Reproduction Notes

Oxford AQA IGCSE Biology: Revision notes

Key facts

  • Sexual reproduction: two parents, gametes fuse, offspring are genetically varied.
  • Asexual reproduction: one parent, no gametes, offspring are identical clones made by mitosis.
  • Variation lets a population adapt if the environment changes.
  • Asexual reproduction is faster and cheaper, but a disease can wipe out every clone.
  • Know four examples: binary fission, budding, runners and bulbs.

Sexual and asexual

Sexual reproduction fuses two gametes to make varied offspring; asexual reproduction needs one parent and makes clones.

In sexual reproduction, haploid gametes made by meiosis fuse at fertilisation to give a diploid zygote with alleles from both parents.

In asexual reproduction one parent divides by mitosis, so there is no mixing of alleles and the offspring are identical.

Sexual

Parents:
Two
Gametes:
Two gametes fuse
Offspring:
Genetically varied
Cell division:
Meiosis, then fertilisation

Asexual

Parents:
One
Gametes:
None
Offspring:
Clones
Cell division:
Mitosis only
  1. 1

    Meiosis

    Haploid gametes made

  2. 2

    Fertilisation

    Two gametes fuse

  3. 3

    Zygote

    Diploid, with alleles from both parents

  4. 4

    Growth

    Mitosis builds the organism

Sexual reproduction

Why are offspring from sexual reproduction varied?

Why variation helps

Sexual reproduction gives variation, so some of a population survive if conditions change, but it is slower and costs more energy.

When disease or climate changes, a varied population contains some individuals that survive and breed, so the population can adapt. Beneficial mutations can combine.

The costs are time and energy to find a mate, the need for two parents, and fewer offspring.

  1. 1

    Conditions change

    disease or climate

  2. 2

    Variation

    some individuals can survive

  3. 3

    Survivors breed

    passing on their advantage

  4. 4

    Population adapts

    beneficial mutations can combine

Why variation helps a population when conditions change.

Advantages

  • Variation, so populations adapt
  • Some survive a change
  • Beneficial mutations combine

Disadvantages

  • Slower: finding a mate
  • Energy for gametes and courtship
  • Needs two parents

A new disease hits a varied population. What is the advantage?

Why clones are risky

Asexual reproduction is quick and cheap, but with no variation a population cannot adapt and one disease can affect every clone.

No mate, courtship or gametes are needed, so one parent can make many offspring quickly using little energy.

Because the offspring are genetically identical, a pathogen that infects one can infect all. With no variation, selection and evolution cannot occur.

  1. 1

    One parent makes identical offspring

    fast and cheap

  2. 2

    No genetic variation

    selection and evolution cannot occur

  3. 3

    A pathogen infects one clone

    it can infect them all

  4. 4

    The population may be wiped out

    it cannot adapt to change

Why clones are at risk.

Advantages

  • Fast and efficient
  • Needs less energy
  • One parent is enough

Disadvantages

  • No genetic variation
  • Cannot adapt to change
  • Disease may kill every clone

Why might a field of clones be wiped out by one pest?

Examples of cloning

Bacteria divide, yeast bud, strawberries send out runners and bulbs split, and all make clones by mitosis from one parent.

Binary fission: one bacterial cell divides into two identical cells, as often as every 20 minutes in ideal conditions.

Budding: a yeast cell grows a bud that detaches.

Runners: a strawberry plant grows horizontal stems that root, forming plantlets.

Bulbs: onions and tulips divide to make daughter bulbs.

Binary fission

Organism:
Bacteria
What happens:
Cell divides into two

Budding

Organism:
Yeast
What happens:
Bud detaches

Runners

Organism:
Strawberry
What happens:
Stem roots, plantlet grows

Bulbs

Organism:
Tulip, onion
What happens:
Daughter bulbs form
  1. 1

    Runner grows

    Horizontal stem from the parent

  2. 2

    Roots form

    Where it touches soil

  3. 3

    Plantlet grows

    Still attached

  4. 4

    Independent

    Runner dies back

A strawberry runner

Which of these is binary fission?

Try an exam question

Compare sexual and asexual reproduction.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.