Reproduction Notes

Edexcel GCSE Biology: Revision notes

Key facts

  • Asexual reproduction: one parent, no gametes, genetically identical offspring.
  • Sexual reproduction: fusion of male and female gametes, giving genetically different offspring.
  • Asexual is rapid and needs no mate, but gives no variation; sexual gives variation, which aids survival if conditions change.
  • Meiosis in the testes and ovaries makes four genetically different haploid gametes.

Asexual reproduction

One parent produces genetically identical offspring, with no fusion of gametes.

The offspring are clones. There is no need to find a mate and the reproductive cycle is rapid.

The disadvantage is no variation, so the whole population is vulnerable to the same threats. Always explain why, for example vulnerability to the same disease.

  1. 1

    One parent

    no gametes and no mate needed

  2. 2

    Mitosis

    produces new cells quickly

  3. 3

    Genetically identical offspring

    clones with no variation

  4. 4

    Same weakness

    one disease can affect the whole population

Asexual reproduction in one line.

Advantages

  • No need to find a mate
  • Rapid reproductive cycle

Disadvantages

  • No variation
  • Whole population vulnerable to the same threats

Why is a lack of variation a disadvantage?

Sexual reproduction

Male and female gametes from two parents fuse at fertilisation, giving genetically different offspring.

Gametes are sex cells. Their fusion is fertilisation.

The advantage is variation, which aids survival if conditions change. The disadvantages are that a mate must be found, and the process is slower.

  1. 1

    Gametes formed

    Male and female sex cells.

  2. 2

    Fertilisation

    The gametes fuse.

  3. 3

    Offspring

    Genetically different from both parents.

Sexual reproduction.

What is fertilisation?

Comparing the two

Asexual gives identical offspring quickly from one parent; sexual gives varied offspring from two parents, more slowly.

Use exact wording: ‘genetically identical’ for asexual offspring, and ‘genetic variation’ for sexual offspring.

  1. 1

    Asexual: one parent

    offspring genetically identical, produced rapidly

  2. 2

    Asexual: no mate needed

    but no variation

  3. 3

    Sexual: two parents

    gametes fuse at fertilisation

  4. 4

    Sexual: genetic variation

    slower, and a mate is needed

Two routes compared.

Asexual

Parents:
1
Variation:
None
Mate needed:
No
Speed:
Rapid

Sexual

Parents:
2
Variation:
Present
Mate needed:
Yes
Speed:
Slower

Which type of reproduction needs two parents?

Meiosis

Meiosis copies DNA then divides twice, giving four genetically different cells with half the chromosomes.

Meiosis occurs in the testes and ovaries. The cell copies its DNA, then divides twice into four daughter cells. Each is haploid (half the chromosome number) and genetically different.

In humans, 46 chromosomes become 23 in each of four gametes. Do not confuse this with mitosis, which gives two identical diploid cells.

  1. 1

    DNA copied

    In the original cell.

  2. 2

    First division

    The cell divides.

  3. 3

    Second division

    The cells divide again.

  4. 4

    Four gametes

    Haploid and genetically different.

Meiosis.

A human cell has 46 chromosomes. How many does a gamete have?

Try an exam question

Compare asexual reproduction with sexual reproduction.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.