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A2 · Practice questions · Meiosis and genetic variation

Two divisions, endless combinations.

Six original Cambridge-style questions on meiosis and the sources of genetic variation, from identifying stages to counting gamete combinations.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Pairs, then chromatids.

01
Recall
[3 marks]

Explain the meaning of the terms haploid, diploid and homologous pair of chromosomes.

  • Haploid (n): a cell or nucleus with one complete set of chromosomes. ✓
  • Diploid (2n): a cell or nucleus with two complete sets of chromosomes, one from each parent. ✓
  • Homologous pair: two chromosomes with the same genes at the same loci (same length and centromere position), one maternal and one paternal; they may carry different alleles. ✓
02
Explain
[4 marks]

Describe the behaviour of the chromosomes, the nuclear envelope and the spindle during prophase I and metaphase I of meiosis in an animal cell.

  • Prophase I: chromosomes condense / become visible as two sister chromatids; homologous chromosomes pair up to form bivalents. ✓
  • Crossing over may occur between non-sister chromatids at chiasmata; the nuclear envelope breaks down. ✓
  • Centrioles move to opposite poles and the spindle forms. ✓
  • Metaphase I: bivalents line up on the equator, attached to spindle fibres by their centromeres, each pair oriented at random. ✓
03
Analysis
[3 marks]

State three ways in which the appearance of a plant cell in telophase II of meiosis differs from that of an animal cell at the same stage.

  • The plant cell divides by a cell plate forming across the middle; the animal cell by the membrane pinching in (cleavage furrow). ✓
  • The plant cell has a cell wall around the dividing cells. ✓
  • The plant cell has no centrioles at the poles of the spindle. ✓
04
Calculation
[2 marks]

The fruit fly Drosophila has 2n = 8. Calculate the number of genetically different gametes one fly can produce by independent assortment alone, and the number of different zygotes possible from two flies.

  • n = 4, so gametes = 24 = 16. ✓
  • Zygotes = 16 × 16 = 256 (ignoring crossing over). ✓
05
Explain
[4 marks]

Explain how meiosis and fertilisation together make the offspring of the same two parents genetically different.

  • Crossing over between non-sister chromatids in prophase I produces new combinations of alleles on chromatids. ✓
  • Independent assortment / random orientation of homologous pairs at metaphase I gives each gamete a different mix of maternal and paternal chromosomes. ✓
  • Random orientation of (non-identical) sister chromatids at metaphase II adds further variation. ✓
  • Random fertilisation: any one of the many types of male gamete can fuse with any one of the many types of female gamete. ✓
06
Data
[3 marks]

The DNA content of a cell (arbitrary units) is measured through meiosis: G1, 10; after S phase, 20; after telophase I, 10; after telophase II, 5. Explain each change.

  • 10 to 20: DNA replicates in S phase of interphase, so each chromosome has two chromatids. ✓
  • 20 to 10: in meiosis I homologous chromosomes separate into two cells (reduction division). ✓
  • 10 to 5: in meiosis II sister chromatids separate into four cells; each gamete has half the DNA of the original G1 cell. ✓

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