Meiosis and Genetic Variation

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Part 1: Knowledge and Meiosis Sequence

NCEA context: This worksheet supports Year 12 Biology learning about meiosis, inheritance and sources of genetic variation, within the New Zealand Curriculum Science, Level 7. It also supports communicating biological explanations using evidence. No specific achievement standard code is assumed.

Learning intentions:

I can compare mitosis and meiosis, describe the sequence of meiosis, and explain how meiosis and fertilisation produce genetic variation.

Key vocabulary: DNA, gene, allele, chromosome, homologous chromosomes, diploid, haploid, gamete, crossing over, independent assortment, random fertilisation and nondisjunction.

1. Complete the meiosis sequence. Use the words below to label the stages and add brief notes about what happens at each stage.

Words: DNA replication, homologous pairs, crossing over, homologous chromosomes separate, sister chromatids separate, four haploid cells

Before meiosis: Chromosomes are copied: ________________________________

Meiosis I: Homologous chromosomes pair and may exchange sections: ________________________________

Meiosis I: Homologous chromosomes separate into two cells: ________________________________

Meiosis II: Sister chromatids separate: ________________________________

Final outcome: Four genetically different cells are produced: ________________________________

Use the space below to draw and label a simplified sequence from one diploid cell to four haploid gametes. Include meiosis I, meiosis II, homologous chromosomes and sister chromatids.

2. A species has 2n = 6 chromosomes in its body cells. How many chromosomes should each normal gamete contain?

3. Which two statements correctly describe meiosis? Select two.

It produces genetically identical body cells.

It produces haploid gametes.

It includes two nuclear divisions.

It maintains the diploid chromosome number in every daughter cell.

Part 2: Explain Genetic Variation

4. Compare mitosis and meiosis by completing the statements.

Mitosis is mainly used for ____________________________________, whereas meiosis is mainly used to produce ____________________________________.

Mitosis usually produces __________ genetically similar daughter cells. Meiosis produces __________ genetically different haploid cells.

Mitosis involves __________ division. Meiosis involves __________ divisions.

5. Explain how crossing over can produce new combinations of alleles in gametes. Use the terms homologous chromosomes and allele.

6. Explain how independent assortment increases genetic variation.

7. Explain how random fertilisation adds variation to the variation already produced by meiosis. Distinguish between a gamete and a zygote in your answer.

8. Two gametes from the same parent contain different combinations of alleles for two genes. Explain how these different gametes could have been produced during meiosis.

9. Extended response: Explain why siblings can be genetically different even though they usually have the same two parents. Give a clear, evidence-based explanation that refers to crossing over, independent assortment and random fertilisation. Use accurate biological vocabulary.

10. Extension: Nondisjunction occurs when chromosomes fail to separate correctly. Predict one possible effect of nondisjunction on a gamete’s chromosome number and explain how this could affect the resulting zygote.

Teacher Guidance and Answer Key

Question 1: Expected sequence: DNA replication; homologous chromosomes pair; crossing over may occur; homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II; four genetically different haploid cells result. Diagrams should show chromosome number reduced during meiosis I.

Question 2: 3 chromosomes. The gametes are haploid, so n = 3.

Question 3: It produces haploid gametes; it includes two nuclear divisions.

Question 4: Growth, repair or asexual reproduction; gametes for sexual reproduction; two; four; one; two. Accept equivalent scientifically accurate wording.

Question 5: Homologous chromosomes pair during meiosis I. Non-sister chromatids can exchange corresponding sections, creating chromosomes with new combinations of alleles. These chromosomes enter different gametes.

Question 6: Each homologous chromosome pair lines up independently of the other pairs. The random orientation and separation produce different combinations of maternal and paternal chromosomes in gametes.

Question 7: A gamete is a haploid sex cell. Random fertilisation means any sperm may combine with any egg, producing a diploid zygote with a new combination of alleles.

Question 8: Credit references to crossing over, independent assortment, or both. The response should explain that different allele combinations can enter gametes as homologous chromosomes and chromatids separate.

Question 9: A strong response links all three processes: crossing over creates new allele combinations; independent assortment distributes chromosome pairs in different combinations; and random fertilisation combines one variable gamete from each parent. The result is genetically different zygotes and siblings.

Question 10: Nondisjunction may produce a gamete with an extra chromosome or one chromosome missing. After fertilisation, the zygote may have an abnormal chromosome number, which can affect development or viability.

Teacher use: Look for accurate use of haploid, diploid, homologous chromosomes, allele, gamete and zygote. Common misconceptions include confusing homologous chromosomes with sister chromatids and stating that crossing over occurs between non-homologous chromosomes.