Sexual and asexual reproduction & Meiosis & Advantages and disadvantages of sexual and asexual reproduction (biology only)

SPEC 4.6.1.1 • 4.6.1.2 • 4.6.1.3 • PAPER 2

Sexual & Asexual Reproduction & Meiosis

Two ways to reproduce. One cell division that halves chromosomes. The trade-offs the examiner asks about every paper.

Why This Matters

In June 2023 Paper 2 Question 8.3, students were asked to explain why meiosis halves the chromosome number. The mark scheme demanded a 3-step chain: meiosis forms gametes → gametes fuse at fertilisation → keeps chromosome number constant.

3

Spec sub-topics covered

★

High-priority Paper 2 topic

#1

Mitosis-vs-meiosis = top failure topic

Why This Lesson

The examiner reports flag mitosis-vs-meiosis confusion as one of the highest-failure topics across the whole spec. This lesson locks down the difference and gives you the exact mark-scheme phrasings for any reproduction question on Paper 2.

Learning Objectives

By the end of this lesson, you'll be able to:

Describe sexual reproduction — fusion of gametes, meiosis, mixing of genetic information, variation. (Spec 4.6.1.1)

Explain how meiosis halves the chromosome number and how fertilisation restores it. (Spec 4.6.1.2)

State the advantages and disadvantages of sexual and asexual reproduction, including organisms that use both. (Spec 4.6.1.3 — biology only)

Spec Note

AQA states clearly: "Knowledge of the stages of meiosis is not required." You don't need prophase, metaphase, etc. Focus on the outcome — chromosome number, number of gametes, variation.

Key Definitions (1)

Three terms the mark scheme is strict on. These exact phrasings score the mark.

Definition

Sexual reproduction

Reproduction involving the fusion (joining) of male and female gametes. There is mixing of genetic information which leads to variation in the offspring. The formation of gametes involves meiosis.

Definition

Asexual reproduction

Reproduction involving only one parent and no fusion of gametes. There is no mixing of genetic information, leading to genetically identical offspring (clones). Only mitosis is involved.

Definition

Gamete

A sex cell. In animals: sperm and egg cells. In flowering plants: pollen and egg cells. Each gamete contains a single set of chromosomes — half the number found in body cells.

Key Definitions (2)

Definition

Meiosis

The cell division that occurs in reproductive organs to produce gametes. Meiosis produces four genetically different gametes, each with a single set of chromosomes (half the normal number).

Definition

Fertilisation

The fusion of male and female gametes. Fertilisation restores the full (normal) number of chromosomes in the new cell. The new cell then divides by mitosis.

Mark Scheme Phrase

The exact wording the spec uses: "halves the number of chromosomes in gametes". Write "halves the number", never just "halves the chromosomes" (which sounds like cutting each chromosome in half).

Sexual vs Asexual — The Spec Contrast

Every compare question on this topic tests these four contrasts. Learn them as paired statements.

FeatureSexual ReproductionAsexual Reproduction

Number of parentsTwo parentsOne parent

GametesFusion of male & female gametesNo gametes, no fusion

Cell divisionMeiosis (gametes) + mitosis (after fertilisation)Only mitosis

Genetic informationMixing of genetic info → variationNo mixing → genetically identical (clones)

Mark Scheme Trap

In a compare question, every sentence must reference both: "In sexual reproduction… but in asexual reproduction…". Describing one then the other separately won't score the compare marks.

Meiosis — How It Works

The spec lists exactly what you need to know. Four bullet points, in this order.

The Four Spec Points

Cells in reproductive organs divide by meiosis.

Copies of the genetic information are made (DNA replicates).

The cell divides twice → four gametes, each with a single set of chromosomes.

All gametes are genetically different from each other.

The Numbers — Humans

Mark schemes test whether you can apply meiosis numerically. Humans are the standard example.

46

Chromosomes in body cells (23 pairs)

23

Chromosomes in each gamete

46

Chromosomes after fertilisation

The Calculation

Body cell: 46 chromosomes → Meiosis halves the number → Sperm has 23, egg has 23 → Fertilisation: 23 + 23 = 46 chromosomes in the new cell. Full number restored. Generation after generation, the chromosome number stays constant.

Why This Matters

If gametes weren't halved by meiosis, every generation would double the chromosome number. 46 → 92 → 184... Meiosis is the mechanism that keeps the chromosome number constant from generation to generation. That exact phrase is in the June 2023 mark scheme.

After Fertilisation — Embryo Development

Two gametes fuse. What happens next? Mitosis takes over. This is on the spec.

The Sequence

Gametes fuse at fertilisation → normal chromosome number restored.

The new cell divides by mitosis → number of cells increases.

As the embryo develops, the cells differentiate (specialise into different cell types).

Synoptic Link (Module 1)

This connects back to mitosis (Lesson 1.05) and cell differentiation (Lesson 1.02). The same processes you met in Module 1 — body cell division and specialisation — are how a single fertilised egg becomes a whole organism.

Common Mistake

Students often write that meiosis continues after fertilisation. It doesn't. Meiosis happens only in reproductive organs to make gametes. Everything else — growth, repair, embryo development — is mitosis.

Mitosis vs Meiosis — The Big One

Examiner reports flag this as the single most-confused topic in the whole module. Learn the table cold.

FeatureMitosisMeiosis

Number of daughter cellsTwoFour

Chromosome numberSame as parent (full set)Halved (single set)

Genetic identityGenetically identicalGenetically different

Where it happensBody (somatic) cellsReproductive organs only

Used forGrowth, repair, asexual reproductionProducing gametes for sexual reproduction

Examiner Alert (MS Jun 2024 P2)

2024 P2 mark schemes do not accept "involves mitosis" for gamete formation, and do not accept "meiosis" for body cell division. Pair the right division with the right cell type: meiosis = gametes; mitosis = body cells.

Why Variation Matters

Meiosis produces four genetically different gametes (spec 4.6.1.2). When two of these gametes fuse at fertilisation, the offspring inherits a unique combination of genes — half from each parent.

Where The Variation Comes From

All gametes are genetically different from each other.

Each offspring receives half its genes from each parent — a new mix every time.

This mixing of genetic information is what produces variety in the offspring.

Why It Matters (Forward Link, Lesson 6.06)

Variation is the raw material for natural selection. Without variation, natural selection has nothing to act on. This is why sexual reproduction is so widespread despite being slower — it generates the variation that helps populations adapt to changing environments.

"In sexual reproduction there is mixing of genetic information which leads to variety in the offspring." — AQA Spec 4.6.1.1

Advantages of Sexual Reproduction

The spec lists three. Learn them word-for-word — these are the mark-scheme phrasings.

Spec 4.6.1.3 — Sexual Advantages

Produces variation in the offspring.

If the environment changes, variation gives a survival advantage by natural selection.

Natural selection can be speeded up by humans in selective breeding to increase food production.

Weak Answer

"Sexual reproduction is good because the offspring are different."

Doesn't name natural selection or survival advantage. Caps at 1 mark.

Strong Answer

"Sexual reproduction produces variation. If the environment changes, this gives a survival advantage by natural selection."

Names variation, environment change, natural selection, survival advantage. Full marks.

Advantages of Asexual Reproduction

The spec lists four. Each one is a single creditable point in a compare question.

Spec 4.6.1.3 — Asexual Advantages

Only one parent needed.

More time and energy efficient — no need to find a mate.

Faster than sexual reproduction.

Many identical offspring can be produced when conditions are favourable.

The Trade-Off

Asexual produces clones — no variation. If the environment changes (e.g. a new disease arrives), none of the offspring are adapted to survive it. The whole population can be wiped out. Sexual reproduction is "insurance" against environmental change.

Disadvantages — The Flipside

The spec lists advantages only — but past papers regularly ask about disadvantages. Disadvantages are simply the inverse of the other method's advantages. Learn them as paired logic.

Disadvantages of Sexual

Needs two parents — must find a mate.

Time- and energy-intensive (courtship, mating, gestation).

Slower than asexual reproduction.

If a mate isn't found, no offspring are produced.

Disadvantages of Asexual

No variation in offspring — all clones.

If the environment changes, the whole population is vulnerable.

A new disease could wipe out the entire population.

Population cannot adapt by natural selection.

Apply It (Spec 4.6.1.3)

AQA states: "Students are expected to be able to explain the advantages and disadvantages of asexual and sexual reproduction for any organism if given appropriate information." Expect to apply this logic to an unfamiliar organism given in the question stem — rabbits, bacteria, aphids, anything.

Organisms That Use Both

The spec names three examples of organisms that switch between sexual and asexual reproduction depending on circumstances. AQA states "Knowledge of reproduction in organisms is restricted to those mentioned" — so learn these three.

OrganismAsexualSexual

Malarial parasitesIn the human hostIn the mosquito

Many fungiBy sporesTo give variation

Many plantsBy runners (e.g. strawberry plants) or bulb division (e.g. daffodils)By producing seeds

Why Switch?

The pattern: organisms reproduce asexually when conditions are favourable (fast production of many identical offspring) and sexually when conditions change or when colonising new environments (variation needed). The mark scheme rewards naming the named example AND explaining the trade-off.

Examiner Alerts

Three traps from the most recent mark schemes and examiner reports.

Alert 1 — Mitosis vs Meiosis (MS Jun 2024 P2)

2024 mark schemes consistently flag students writing "involves mitosis" for gamete formation as wrong. The mark scheme accepts meiosis for gametes only; mitosis for body cells only. Get the pairing wrong, lose the mark.

Alert 2 — "Halves the chromosomes" wording

The spec phrase is "halves the number of chromosomes". Avoid "halves the chromosomes" — it sounds like cutting each chromosome in half. The word "number" is what scores the mark.

Alert 3 — Chain reasoning (MS Jun 2023 P2 Q8.3)

The June 2023 mark scheme for "explain why meiosis halves chromosome number" demanded a 3-step chain: meiosis forms gametes → gametes fuse at fertilisation → chromosome number stays constant from generation to generation. Skip any step, lose that mark.

PEE Worked Example

Question: Compare meiosis and mitosis. 4 marks

P

Point: Both meiosis and mitosis are types of cell division and both start by making copies of the genetic information.

E

Evidence: Mitosis produces two genetically identical daughter cells with the same chromosome number, but meiosis produces four genetically different gametes with half the chromosome number.

E

Explain: Mitosis is used for growth, repair and asexual reproduction in body cells, whereas meiosis happens only in reproductive organs to produce gametes for sexual reproduction.

Why This Scores 4/4

Four paired comparative points: (1) cell division + DNA copies, (2) 2 vs 4 daughter cells, (3) same vs halved chromosomes, (4) location and purpose. Every sentence uses "but" or "whereas" — the comparative linkers AQA demands.

Practice Question

A species of frog has 26 chromosomes in each of its body cells.

(a) State the number of chromosomes you would expect to find in one frog sperm cell. 1 mark

(b)Explain why it is important that gametes are produced by meiosis rather than by mitosis. 3 marks

(a)

13 chromosomes. Meiosis halves the number of chromosomes, so 26 ÷ 2 = 13.

P

Point: Meiosis produces four gametes, each with a single set of chromosomes (half the number found in the parent cell).

E

Evidence: When two gametes fuse at fertilisation, their chromosomes combine, restoring the full chromosome number in the new cell.

E

Explain: If mitosis were used to make gametes, each gamete would have the full chromosome number — so fertilisation would double the chromosome number each generation. Meiosis keeps the chromosome number constant from generation to generation.

Mark scheme (4 marks):

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(a) 13 chromosomes (1)

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(b) Meiosis halves the chromosome number / gametes have a single set (1)

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(b) Fertilisation restores the full chromosome number (1)

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(b) Keeps chromosome number constant / prevents doubling each generation (1)

Recap — 3 Takeaways

Two Reproduction Types

Sexual = 2 parents, meiosis, gamete fusion, variation. Asexual = 1 parent, mitosis only, genetically identical clones.

Meiosis = Halving

Cells in reproductive organs divide twice → 4 gametes, each genetically different, each with half the chromosome number. Fertilisation restores the full number.

The Trade-Off

Sexual gives variation for natural selection. Asexual is fast and needs no mate. Many organisms (malaria, fungi, plants) use both.

One Mark-Scheme Trap To Avoid

Never write "halves the chromosomes" — it sounds like cutting each chromosome in half. Always write "halves the number of chromosomes". That one extra word, "number", is what AQA's mark scheme awards the mark for.

NEXT: Lesson 6.02 — DNA and the genome & DNA structure