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Inheritance: Question 6

Syllabus 16.1

Multiple choice A2 1 mark

During meiosis in a diploid cell, two separate events contribute to genetic variation before fertilisation even occurs. In prophase I, non-sister chromatids of each pair of homologous chromosomes can exchange sections of DNA at a chiasma. Later, in metaphase I, each pair of homologous chromosomes lines up at the equator of the cell independently of every other pair, so that the maternal and paternal chromosomes are distributed to the two poles of the cell in random combinations.

Which statement correctly distinguishes the genetic variation produced by these two events?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: What crossing over does

During prophase I, homologous chromosomes pair up closely (forming a bivalent), and their non-sister chromatids can exchange corresponding sections of DNA at points called chiasmata. Because the two homologous chromosomes usually carry different alleles at some gene loci, this exchange creates chromatids with new combinations of alleles that were not present on either original parental chromosome. Crossing over therefore generates variation within a chromosome.

Step 2: What independent assortment does

Later, at metaphase I, each pair of homologous chromosomes lines up at the equator independently of how every other pair lines up. Which homologue of a pair faces which pole is random and is not influenced by the orientation of any other pair. When the homologues separate at anaphase I, this randomness means each gamete ends up with an essentially random mixture of maternal and paternal chromosomes. Independent assortment does not change what is on any individual chromosome. It changes only which whole chromosomes travel together into the same gamete.

Step 3: Evaluate each option

  • A: correct, this matches the distinction above: crossing over recombines alleles within a chromosome; independent assortment recombines whole chromosomes.
  • B: incorrect, this swaps the two definitions around.
  • C: incorrect. Crossing over does change the allele combination along a chromosome; only independent assortment leaves individual chromosomes unchanged.
  • D: incorrect. Crossing over occurs during prophase I (between non-sister chromatids of homologous chromosomes), not during meiosis II between sister chromatids.

Final answer

  • Crossing over recombines alleles within a chromosome; independent assortment recombines whole chromosomes between gametes, option A.