The big idea: In sexual reproduction, two gametes (sex cells) join to make a new individual.
The male gamete is the sperm, made in the testes. The female gamete is the egg (ovum), made in the ovaries.
Both gametes are made by meiosis, so each is haploid — it carries half the normal number of chromosomes.
When a sperm and an egg fuse — an event called fertilization — their nuclei join to form a single diploid cell called a zygote, which grows into a new organism.
- Gamete
- A sex cell (sperm or egg) used in sexual reproduction. Gametes are haploid — they carry one set of chromosomes.
- Haploid (n)
- Having only one set of chromosomes. Human gametes are haploid (23 chromosomes).
- Diploid (2n)
- Having two sets of chromosomes — one from each parent. Most human body cells are diploid (46 chromosomes).
- Meiosis
- The type of cell division that makes gametes. It halves the chromosome number, so a diploid cell produces haploid cells.
- Fertilization
- The fusion of a sperm nucleus with an egg nucleus to form a diploid zygote.
- Zygote
- The single diploid cell formed when a sperm fertilizes an egg. It divides to form an embryo.
Why gametes must be haploid: If gametes were diploid, fertilization would double the chromosome number every generation.
Making gametes haploid by meiosis means that when two gametes fuse, the diploid number is restored — kept constant from one generation to the next.
The two reproductive systems are built for different jobs.
The male system makes and delivers huge numbers of small, swimming sperm. The female system makes a few large eggs, and provides a place — the uterus — where a fertilized egg can develop.
The key structures: In the male system, the testes make sperm; the sperm duct carries them to the urethra, which passes through the penis and releases them.
In the female system, the ovaries make eggs; an egg travels down the oviduct (fallopian tube) to the uterus. The oviduct is usually where fertilization takes place.
| Structure | Male or female? | Function |
|---|---|---|
| Testis | Male | Produces sperm (by meiosis) and the hormone testosterone |
| Sperm duct | Male | Carries sperm away from the testis towards the urethra |
| Urethra (in the penis) | Male | Carries sperm out of the body during ejaculation |
| Ovary | Female | Produces eggs (by meiosis) and the hormones estrogen and progesterone |
| Oviduct (fallopian tube) | Female | Carries the egg towards the uterus; usually the site of fertilization |
| Uterus | Female | Where the embryo implants and develops |
| Vagina | Female | Receives sperm; the birth canal |
Sperm and egg are built differently: Both gametes are haploid and made by meiosis, but their structures suit their roles.
Sperm are tiny, made in huge numbers, and swim to the egg using a tail (flagellum), carrying almost no food store.
The egg is large — the biggest human cell — released in small numbers, cannot swim, and carries a large food store to nourish the early embryo.
| Feature | Sperm (male gamete) | Egg / ovum (female gamete) |
|---|---|---|
| Made in | The testes | The ovaries |
| Made by | Meiosis | Meiosis |
| Chromosome number | Haploid (n) | Haploid (n) |
| Size | Very small | Large (the largest human cell) |
| Number produced | Very many (millions) | Very few (usually one released per cycle) |
| Movement | Swims using a tail (flagellum) | Cannot swim — moved along the oviduct |
| Food store | Almost none | Large food store (cytoplasm / yolk) |
Fertilization — step by step: Many sperm swim up the oviduct towards the egg, but only one fertilizes it.
That one sperm enters the egg; the egg membrane then changes to block any other sperm (so the zygote does not get too many chromosomes).
Finally the haploid sperm nucleus fuses with the haploid egg nucleus, combining the genetic material of both parents into a single diploid zygote.
| Step | What happens | Why it matters |
|---|---|---|
| 1. Sperm reach the egg | Many sperm swim up the oviduct towards the egg | Only a tiny fraction survive the journey |
| 2. One sperm enters | A single sperm penetrates the egg's outer layer | Only ONE sperm fertilizes the egg |
| 3. Block to others | The egg membrane changes so no further sperm can enter | Stops the zygote getting too many chromosomes (polyspermy) |
| 4. Nuclei fuse | The haploid sperm nucleus fuses with the haploid egg nucleus | Combines genetic material from both parents |
| 5. Zygote forms | A single diploid cell — the zygote — is produced | The diploid number is restored; the zygote can divide to form an embryo |
Before fertilization
- Two separate gametes (sperm + egg)
- Each is haploid (n) — half the chromosome number
- Each was made by meiosis
- Genetic material is in two separate nuclei
After fertilization
- One cell — the zygote
- It is diploid (2n) — the full chromosome number
- The two nuclei have fused into one
- It carries genes from both parents → variation
A memory hook: Sperm = small + swims (many, with a tail). Egg = enormous + energy store (few, stays put).
And remember the equation of fertilization: haploid + haploid → diploid (n + n → 2n).
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How this is tested: On Paper 1A a single-mark question often asks you to label a structure on a diagram of the male or female reproductive system, or to identify what happens during fertilization — the answer being that a sperm nucleus fuses with an egg nucleus to form a diploid zygote.
On Paper 2 a short Outline question commonly asks why diploid organisms make their gametes by meiosis — the answer centres on halving the chromosome number so that the diploid number is restored, not doubled, at fertilization.
IB-style question — outline why gametes are made by meiosis
Outline why diploid organisms produce their gametes by meiosis rather than by mitosis. [2]
How to score both marks
- Meiosis halves the chromosome number. Meiosis produces haploid gametes — each carries half the chromosome number of the parent cell.
- This keeps the diploid number constant. When two haploid gametes fuse at fertilization, the diploid number is restored (not doubled), so the chromosome number stays constant from one generation to the next. (Award 1 mark for 'meiosis makes haploid gametes / halves the number', 1 mark for 'so fertilization restores the diploid number'.)
Final answer
Meiosis halves the chromosome number, making haploid gametes; when two gametes fuse at fertilization the diploid number is restored, so the chromosome number stays constant each generation (and meiosis also creates variation).
✓ Why this scores full marks: It links cause to effect: meiosis makes gametes haploid, and that is why fertilization restores — rather than doubles — the diploid number.
Answering only 'meiosis makes gametes' without the chromosome-number point would score just one mark.
IB-style question — draw and label gamete production by meiosis
Draw a simple diagram to show how one diploid cell gives rise to four haploid gametes, and label the diploid and haploid cells. [3]
How to score all three marks
- Start with one diploid cell. Draw a single starting cell and label it diploid (2n) — it contains two sets of chromosomes.
- Show two divisions. Show the cell dividing twice (meiosis I then meiosis II), giving four cells in total.
- Label the products. Label the four end cells as haploid (n) gametes. (Award 1 mark for one diploid starting cell, 1 mark for two divisions producing four cells, 1 mark for labelling the four products as haploid gametes.)
Final answer
One diploid (2n) cell → divides twice (meiosis) → four haploid (n) gametes.
Gametes are made by meiosis: one diploid cell (2n) divides twice to give four haploid (n) cells. Halving the chromosome number means that, when two gametes fuse at fertilization, the diploid number is restored — not doubled.
Interactive diagram
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| Structure | Male or female? | Function |
|---|---|---|
| Testis | Male | Produces sperm (by meiosis) and the hormone testosterone |
| Sperm duct | Male | Carries sperm away from the testis towards the urethra |
| Urethra (in the penis) | Male | Carries sperm out of the body during ejaculation |
| Ovary | Female | Produces eggs (by meiosis) and the hormones estrogen and progesterone |
| Oviduct (fallopian tube) | Female | Carries the egg towards the uterus; usually the site of fertilization |
| Uterus | Female | Where the embryo implants and develops |
| Vagina | Female | Receives sperm; the birth canal |