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Biology · Grades 9–12

Meiosis and chromosome sets

Track sets of chromosomes rather than assuming gametes are identical.

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Reduction

Meiosis reduces chromosome-set number in the usual diploid-to-haploid model.

Divisions

One DNA replication is followed by two division stages.

Variation

Crossing over and independent assortment can create different genetic combinations.

Fertilization

Fusion of haploid gametes can restore the diploid chromosome number.

Worked examples

A diploid cell has 12 chromosomes. In the standard model, how many are in a haploid gamete?

Half of 12 is 6.

6

What separates in meiosis I in the standard model?

The first division separates homologous pairs.

Homologous chromosomes.

Check your understanding

  1. How many DNA replications precede the two meiotic divisions in the usual model?

    1. None.
    2. One.
    3. Two separate replications, one before each division.

  2. Two gametes each have 5 chromosomes. What chromosome number follows their fusion in the model?

  3. Why are the products of meiosis not generally genetically identical?

Answers and marking notes

Answers and marking notes
  1. One.

    DNA replication occurs once before meiosis I.

  2. 10

    5 + 5 = 10.

  3. Recombination and chromosome assortment can distribute different allele combinations.

    Mention at least one valid variation mechanism.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.