QUESTION 136 (5 marks)
Letters in the diagram represent chromosome segments containing genes. A misaligned exchange between homologous chromosomes produces two chromatids from an original segment order ABCDE. The centromeres remain intact.
a) Identify the abnormality in each product and specify the segment involved. [2 marks]
b) Explain why the altered products need not have a different chromosome number from the originals. [2 marks]
c) Give one way these changes could affect gene expression or phenotype. [1 mark]
Practice marking scheme
Answer
Product 1, ABCCDE, has a duplication of C. Product 2, ABDE, has a deletion of C. Neither change necessarily alters chromosome number.
Working
The exchange transfers an extra copy of segment C to one product while the other loses C. Chromosome number is counted by centromeres; these structural changes alter the content of existing chromosomes rather than necessarily adding or removing an entire chromosome. A duplicated or deleted region can change gene dosage.
Marking criteria
- Identifies duplication of C in product 1. [1 mark]
- Identifies deletion of C in product 2. [1 mark]
- States that the centromeres/chromosome units remain. [1 mark]
- Distinguishes structural segment changes from whole-chromosome number changes. [1 mark]
- Explains altered gene dosage or loss of a gene product. [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.