva that settles on rock, becomes a polyp and starts a new colony by budding [1].
Evaluate the use of tissue culture to produce 50 000 banana plants for a new plantation.
Reveal answer
Advantages: many plants quickly from one parent in a small space [1]; plantlets are free of disease when started from a clean growing tip, and all have the known yield and quality of the parent [1].
Disadvantages: all 50 000 are one clone with no genetic variation, so a single disease or pest (for example Fusarium wilt) that attacks one attacks all; the laboratory and sterile conditions are expensive [1].
Judgement: worth doing for a uniform, disease-free start, but only if the plantation also plants more than one variety or keeps resistant stock, because one clone is a single point of failure [1].
7. The water flea Daphnia reproduces asexually in ponds in spring and summer. When the pond becomes crowded or begins to dry, it produces males, and females lay a few fertilised “resting eggs” that survive drying and freezing. A student says “switching to sex is a mistake, because the population grows more slowly.” Evaluate this statement, using the idea of the cost of males and of variation.
Reveal answer
The student is right about speed: when half the offspring are males, the number of females grows about half as fast per generation, so a sexual population falls far behind an asexual one within a few generations [1].
But crowding and drying signal that conditions are about to change or become lethal; in these conditions speed is worth nothing if the whole clone dies [1].
Sexual reproduction produces genetically varied resting eggs, so the next generation includes new combinations of alleles, some of which may suit the changed pond — and the resting egg is a stage that survives drying [1].
So the switch is not a mistake: asexual reproduction when conditions are good, sexual when they deteriorate, gets the benefit of each at the time it matters [1].
8. A human can make 2²³ genetically different gametes from the independent assortment of whole chromosomes alone. Calculate this number in standard form, then calculate how many different combinations one couple could produce in a zygote from this source alone, and explain why the true number of possible children is larger still.
Reveal answer
Gametes per person = 2²³
Gametes per person = 8 388 608 ≈ 8.4 × 10⁶ [1]
Zygotes = 2²³ × 2²³ = 2⁴⁶
Zygotes ≈ 7.0 × 10¹³ [1]
That is about 70 trillion, roughly 8 000–9 000 times the number of people alive.
The true number is far larger because in meiosis the two chromosomes of each pair also swap sections (crossing over), creating new combinations of alleles within each chromosome [1]; and new mutations add further variation — so apart from identical twins, no two children of the same couple are genetically the same [1].
Summary
- Asexual reproduction: one parent, no gametes, mitosis only; offspring are genetically identical clones. Sexual reproduction: two haploid gametes fuse to form a diploid zygote; meiosis makes the gametes; offspring vary.
- Forms of asexual reproduction: binary fission (bacteria), budding (yeast, Hydra, coral), fragmentation (sea stars, corals), spores (moulds), vegetative propagation (runners, tubers, bulbs, rhizomes, suckers), parthenogenesis (aphids).
- Variation comes from meiosis (every gamete different) and random fertilisation (any sperm with any egg). It costs time, energy and the “two-fold cost of males”, but lets a population survive and adapt when conditions change.
- Every sexual life cycle has one meiosis and one fertilisation per generation: human — adult, gametes, zygote, embryo; flowering plant — plant, pollen and egg cell, pollination and fertilisation, seed, germination.
- Aphids reproduce asexually through summer and sexually in autumn (overwintering eggs); corals bud to grow colonies and spawn en masse to reproduce sexually. People clone plants by cuttings, grafting and tissue culture, and animals by nuclear transfer (Dolly, 1996).
Next time → The flower as a reproductive system: the parts of an insect-pollinated flower, how pollen reaches a stigma, and how a pollen tube carries the male gamete to the egg — with a virtual lab to find how much sugar a pollen grain needs.