How does meiosis produce genetic variation and how does fertilisation restore the chromosome number?
The events of meiosis, how crossing over and independent assortment generate genetic variation, the role of gametes and fertilisation, and the difference between meiosis and mitosis.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on meiosis and fertilisation, covering the events of meiosis, crossing over and independent assortment, the role of gametes and fertilisation, and how meiosis differs from mitosis.
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What this dot point is asking
Edexcel wants you to describe the events of meiosis, explain how crossing over and independent assortment produce genetic variation, explain the role of gametes and fertilisation, and contrast meiosis with mitosis. The recurring exam theme is the source of variation, which feeds directly into natural selection and evolution.
The events of meiosis
Meiosis has two divisions. Before it begins, DNA is replicated in interphase so each chromosome consists of two sister chromatids.
- Meiosis I (reduction division) separates the homologous chromosomes, halving the chromosome number from diploid to haploid. In prophase I homologous chromosomes pair up to form bivalents and crossing over occurs; in metaphase I the bivalents line up on the equator; in anaphase I whole chromosomes (each still two chromatids) are pulled to opposite poles.
- Meiosis II separates the sister chromatids, like mitosis, in each of the two cells produced by meiosis I.
The result is four haploid cells, each genetically different. In humans a diploid cell with produces gametes with chromosomes.
Sources of genetic variation
Gametes and fertilisation
The random fusion of any two gametes is a third source of variation. Taken together, crossing over, independent assortment and random fertilisation make every individual (except identical twins) genetically unique.
Meiosis compared with mitosis
- Mitosis produces two genetically identical diploid cells in one division, for growth and repair.
- Meiosis produces four genetically different haploid gametes in two divisions, and halves the chromosome number.
Examples in context
Example 1. Non-disjunction and Down syndrome. If homologous chromosomes fail to separate in meiosis I (non-disjunction), a gamete can receive two copies of chromosome 21. Fertilisation then produces a zygote with three copies (trisomy 21), giving Down syndrome. This shows why the orderly separation in meiosis I matters, and why errors there change the chromosome number rather than the base sequence.
Example 2. Why siblings differ. Two children of the same parents inherit different combinations because each gamete is the product of independent assortment ( combinations in humans, over 8 million) plus crossing over, and fertilisation pairs any sperm with any egg. The number of genetically distinct offspring is astronomically large, which is the raw material for natural selection.
Try this
Q1. Explain how independent assortment produces genetic variation. [2 marks]
- Cue. Homologous pairs line up randomly at metaphase I, so gametes get different combinations of maternal and paternal chromosomes.
Q2. Explain why fertilisation must follow meiosis to maintain a constant chromosome number. [2 marks]
- Cue. Meiosis halves the number to haploid; fusion of two haploid gametes restores the diploid number in the zygote.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20184 marksDescribe how crossing over and independent assortment during meiosis lead to genetic variation in gametes.Show worked answer →
Two named mechanisms, each linked to variation.
Crossing over occurs in prophase I: homologous chromosomes pair up (forming bivalents) and exchange sections of DNA at chiasmata, producing new combinations of alleles on each chromosome. Independent assortment occurs at metaphase I: each homologous pair lines up on the equator randomly and independently of the other pairs, so maternal and paternal chromosomes are shuffled into the gametes in different combinations.
Markers reward: crossing over at chiasmata exchanges alleles; independent assortment is the random arrangement of pairs; both produce gametes that are genetically different from each other.
Edexcel 20215 marksA species has a diploid number of . Calculate the number of genetically different gametes that can be produced from independent assortment alone, and explain why the actual number of different gametes is far greater.Show worked answer →
A worked calculation with explanation.
With there are homologous pairs. The number of combinations from independent assortment is different gametes. The actual number of genetically different gametes is far greater because crossing over recombines alleles within chromosomes (producing chromosomes never seen in the parent), and the gene loci can carry many different alleles. Random fertilisation then multiplies the variation further.
Markers reward: correctly calculated; crossing over named as the extra source; mention of random fertilisation multiplying variation.
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)