QUESTION 142 (6 marks)
The gel profiles show alleles at one autosomal locus. A single diploid individual can have at most two allele bands at this locus. The unknown sample U is compared with references R and S. All four size-marker values label alleles, and distinct alleles have distinct sizes. Assume complete amplification, no new mutations and no additional artefact bands.
a) Determine whether U can be a single-person profile at this locus. Justify from the gel. [2 marks]
b) Identify a combination of reference profiles consistent with U. [1 mark]
c) Explain why the 120 bp band travels farther than the 210 bp band. [2 marks]
d) Explain why consistency with the references does not prove that these exact individuals supplied U. [1 mark]
Practice marking scheme
Answer
U has four allele bands and cannot be from one diploid individual under the model. It is consistent with a mixture of R and S, but the matching alleles do not uniquely identify those individuals.
Working
R has 210 and 150 bp bands; S has 180 and 120 bp; U has all four. More than two distinct alleles at a diploid locus requires at least two DNA sources under the assumptions. DNA is negatively charged and travels towards the positive electrode, with smaller fragments migrating farther. Other individuals may share the tested alleles.
Marking criteria
- States U cannot be one diploid individual under the assumptions. [1 mark]
- Uses four alleles versus a maximum of two. [1 mark]
- Identifies a mixture of R and S. [1 mark]
- States migration of negatively charged DNA towards the positive electrode. [1 mark]
- Links smaller size to easier/farther movement through the gel matrix. [1 mark]
- Explains that other individuals may share alleles at the single tested locus. [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.