# Reproduction

> Sexual and asexual reproduction in animals, plants, fungi, bacteria and protists, fertilisation, and the hormones that control pregnancy.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/reproduction/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The shark that had pups alone
In 2016, a female zebra shark called Leonie, at an aquarium in Townsville, gave birth to three pups. The surprise: she had not been near a male for three years.

Scientists tested the pups' DNA. They had genes only from their mother. Leonie had switched to making babies without a father, a type of asexual reproduction called parthenogenesis. Most animals reproduce sexually, but life has found many ways to make the next generation.

**In short:** Reproduction makes new organisms. Asexual reproduction uses one parent and gives identical offspring, while sexual reproduction joins gametes from two parents and gives varied offspring. In flowering plants, pollination is followed by fertilisation, and ovules become seeds.

## Starting point: what you already know

- Offspring inherit DNA from their parents.
- Sperm and egg join at fertilisation (Year 9 and 10 science).
- Flowers are the reproductive organs of flowering plants.

## Sexual and asexual reproduction

### Key idea: The big difference
**Asexual** reproduction: **one** parent, offspring are **genetically identical** (clones). Fast, no mate needed.

**Sexual** reproduction: **two** parents, gametes (sex cells) join, offspring are **genetically different**. Slower, but creates **variation**, which helps a species survive change.

| Organism | Asexual methods | Sexual reproduction? |
|---|---|---|
| **Bacteria** | **Binary fission**: the cell copies its DNA and splits in two, as often as every 20 minutes | No, but they can swap DNA (conjugation) |
| **Protists** (*Amoeba*, *Paramecium*) | Binary fission | Some do |
| **Fungi** (yeast, moulds) | Yeast **budding**. Moulds make huge numbers of **spores** | Yes, many fungi also make sexual spores |
| **Plants** | **Vegetative propagation**: runners (strawberries), tubers (potatoes), bulbs (onions), cuttings | Yes, through flowers |
| **Animals** | Budding (*Hydra*), fragmentation (sea stars), parthenogenesis (some sharks, lizards, bees) | Yes, mostly |

## Fertilisation in animals

**Fertilisation** is the joining of a male gamete (sperm) and a female gamete (egg) to form a **zygote**.

| | External fertilisation | Internal fertilisation |
|---|---|---|
| Where | Outside the body, usually in water | Inside the female's body |
| Gametes released | Huge numbers, because most are lost | Fewer |
| Examples | Most fish, frogs, corals | Mammals, birds, reptiles, sharks |
| Advantage | Little energy spent finding a mate | Higher chance of fertilisation, embryo is protected |

The annual mass spawning of corals on the Great Barrier Reef, a few nights after a full moon in spring, is external fertilisation on a huge scale.

## Reproduction in flowering plants

1. **Pollination**: pollen is carried from the **anther** (male) to the **stigma** (female) by wind, insects, birds or mammals. Banksias and grevilleas are pollinated by honeyeaters and even small possums.
2. A **pollen tube** grows down the style to the **ovary**.
3. **Fertilisation**: a male gamete from the pollen joins the egg in the **ovule**.
4. The ovule becomes a **seed** and the ovary becomes a **fruit**.
5. Seeds are **dispersed** by wind, water, animals or explosion.

## Hormones in human pregnancy

Hormones are chemical messengers carried in the blood.

| Hormone | Made by | Job in pregnancy |
|---|---|---|
| **hCG** (human chorionic gonadotropin) | The early embryo | Keeps the corpus luteum making progesterone. Pregnancy tests detect it in urine |
| **Progesterone** | Corpus luteum, then placenta | Keeps the uterus lining thick. Stops new eggs being released and the uterus contracting |
| **Oestrogen** | Placenta | Helps the uterus and breasts grow |
| **Oxytocin** | Pituitary gland | Causes contractions in birth and milk release |
| **Prolactin** | Pituitary gland | Makes milk |

Near birth, progesterone falls and oxytocin rises, so the uterus begins to contract.

**Mind map**

```text
Reproduction
  Asexual
    One parent, clones
    Binary fission: bacteria
    Budding: yeast, Hydra
    Spores: fungi
    Runners, tubers: plants
  Sexual
    Two parents, variation
    Gametes form a zygote
  Animals
    External: fish, frogs, coral
    Internal: mammals, birds
  Plants
    Pollination
    Fertilisation in ovule
    Seed and fruit
  Pregnancy hormones
    hCG
    Progesterone
    Oxytocin
```

### Year 11: Weighing up the two strategies
| | Asexual | Sexual |
|---|---|---|
| Speed | Fast, can colonise quickly | Slow |
| Energy | Low | High (finding mates, making gametes) |
| Variation | None, unless a mutation | High |
| In a stable environment | Very successful | Less efficient |
| When the environment changes | Whole population may be wiped out | Some offspring likely to survive |

Many organisms switch between the two. Aphids reproduce asexually all summer, then sexually before winter. Strawberries spread by runners but also make seeds.

**Many eggs or few?** Animals with external fertilisation release millions of eggs because most are never fertilised or are eaten. Animals with internal fertilisation make far fewer eggs, because each has a much better chance of being fertilised and protected.

**Asexual parasites.** A parasite such as the trypanosome copies itself asexually inside its host, so it can build up huge numbers very quickly without needing a mate.

### Year 12: Manipulating reproduction in agriculture
Farmers and scientists change how plants and animals reproduce:

- **Artificial insemination** (AI): semen from a chosen bull is used to fertilise many cows, spreading wanted traits fast.
- **IVF** (in vitro fertilisation) and **embryo transfer** in cattle and in people.
- **Cross-pollination** by hand to breed new crop varieties.
- **Cloning** by cuttings, tissue culture or somatic cell nuclear transfer (Dolly the sheep, 1996).

Benefits: faster improvement of crops and livestock. Risks: lower **genetic diversity**, so a whole crop or herd may be hit by one disease. These are developed further in Module 6.

**The menstrual cycle.** The pituitary gland releases **FSH**, which makes a follicle grow in the ovary. The growing follicle makes **oestrogen**, which thickens the uterus lining. When oestrogen gets high, it triggers a **surge of LH** from the pituitary, and the LH surge causes **ovulation** (the egg is released).

The empty follicle becomes the **corpus luteum**, which makes **progesterone** to keep the lining thick and stop new follicles developing. If no embryo arrives, the corpus luteum breaks down, progesterone falls, and the lining is shed (**menstruation**). If an embryo implants, its hCG keeps the corpus luteum going until the **placenta** takes over making progesterone.

**Reasoning with hormones.**
- No FSH or LH means no follicles grow and no ovulation happens, so there is no cycle.
- Low levels of oestrogen normally hold back FSH and LH (negative feedback). Clomiphene blocks the brain's oestrogen receptors, so the brain "sees" too little oestrogen and releases more FSH and LH, which brings on ovulation.
- An older woman's eggs are more likely to have chromosome errors, so IVF with eggs donated by younger women succeeds more often.

## Quick summary

- Asexual reproduction: one parent, identical offspring. Sexual: two parents, varied offspring.
- Bacteria use binary fission. Yeast buds. Fungi make spores. Plants use runners, tubers and bulbs.
- Fertilisation can be external (fish, frogs, coral) or internal (mammals, birds, reptiles).
- In flowering plants, pollination is followed by fertilisation in the ovule. Ovules become seeds, ovaries become fruit.
- hCG and progesterone keep a pregnancy going. Oxytocin causes contractions at birth.

## Quiz

Try these questions from the chapter quiz.

1. How many parents are needed for asexual reproduction?
   - A. One
   - B. Two
   - C. Three
   - D. None
   - Answer: A. One. Asexual reproduction needs one parent.

2. Bacteria reproduce asexually by
   - A. budding
   - B. binary fission
   - C. spores only
   - D. pollination
   - Answer: B. binary fission. The cell copies its DNA and splits in two.

3. Which is the main advantage of sexual reproduction?
   - A. It is faster
   - B. It needs no mate
   - C. It produces variation
   - D. It uses less energy
   - Answer: C. It produces variation. Variation helps species survive change.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/reproduction/#quiz