Explain how the process of meiosis contributes to genetic variation in sexually reproducing organisms, with reference to two distinct mechanisms that occur during meiosis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Meiosis introduces genetic variation mainly through two mechanisms.
1. Crossing over in prophase I: homologous chromosomes exchange non‑sister chromatids, creating recombinant chromatids that carry new allele combinations not found in either parent.
2. Independent assortment in metaphase I: bivalents align randomly at the metaphase plate, so the assortment of maternal and paternal chromosomes into gametes is independent, giving 2ⁿ possible chromosome combinations (n = haploid number).
1. Crossing over in prophase I: homologous chromosomes exchange non‑sister chromatids, creating recombinant chromatids that carry new allele combinations not found in either parent.
2. Independent assortment in metaphase I: bivalents align randomly at the metaphase plate, so the assortment of maternal and paternal chromosomes into gametes is independent, giving 2ⁿ possible chromosome combinations (n = haploid number).
Examiner tips
- Use the word ‘crossing over’ and ‘independent assortment’ explicitly; explain the stage and the outcome. Include the 2ⁿ figure for independent assortment. Keep each point concise.
Common mistakes
- Confusing crossing over with crossing over during anaphase; describing only one mechanism; omitting the 2ⁿ calculation.
Mark scheme (4 marks)
- Crossing over (recombination) occurs between non-sister chromatids of homologous chromosomes during prophase I
- Crossing over produces recombinant chromatids with new combinations of alleles not present in either parental chromosome
- Independent assortment occurs during metaphase I when bivalents align randomly at the metaphase plate
- Independent assortment produces 2ⁿ possible combinations of chromosomes in gametes (where n = haploid number), greatly increasing genetic diversity
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Reproduction questions
- Explain the hormonal control of ovulation during the human menstrual cycle.
- Explain the roles of FSH and LH in spermatogenesis in the human male.
- Explain the biological advantages of internal fertilisation over external fertil…
- Explain the significance of the acrosome reaction and the cortical reaction in e…