Why is "Confusing fertilisation with mating." a common mistake in Sexual Reproduction?

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 is "Saying gametes are diploid." a common mistake in Sexual Reproduction?

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.

ReproductionCambridge IGCSE Biology 0610 Extended

Sexual Reproduction

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Sexual Reproduction — Cambridge IGCSE 0610 Biology Extended (2026)

Two parents combine genes via gametes. Slower than asexual but creates VARIATION — the raw material of evolution.

What you’ll learn

Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).

16.2 — Define sexual reproduction.
16.2 — Explain the role of gametes and fertilisation.
16.2 — Distinguish haploid and diploid.
16.2 — Compare sexual and asexual reproduction.

What is sexual reproduction?

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?

If gametes were diploid (46), zygote would be 92 — chromosome number doubling each generation. Impossible.
Halving in gamete formation (MEIOSIS — see Inheritance topic) means fertilisation restores the right number.
23 + 23 = 46. Maintained across generations.

Two haploid gametes (23 each) fuse so the diploid zygote has the full 46 chromosomes.

Sources of variation.

Gametes are different from each other (meiosis shuffles the genes).
Random fertilisation — any sperm can fuse with any egg.
Combination of two parents' genes — child gets a mix.

Worked qualitative. Why are siblings (same parents) NOT identical, except for identical twins?

Each gamete has a unique random combination of parental chromosomes.
Random which sperm fertilises which egg.
Each fertilisation = unique combination → unique child.
Identical twins: one fertilised egg splits into two early in development → genetically identical.

Cambridge tip. Always say 'fusion of gamete NUCLEI'. Don't just say 'sperm and egg join'.

Two gametes fuse.
Gametes haploid; zygote diploid.
Variation from chromosome shuffling + random fertilisation.
Each child a unique combination.

Sexual vs asexual reproduction

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?

Asexual: stable environments, plentiful resources, when speed matters more than adaptability. Bacteria, agriculture, weeds.
Sexual: changing environments, when novel diseases or pressures emerge. Most multi-cellular life.

Sexual reproduction mixes two parents' genes for variety; asexual reproduction copies one parent exactly.

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)?

Asexual (runners): fast colonisation of nearby ground; reliable success.
Sexual (seeds): dispersal far from the parent plant; variation in case conditions change.
Combined: plant doesn't put all eggs in one basket.

Cambridge tip. When asked WHY sexual reproduction is advantageous, the key answer is GENETIC VARIATION. This enables adaptation.

Sexual: 2 parents, varied offspring, slow.
Asexual: 1 parent, clones, fast.
Variation = adaptation potential.
Many organisms do both.

How it’s examined

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.

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Step-by-step worked examples — Sexual Reproduction

Step-by-step solutions to past-paper-style questions on sexual reproduction, written exactly the way a tutor would explain them at the board.

1Define sexual reproduction
Getting started• definition
Question
Define sexual reproduction and state what a gamete is.
Step-by-step solution
1. Step 1
  Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote, producing genetically different offspring.
2. 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.
3. 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.
2Gametes, zygotes, haploid and diploid
Getting started• identify
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
1. Step 1
  (a) A sperm cell is a gamete → haploid.
2. Step 2
  (b) A body cell → diploid.
3. Step 3
  (c) An egg cell is a gamete → haploid.
4. 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.
3Advantages and disadvantages
Building confidence• Adapted from 0610/42 Oct/Nov 2024 Q14• pros and cons
Question
Compare sexual reproduction with asexual reproduction, giving advantages and disadvantages.
Step-by-step solution
1. 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.
2. Step 2
  Disadvantage: it usually needs two parents, so a mate must be found, and it is slower than asexual reproduction.
3. 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.
4Why sexual reproduction produces variation
Building confidence• variation
Question
Explain why the offspring produced by sexual reproduction show genetic variation.
Step-by-step solution
1. Step 1
  Each offspring is formed from two gametes, one from each parent.
2. Step 2
  The gametes carry different combinations of genes, and which sperm fertilises which egg is a matter of chance.
3. Step 3
  So each offspring receives a new mixture of genes — half from each parent.
4. 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.
5Why gametes must be haploid
Stretch• haploid, chromosome number
Question
Explain why it is important that gametes are haploid.
Step-by-step solution
1. Step 1
  A gamete is haploid, meaning it has half the normal (diploid) number of chromosomes.
2. Step 2
  At fertilisation, the nuclei of two gametes fuse.
3. Step 3
  Because each gamete has half the number, the zygote formed has the full diploid number restored (half from each parent).
4. 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.
6Why variation helps a species survive
Stretch• variation, natural selection
Question
Explain how the genetic variation produced by sexual reproduction helps a species to survive when its environment changes.
Step-by-step solution
1. Step 1
  Sexual reproduction produces offspring with a range of different genes (variation).
2. 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.
3. Step 3
  These individuals are more likely to survive and reproduce, passing on their useful genes.
4. 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.

Model Answers — Sexual Reproduction

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.

Question 1
Paper 4 short-answer style1 mark
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.
Question 2
Paper 4 short-answer style2 marks
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'.
Question 3
Paper 4 (Extended) structured style3 marks
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.
Question 4
Paper 4 (Extended) structured style4 marks
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.
Question 5
Paper 4 (Extended) structured style5 marks
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.
Question 6
Paper 4 (Extended) extended-response style6 marks
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.

Key Definitions and Keywords — Sexual Reproduction

Definitions to memorise and the exact keywords mark schemes credit for sexual reproduction answers — sharpened from recent examiner reports for the 2026 0610 sitting.

Sexual reproduction
Examiner keyword
A process involving the fusion of the nuclei of two gametes to form a zygote, producing genetically different offspring.
Gamete
Examiner keyword
A sex cell (e.g. sperm, egg, pollen). It is haploid — half the chromosome number of a body cell.
Fertilisation
Examiner keyword
The fusion of the nuclei of a male gamete and a female gamete to form a diploid zygote.
Zygote
The diploid cell formed by fertilisation. It divides and develops into a new organism.
Haploid / diploid
Examiner keyword
Haploid = one set of chromosomes (gametes). Diploid = two sets of chromosomes (body cells and the zygote).

Common Mistakes and Misconceptions — Sexual Reproduction

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.

✕Confusing fertilisation with mating.
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.
✕Saying gametes are diploid.
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.

Practice questions

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Sexual Reproduction

Short Study Notes in the form of Slides

Short Study Notes — Sexual Reproduction

Detailed Notes

What you’ll learn

How it’s examined

1Define sexual reproduction

2Gametes, zygotes, haploid and diploid

3Advantages and disadvantages

4Why sexual reproduction produces variation

5Why gametes must be haploid

6Why variation helps a species survive

Sexual reproduction

Gamete

Fertilisation

Zygote

Haploid / diploid

✕Confusing fertilisation with mating.

✕Saying gametes are diploid.

Practice questions

Past paper style quiz

Assess your understanding

Video lesson