Reproduction: sexual & asexual

What you will learn

distinguish sexual and asexual reproduction in terms of parents, gametes, and offspring,
relate the structure of reproductive cells and organs to their function,
compare mitosis and meiosis as cell-division processes,
identify the parts of a flower and describe pollination and fertilisation,
evaluate advantages and disadvantages of each reproductive strategy.

Worked example 0 Real-world example: strawberry plants in two modes

A strawberry plant produces flowers each spring. Bees carry pollen from one flower to another; fertilised ovules develop into seeds inside the fruit. Later in summer, the same plant sends out runners — long stems that sprout new plantlets where they touch the ground.

The seeds formed via pollination come from sexual reproduction: two parents, genetic variation.
The runner plantlets are asexual: a single parent, identical clones.
A grower who wants a reliable crop plants the runners (predictable fruit). A breeder who wants a new variety uses seeds (variation to select from).

Key idea: the same organism can use both strategies — each has a different pay-off.

1. Asexual reproduction

One parent produces genetically identical offspring (clones). No gametes, no fertilisation — just mitosis.

Mode	Example	Description
Binary fission	bacteria, amoeba	cell splits into two equal daughter cells
Budding	hydra, yeast	a small outgrowth pinches off to form a new individual
Vegetative	strawberry runners, potato tubers	a plant part grows into a new plant
Fragmentation	starfish, sea sponges	a piece breaks off and regrows the whole body
Spore formation	fungi, ferns	specialised cells disperse and grow into new organisms

Worked example 1 Binary fission in bacteria

A single E. coli cell divides every 20 minutes under ideal conditions. Starting from one cell, how many bacteria are there after 2 hours?

Number of divisions: $\dfrac{120}{20} = 6$ .
Each division doubles the population: $1 \to 2 \to 4 \to 8 \to 16 \to 32 \to 64$ .
After 2 hours there are 64 cells, all genetically identical.

Key idea: asexual reproduction produces explosive population growth, but no variation.

2. Sexual reproduction: gametes and fertilisation

Two parents each contribute a gamete (sex cell). Gametes are haploid ( $n$ ) — they carry half the usual number of chromosomes. At fertilisation, two gametes fuse to form a zygote, which is diploid ( $2n$ ).

In humans: $n = 23$ chromosomes in egg and sperm; $2n = 46$ in the zygote.
Egg cells are large, packed with nutrients, few in number.
Sperm cells are small, motile (have tails), produced in huge numbers.

Simplified animal life cycle: diploid adult body cells produce haploid gametes by meiosis; two gametes fuse at fertilisation to produce a diploid zygote.

3. Mitosis vs meiosis

Both are cell divisions, but they do different jobs.

Feature	Mitosis	Meiosis
Purpose	growth, repair, asexual reproduction	produce gametes
Divisions	1	2
Daughter cells	2	4
Chromosome number	same as parent ( $2n \to 2n$ )	halved ( $2n \to n$ )
Genetic variation	none (clones)	yes (crossing over, random assortment)
Where	all body (somatic) cells	ovaries, testes (or anthers, ovaries in plants)

Worked example 2 Counting chromosomes

A dog has 78 chromosomes in its body cells. State the chromosome number in (a) a dog skin cell after mitosis, (b) a dog sperm cell, (c) a fertilised dog egg.

Skin cell after mitosis: $2n = 78$ — mitosis keeps the number the same.
Sperm: $n = 39$ — meiosis halves it.
Zygote (fertilised egg): $39 + 39 = 78 = 2n$ .

Key idea: meiosis halves chromosomes so that fertilisation restores the diploid number.

4. Reproduction in flowering plants

A flower contains both male and female organs in many species.

Stamen (male): anther produces pollen; filament supports it.
Carpel / pistil (female): stigma (receives pollen), style, ovary (contains ovules).
Petals attract pollinators; sepals protect the bud.

Pollination: pollen is carried from anther to stigma (by wind, insects, birds, or water). Fertilisation: the pollen grain grows a tube down the style; a male nucleus fuses with an egg cell in the ovule. The ovule becomes a seed; the ovary becomes the fruit.

Parts of a generalised flower. The stamen (anther + filament) is the male organ; the carpel (stigma + style + ovary) is the female organ.

5. Comparing the two modes

Property	Asexual	Sexual
Parents	1	2
Offspring genetics	identical clones	variable
Speed	fast	slower (finding a mate, developing gametes)
Energy cost	low	high
Benefit	exploits stable environments	survives changing environments
Example	bacterium dividing	two humans producing a child

Practice: Year 9

Fluency

Terminology and basics

Define gamete, zygote, haploid, and diploid.
List three examples of asexual reproduction, naming the organism and the mode.
State the chromosome number in a human (a) skin cell, (b) sperm, (c) zygote.
Name the male and female reproductive organs of a flower.
Which cell division produces gametes? Which produces body cells?
Give two advantages and two disadvantages of asexual reproduction.

Reasoning

Apply the ideas

A mushroom releases millions of spores. Is this sexual or asexual reproduction? Justify.
Explain why offspring of sexual reproduction are not identical to either parent.
Fruit growers prefer to propagate apple trees by grafting (asexual) rather than from seed. Give one reason why.
A pond suddenly becomes contaminated with a new bacterium. Which population is more likely to survive: a pond full of genetically identical clones of a frog, or a pond with a variable frog population? Explain.
Compare the roles of meiosis and fertilisation in maintaining a constant chromosome number across generations.

Problem solving

Reasoning from data

A bacterium with generation time 30 minutes starts as a single cell. How many cells after 4 hours? Show your working.
A lizard species reproduces asexually in stable desert conditions but can reproduce sexually during droughts. Suggest why.
Explain why sperm cells are produced in very large numbers while egg cells are produced in small numbers.
A plant breeder crosses two varieties to produce a new hybrid. Explain each of the following steps in terms of sexual reproduction: pollination, fertilisation, seed formation.

Challenge

Reasoning

Harder reasoning

Some species (aphids, water fleas) switch between sexual and asexual reproduction during the year. Suggest an evolutionary explanation for this dual strategy.
A honeybee queen produces fertilised eggs that become female workers (diploid) and unfertilised eggs that become male drones (haploid). Explain how this “haplodiploid” system is a mixture of sexual and asexual reproduction.
Cloning a mammal (e.g. Dolly the sheep) bypasses meiosis and fertilisation. Predict two problems you might expect in a population produced entirely by cloning.
Gregor Mendel’s pea experiments worked partly because pea flowers are usually self-pollinating. Explain why self-pollination still counts as sexual reproduction even though there is only one plant involved.