Meiosis and the Causes of Variation
Meiosis is the type of cell division that produces gametes (sperm and egg cells), and is tested in more structural detail at IGCSE than at GCSE. Students must give meiosis as two divisions producing four genetically different haploid daughter cells from one diploid parent cell, and explain the two named sources of genetic variation it creates: independent assortment (the random way pairs of chromosomes line up and separate) and crossing over (the exchange of sections of DNA between paired chromosomes). This links directly to why offspring produced by sexual reproduction vary, and to the causes of continuous and discontinuous variation more broadly.
Method
- State what meiosis is and where it happens: a type of cell division that occurs in the reproductive organs (testes and ovaries) to produce gametes (sperm and egg cells), involving two divisions (meiosis I and meiosis II) of one diploid parent cell, producing four haploid daughter cells, each genetically different from one another and from the parent cell.
- Contrast meiosis with mitosis on three points, since exam questions frequently ask for this: number of divisions (meiosis: two; mitosis: one), number and type of daughter cells (meiosis: four haploid cells; mitosis: two diploid cells), and genetic outcome (meiosis: four genetically different cells; mitosis: two genetically identical cells).
- Explain independent assortment: during meiosis I, homologous chromosome pairs (one from each parent) line up randomly at the middle of the cell, and it is random which chromosome of each pair ends up in which daughter cell, so each gamete gets a different random mixture of the original maternal and paternal chromosomes.
- Explain crossing over: while homologous chromosomes are paired up early in meiosis I, sections of DNA can be exchanged between the paired (non-identical) chromosomes, creating new combinations of alleles on a chromosome that were not present on either original parental chromosome.
- Link both mechanisms to the outcome examiners want stated explicitly: independent assortment and crossing over both increase genetic variation among the gametes produced, and because any sperm can fertilise any egg (random fertilisation, a third source of variation), sexual reproduction produces offspring that are all genetically different from each other and from their parents.
- For questions distinguishing continuous and discontinuous variation, link back to genetics: discontinuous variation (for example, blood group, or a single-gene trait) usually results from one or a small number of genes, giving a small number of distinct categories; continuous variation (for example, height, or skin colour) usually results from the combined, additive effect of many genes plus environmental factors, giving a full range of values.
- When a question asks 'why are siblings not identical (except identical twins)', give the full causal chain: meiosis in each parent produces genetically varied gametes (via independent assortment and crossing over), and fertilisation combines a random gamete from each parent, so each sibling inherits a different combination of parental alleles.
Worked example
A student is asked: 'Explain why the four sperm cells produced from a single primary sperm-forming cell by meiosis are not genetically identical to each other.' Answer this question.
- Identify the process: meiosis (which produces gametes), not mitosis.
- State the general point: meiosis produces genetic variation between the resulting cells, unlike mitosis.
- Give the first named mechanism: during meiosis I, homologous chromosome pairs line up and separate randomly (independent assortment), so each of the resulting cells gets a different random mixture of the original chromosomes.
- Give the second named mechanism: crossing over also occurs between paired homologous chromosomes early in meiosis I, exchanging sections of DNA and creating new combinations of alleles on the chromosomes.
- Combine both mechanisms into the final explanation: because of both independent assortment and crossing over, each of the four sperm cells produced ends up with a different, unique combination of genetic material, so they are not genetically identical to each other.
Practice questions
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Q1State the number of daughter cells produced by meiosis from one parent cell.Show answer
Answer: Four.
Q2State whether the daughter cells produced by meiosis are haploid or diploid.Show answer
Answer: Haploid.
Q3Name the two organs in the human body where meiosis occurs.Show answer
Answer: The testes and the ovaries.
Q4State one structural difference between mitosis and meiosis in terms of the number of cell divisions involved.Show answer
Answer: Mitosis involves one division; meiosis involves two divisions.
Q5Define crossing over.Show answer
Answer: The exchange of sections of DNA between paired (homologous) chromosomes during meiosis, creating new combinations of alleles.
Q6Explain why two siblings (who are not identical twins) are not genetically identical, even though they share the same two parents.Show answer
Answer: Each parent produces genetically varied gametes by meiosis (due to independent assortment and crossing over), and fertilisation combines a random gamete from each parent for each sibling, so each sibling inherits a different combination of parental alleles.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
State two ways in which meiosis differs from mitosis in terms of the daughter cells produced.
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A geneticist studies a species with 8 chromosomes (4 homologous pairs) in each body cell. Calculate the number of chromosomes present in a single gamete produced by meiosis in this species, and explain your reasoning.
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Explain how meiosis produces genetic variation among gametes, describing both independent assortment and crossing over, and explain why this variation is important for a species.
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Free printable worksheet
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