QUESTION 65 (5 marks)
Two loci are linked on a homologous chromosome pair. The alleles are arranged on one homologue and on the other. A crossover occurs between the loci in two non-sister chromatids.
a) Identify the two recombinant allele combinations produced. [2 marks]
b) Explain how crossing over can increase genetic variation in offspring. [2 marks]
c) Identify the stage of meiosis in which crossing over occurs. [1 marks]
Practice marking scheme
Answer
a) Ab and aB. b) It exchanges DNA between homologues and produces new combinations of existing alleles in gametes. c) Prophase I.
Working
A crossover exchanges corresponding DNA segments between non-sister chromatids. When the crossover lies between A and B, the participating chromatids acquire Ab and aB combinations, while the non-participating chromatids retain AB and ab. Fertilisation involving these gametes can produce different offspring genotypes. Crossing over rearranges existing alleles rather than necessarily creating new alleles.
Marking criteria
- Identifies Ab. [1 mark]
- Identifies aB. [1 mark]
- Describes exchange between non-sister chromatids of homologues. [1 mark]
- Links new allele combinations in gametes to offspring variation. [1 mark]
- Names prophase I. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.