Meiosis and genetic variation

What you'll learn

This guide covers how meiosis produces genetically different gametes (sex cells) and explains why sexual reproduction leads to variation in offspring. You'll understand the key differences between meiosis and mitosis, and how genetic variation arises through both meiosis and random fertilisation — essential knowledge for AQA GCSE Biology Paper 2.

Key terms and definitions

Meiosis — a type of cell division that produces four non-identical daughter cells (gametes), each with half the number of chromosomes of the parent cell

Gamete — a sex cell (sperm in males, egg cells in females) containing half the normal number of chromosomes (haploid)

Haploid — a cell containing a single set of chromosomes (n); in humans, gametes contain 23 chromosomes

Diploid — a cell containing two complete sets of chromosomes (2n); in humans, body cells contain 46 chromosomes (23 pairs)

Fertilisation — the fusion of male and female gametes to form a zygote, restoring the diploid number

Chromosome — a structure made of DNA that carries genetic information in the form of genes

Genetic variation — differences in the characteristics of individuals in a population caused by differences in genes

Zygote — the diploid cell formed when two gametes fuse during fertilisation

Core concepts

Purpose of meiosis

Meiosis is essential for sexual reproduction. It produces gametes (sex cells) that contain half the normal number of chromosomes. This halving is crucial because:

• Body cells in humans are diploid and contain 46 chromosomes (23 pairs)
• Gametes must be haploid and contain only 23 chromosomes (one from each pair)
• During fertilisation, a sperm (23 chromosomes) fuses with an egg cell (23 chromosomes)
• The resulting zygote has the full diploid number restored (46 chromosomes)

Without meiosis, chromosome numbers would double with each generation, which would be fatal to organisms. Meiosis maintains a constant chromosome number across generations.

The process of meiosis

Meiosis involves two divisions (meiosis I and meiosis II) that produce four haploid cells from one diploid parent cell. At GCSE level, you need to understand the overall process rather than detailed stages:

Before meiosis begins:

• DNA replication occurs — each chromosome is copied to form two identical strands (called chromatids) joined at the centromere
• The cell now contains 46 chromosomes, but each consists of two identical chromatids

First division (meiosis I):

• Chromosome pairs line up along the centre of the cell
• The pairs are pulled apart to opposite poles of the cell
• Each new cell receives one chromosome from each pair (still consisting of two chromatids)
• Two haploid cells are formed, each with 23 chromosomes

Second division (meiosis II):

• The chromosomes line up along the centre of each cell
• The chromatids are pulled apart to opposite poles
• Four haploid daughter cells are produced
• Each cell contains 23 single chromosomes
• All four cells are genetically different from each other and from the parent cell

Key differences between mitosis and meiosis

Understanding the differences between these two types of cell division is essential for exam success:

A common exam question asks you to state how many cells are produced by meiosis and whether they are genetically identical. Remember: meiosis produces four genetically different haploid cells.

How meiosis creates genetic variation

Meiosis produces genetic variation in two main ways:

1. Independent assortment (random distribution of chromosomes)

During the first division of meiosis, chromosome pairs line up randomly along the centre of the cell. When they separate:

• Either chromosome from each pair can go to either daughter cell
• This random distribution creates many different combinations
• In humans, with 23 pairs of chromosomes, this creates over 8 million possible combinations in gametes

2. Formation of novel combinations

Each gamete receives a mixture of maternal (from your mother) and paternal (from your father) chromosomes:

• You inherited one chromosome in each pair from your mother and one from your father
• When your gametes form, they get a random mix of these
• This means your gametes don't contain only "your mother's" or only "your father's" chromosomes — they contain a unique combination

At GCSE, you don't need to know about crossing over (exchange of genetic material between chromosomes), though this is another source of variation.

Fertilisation and genetic variation

Fertilisation adds another layer of randomness that increases genetic variation:

Random fertilisation:

• Any sperm can fertilise any egg cell
• Each sperm is genetically unique (due to meiosis)
• Each egg cell is genetically unique (due to meiosis)
• The combination of gametes during fertilisation is random

Calculating possible combinations:

If one parent can produce 8 million different gametes and the other can also produce 8 million different gametes:

• Number of possible offspring combinations = 8 million × 8 million
• This equals approximately 64 trillion different combinations

This explains why siblings (except identical twins) look different — each received a different combination of chromosomes during fertilisation.

Advantages of genetic variation

Genetic variation produced through sexual reproduction benefits species:

Survival advantage:

• Variation increases the chance that some individuals will survive if the environment changes
• For example, if a new disease appears, some individuals may have genetic resistance
• These individuals survive and reproduce, passing on their genes

Evolution:

• Genetic variation provides the raw material for natural selection
• Individuals with advantageous characteristics are more likely to survive and reproduce
• Over many generations, beneficial characteristics become more common

Example in context: Caribbean banana plants reproduced through asexual reproduction (cloning) are all genetically identical. When Panama disease arrived, it devastated entire plantations because no plants had genetic resistance. If these plants reproduced sexually, creating variation, some individuals might have survived.

Sexual versus asexual reproduction

Understanding the differences helps explain the advantages and disadvantages of each:

Sexual reproduction:

• Involves meiosis and fertilisation
• Produces genetically varied offspring
• Requires two parents (usually)
• Slower and requires more energy
• Advantages: variation aids survival in changing environments
• Disadvantages: takes time and energy to find a mate

Asexual reproduction:

• Involves only mitosis
• Produces genetically identical offspring (clones)
• Requires only one parent
• Faster and requires less energy
• Advantages: rapid reproduction, no mate needed, successful characteristics always passed on
• Disadvantages: no genetic variation, whole population vulnerable to environmental changes

Worked examples

Example 1: Explaining chromosome numbers (4 marks)

Question: A human skin cell contains 46 chromosomes. Explain why a human sperm cell contains only 23 chromosomes.

Mark scheme answer:

• Sperm cells are gametes (1 mark)
• Gametes are produced by meiosis (1 mark)
• Meiosis halves the chromosome number (1 mark)
• So that when fertilisation occurs / gametes fuse, the zygote / offspring has 46 chromosomes / the diploid number is restored (1 mark)

Exam tip: This type of question requires you to make the link between meiosis, gametes, and maintaining chromosome number across generations. Don't just state facts — explain the consequence.

Example 2: Comparing mitosis and meiosis (6 marks)

Question: Compare the process and outcomes of mitosis and meiosis.

Mark scheme answer:

• Mitosis involves one division, meiosis involves two divisions (1 mark)
• Mitosis produces two daughter cells, meiosis produces four daughter cells (1 mark)
• Mitosis produces diploid cells, meiosis produces haploid cells (1 mark)
• Mitosis produces genetically identical cells, meiosis produces genetically different cells (1 mark)
• Mitosis is used for growth and repair, meiosis is used to produce gametes (1 mark)
• Mitosis occurs in body cells, meiosis occurs in reproductive organs / testes / ovaries (1 mark)

Exam tip: "Compare" questions require you to make direct comparisons. Use comparative language: "whereas," "however," "in contrast." Table format can be effective if time allows.

Example 3: Explaining genetic variation (5 marks)

Question: Offspring produced by sexual reproduction show genetic variation. Explain how meiosis and fertilisation lead to genetic variation in offspring.

Mark scheme answer:

• Meiosis produces gametes that are genetically different (1 mark)
• Due to independent / random assortment of chromosomes (1 mark)
• Each gamete contains a different combination of maternal and paternal chromosomes (1 mark)
• Fertilisation is random / any sperm can fertilise any egg (1 mark)
• This produces a unique combination of chromosomes / genes in the offspring (1 mark)

Exam tip: Questions on genetic variation often require you to link meiosis AND fertilisation. Don't forget to mention both processes and explain how each contributes to variation.

Common mistakes and how to avoid them

• Confusing the number of cells produced: Mitosis produces 2 identical cells; meiosis produces 4 different cells. Draw a quick diagram if needed to check your answer.

• Stating that meiosis occurs in gametes: Meiosis produces gametes — it doesn't occur in gametes. Meiosis happens in reproductive organs (testes and ovaries).

• Forgetting that chromosomes are doubled before division: Before meiosis begins, DNA replicates so each chromosome consists of two chromatids. This is why two divisions are needed to produce haploid cells.

• Writing that meiosis "creates" variation: Meiosis and fertilisation produce variation by shuffling existing genetic material into new combinations — they don't create new genes or characteristics.

• Using imprecise language about chromosome numbers: Be specific: "haploid" means half the diploid number, not "half the chromosomes." In humans, haploid = 23 chromosomes and diploid = 46 chromosomes.

• Forgetting to link concepts: When explaining why meiosis is necessary, you must mention that fertilisation restores the diploid number — otherwise the examiner doesn't know you understand the full process.

Exam technique for "Meiosis and genetic variation"

• Command words matter: "Describe" requires you to state facts about what happens. "Explain" requires you to give reasons why something happens or describe a mechanism. For example: "Describe meiosis" = state the steps; "Explain why meiosis is important" = link it to maintaining chromosome number across generations.

• Use correct terminology consistently: Don't interchange "chromosomes" and "genes" — they're different. Don't write "sex cells" if you mean specifically sperm or egg cells. Precision earns marks.

• Link meiosis to variation in extended answers: Six-mark questions often ask you to explain how sexual reproduction leads to variation. Structure your answer: (1) meiosis produces different gametes due to independent assortment, (2) fertilisation is random, (3) therefore offspring receive unique combinations of chromosomes. This logical flow demonstrates understanding.

• Practice calculations: You may be asked to calculate possible combinations of chromosomes. If an organism has n pairs of chromosomes, each parent can produce 2^n different gametes. For offspring, multiply the gamete possibilities from each parent together.

Quick revision summary

Meiosis is cell division producing four genetically different haploid gametes from one diploid parent cell through two divisions. It creates variation through independent assortment of chromosomes during division. Random fertilisation adds further variation as any sperm can fuse with any egg. This genetic variation helps populations survive environmental changes and provides material for evolution. Sexual reproduction (using meiosis) produces varied offspring but is slower than asexual reproduction (using mitosis), which produces identical clones rapidly.