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Learn9700Paper 4 · A Level Structured QuestionsInheritanceMeiosis and variation
Inheritance · Subtopic 1 of 2

Meiosis and Variation

Meiosis makes four genetically different haploid cells from one diploid cell, by two divisions. Meiosis I separates homologous chromosomes (pairs carrying the same genes): in prophase I homologues pair as bivalents and cross over at chiasmata, swapping sections of chromatids; in metaphase I bivalents line up randomly (independent assortment); in anaphase I homologues separate. Meiosis II separates sister chromatids, like mitosis. Variation comes from crossing over, independent assortment (2n2^n combinations) and random fertilisation. Non-disjunction (failure to separate) gives gametes with an extra or missing chromosome, e.g. Down syndrome (trisomy 21). Meiosis halves the chromosome number so fertilisation restores diploidy.

Cambridge 9700 Paper 4Edexcel YBI11 WBI12AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the stages of meiosis.
  • Explain how meiosis produces variation.
  • Calculate combinations from independent assortment.
  • Explain non-disjunction.
Pause and think

Siblings with the same parents can look very different. Where does that variation come from?

Section 2 of 14

Core idea

Start hereWhy siblings differ

Brothers and sisters share parents but rarely look identical. Meiosis shuffles the genetic deck each time.

The big idea

Meiosis halves chromosome number and creates new combinations by crossing over and independent assortment.

Meiosis in outline

Two divisions, four cells.

Worked exampleIndependent assortment

2n = 6. Gamete combinations?

Goal
Number.
Strategy
2ⁿ with haploid n.
  1. n=3n=3
    Haploid number.
  2. 23=82^3=8
    Each pair orients two ways.
Answer8.
✓ CheckBefore crossing over adds more ✓.

Summary

  • Two divisions.
  • Crossing over; independent assortment.
  • Non-disjunction.
Key words
Homologous chromosomes
Pair with the same genes, one from each parent.
Chiasma
Point where crossing over occurs.
HistoryDown syndrome identified. In 1959 Jérôme Lejeune showed that Down syndrome is caused by an extra copy of chromosome 21.
Fun factYou are one in 70 trillion. Independent assortment alone gives 223≈8.42^{23}\approx8.4 million kinds of gamete per parent, so about 70 trillion combinations for a child, before crossing over.
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