Question 1
Paper 4 short-answer style1 markState what is meant by a gamete. (1 mark)
Model answer
A gamete is a sex cell (such as a sperm or egg), which is haploid.
Why this scores
One mark for 'sex cell'. Mentioning haploid strengthens it.
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Detailed notes on Reproduction for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Two parents combine genes via gametes. Slower than asexual but creates VARIATION — the raw material of evolution.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Two haploid gametes fuse → diploid zygote with a mix of both parents' genes.
Definition. Sexual reproduction involves the FUSION of the nuclei of TWO GAMETES (sex cells) to form a ZYGOTE → genetically VARIED offspring.
The chromosomal arithmetic.
| Cell | Type | Chromosome number |
|---|---|---|
| Body cell | Diploid (2n) | 46 (humans) |
| Gamete | Haploid (n) | 23 (humans) |
| Zygote | Diploid (2n) | 46 (humans) |
Why halve then double?
Sources of variation.
Worked qualitative. Why are siblings (same parents) NOT identical, except for identical twins?
Cambridge tip. Always say 'fusion of gamete NUCLEI'. Don't just say 'sperm and egg join'.
Sexual: variation, adaptation. Asexual: speed, reliability.
| Feature | Sexual | Asexual |
|---|---|---|
| Parents | TWO | ONE |
| Gametes | YES (sperm + egg) | NO |
| Variation | YES (mixed genes) | NO (clones) |
| Speed | Slow | Fast |
| Energy cost | High | Low |
| Vulnerability to environmental change | LOW (variation helps) | HIGH (all identical) |
| Vulnerability to disease | LOW | HIGH |
| Use of resources | Needs flowers, courtship, etc. | Minimal |
When does each WORK BEST?
Most plants and many simpler animals do BOTH — sexual when conditions are uncertain (winter ahead, water shortage), asexual when conditions are stable. Best of both worlds.
Worked qualitative. Why do strawberry plants reproduce asexually (runners) AND sexually (flowers + seeds)?
Cambridge tip. When asked WHY sexual reproduction is advantageous, the key answer is GENETIC VARIATION. This enables adaptation.
Verbatim phrases and definitions Cambridge mark schemes credit.
Sexual reproduction appears Paper 4 (5-7 marks: define, gamete arithmetic, contrast with asexual). Examiner reports flag students saying gametes are diploid and missing 'fusion of nuclei'.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (16.2); 0610/42 Oct/Nov 2024 — Q14 (sexual reproduction); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on sexual reproduction, written exactly the way a tutor would explain them at the board.
Question
Define sexual reproduction and state what a gamete is.
Step-by-step solution
Step 1
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote, producing genetically different offspring.
Step 2
A gamete is a sex cell — for example a sperm and an egg in animals, or pollen and an egg cell in plants.
Step 3
Gametes are haploid — they have half the normal chromosome number.
Answer
Sexual reproduction is the fusion of the nuclei of two gametes to form a zygote, giving varied offspring. A gamete is a haploid sex cell.
Question
State whether each is haploid or diploid: (a) a sperm cell; (b) a body (skin) cell; (c) an egg cell; (d) a zygote.
Step-by-step solution
Step 1
(a) A sperm cell is a gamete → haploid.
Step 2
(b) A body cell → diploid.
Step 3
(c) An egg cell is a gamete → haploid.
Step 4
(d) A zygote (formed when two gametes fuse) → diploid.
Answer
(a) Haploid; (b) diploid; (c) haploid; (d) diploid.
Examiner tip
Gametes are haploid; the zygote formed when they fuse is diploid, restoring the full chromosome number.
Question
Compare sexual reproduction with asexual reproduction, giving advantages and disadvantages.
Step-by-step solution
Step 1
Advantage: sexual reproduction produces genetic variation in the offspring, so the population can adapt to a changing environment, and some offspring may resist a disease that affects others.
Step 2
Disadvantage: it usually needs two parents, so a mate must be found, and it is slower than asexual reproduction.
Step 3
It also uses more energy (e.g. in producing flowers, attracting mates, making gametes).
Answer
Advantage: variation lets the population adapt and some offspring resist disease. Disadvantage: needs two parents, is slower and uses more energy than asexual reproduction.
Question
Explain why the offspring produced by sexual reproduction show genetic variation.
Step-by-step solution
Step 1
Each offspring is formed from two gametes, one from each parent.
Step 2
The gametes carry different combinations of genes, and which sperm fertilises which egg is a matter of chance.
Step 3
So each offspring receives a new mixture of genes — half from each parent.
Step 4
Because the combination is different each time, the offspring differ from each other and from their parents — they show genetic variation.
Answer
Each offspring gets a random mixture of genes from two gametes (one from each parent), so each has a different combination of genes — producing genetic variation.
Examiner tip
The key ideas: genes from two parents; random fusion of gametes; a new combination each time.
Question
Explain why it is important that gametes are haploid.
Step-by-step solution
Step 1
A gamete is haploid, meaning it has half the normal (diploid) number of chromosomes.
Step 2
At fertilisation, the nuclei of two gametes fuse.
Step 3
Because each gamete has half the number, the zygote formed has the full diploid number restored (half from each parent).
Step 4
If the gametes were diploid, the zygote would have double the correct number of chromosomes, which would keep doubling each generation — so being haploid keeps the chromosome number constant.
Answer
Gametes are haploid so that, when two fuse at fertilisation, the diploid chromosome number is restored. If they were diploid, the number would double each generation.
Examiner tip
Link haploid gametes to keeping the chromosome number constant from one generation to the next.
Question
Explain how the genetic variation produced by sexual reproduction helps a species to survive when its environment changes.
Step-by-step solution
Step 1
Sexual reproduction produces offspring with a range of different genes (variation).
Step 2
If the environment changes (e.g. a new disease, or a change in climate), some individuals may by chance have features that suit the new conditions.
Step 3
These individuals are more likely to survive and reproduce, passing on their useful genes.
Step 4
Over generations the species can therefore adapt to the new conditions and survive — which would be unlikely if all individuals were identical.
Answer
Variation means some offspring may suit new conditions; these survive and reproduce, passing on their genes, so the species can adapt to a changing environment and survive.
Examiner tip
This connects variation to natural selection — variation provides the differences that selection acts on.
High-scoring sample answers for sexual reproduction on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State what is meant by a gamete. (1 mark)
Model answer
A gamete is a sex cell (such as a sperm or egg), which is haploid.
Why this scores
One mark for 'sex cell'. Mentioning haploid strengthens it.
State what happens at fertilisation and name the cell that is formed. (2 marks)
Model answer
At fertilisation, the nuclei of two gametes fuse together. The cell formed is called a zygote.
Why this scores
One mark for the fusion of gamete nuclei; one for 'zygote'.
Explain why gametes are haploid. (3 marks)
Model answer
Gametes are haploid, with half the normal number of chromosomes, so that when two gametes fuse at fertilisation, the full diploid number of chromosomes is restored in the zygote. If the gametes were diploid, the chromosome number would double each generation, which would be harmful.
Why this scores
Three marks: gametes haploid/half the number; fusion restores the diploid number; otherwise the number would double each generation.
Explain why offspring produced by sexual reproduction show genetic variation. (4 marks)
Model answer
Each offspring is produced by the fusion of two gametes, one from each parent, so it receives genes from both parents. The gametes carry different combinations of genes, and which gametes fuse is a matter of chance. As a result, each offspring receives a different, new combination of genes, so the offspring differ from one another and from their parents — they show genetic variation.
Why this scores
Four marks: two gametes fuse; genes come from both parents; random which gametes fuse; new combination of genes each time → variation.
Explain the advantages of sexual reproduction over asexual reproduction. (5 marks)
Model answer
Sexual reproduction produces offspring that show genetic variation, because each offspring receives a new mixture of genes from two parents, whereas asexual reproduction produces identical clones. This variation is an advantage because, if the environment changes, some of the varied offspring may by chance have features that make them better suited to the new conditions, or resistant to a new disease. These individuals are more likely to survive and reproduce, so the population can adapt and is less likely to be wiped out than an identical, asexually produced population would be.
Why this scores
Five marks: produces variation; from a mixture of genes from two parents; some offspring better suited to new conditions / disease-resistant; these survive and reproduce; population can adapt/survive change.
Explain how the variation produced by sexual reproduction can allow a species to survive a change in its environment, such as the appearance of a new disease. (6 marks)
Model answer
In sexual reproduction, each offspring is formed by the fusion of two gametes, one from each parent, so it receives a new combination of genes. Because the gametes carry different genes and fuse at random, the offspring show a wide range of genetic variation. When a new disease appears, the individuals in the population are not all the same: by chance, some of them may have genes that make them resistant to the disease, while others are not. The non-resistant individuals may die, but the resistant individuals survive. These survivors reproduce and pass on the genes for resistance to their offspring, so over the generations the proportion of resistant individuals increases and the species adapts to the presence of the disease. In this way, the variation produced by sexual reproduction allows at least some individuals to survive and the species to continue — something that would be very unlikely if all the individuals were genetically identical.
Why this scores
Up to 6 marks: sexual reproduction produces variation; offspring differ; when disease appears, some have resistance by chance; non-resistant die / resistant survive; survivors reproduce and pass on resistance; species adapts/survives. Reward the natural selection reasoning.
Definitions to memorise and the exact keywords mark schemes credit for sexual reproduction answers — sharpened from recent examiner reports for the 2026 0610 sitting.
A process involving the fusion of the nuclei of two gametes to form a zygote, producing genetically different offspring.
A sex cell (e.g. sperm, egg, pollen). It is haploid — half the chromosome number of a body cell.
The fusion of the nuclei of a male gamete and a female gamete to form a diploid zygote.
The diploid cell formed by fertilisation. It divides and develops into a new organism.
Haploid = one set of chromosomes (gametes). Diploid = two sets of chromosomes (body cells and the zygote).
The traps other students keep falling into on sexual reproduction questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Both involve sex cells.
How to avoid it
Mating brings the gametes together; FERTILISATION is the actual fusion of the gamete nuclei.
Why it happens
Most cells are diploid.
How to avoid it
Gametes are HAPLOID — half the number. When two fuse, the diploid number is restored in the zygote.
The things students keep getting wrong in this sub-topic, answered.