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Date	November 2020	Marks available	1	Reference code	20N.1.HL.TZ0.12
Level	Higher level	Paper	Paper 1	Time zone	TZ0 / no time zone
Command term		Question number	12	Adapted from	N/A

Question

In DNA profiling, short tandem repeats (STRs) of DNA are used, as these vary a great deal between individuals. Some skin cells were retrieved from a crime scene. Three STR sections of the DNA from these cells, from the victim and from a suspect, were analysed and the results are shown in the table.

What can be concluded from the gel electrophoresis data?

A. The suspect was at the crime scene.

B. The victim has the band moving the least distance.

C. The suspect and the victim have bands moving the same distance.

D. The skin cells have the band moving the greatest distance.

Markscheme

Examiners report

This was the other question that attracted most comments from teachers. Those that thought the question would prove difficult were right, with fewer than 30 % of candidates choosing the correct answer. There was a lot of information to digest and the presentation of the data as numbers of repeats differed rather than bands on a gel was unusual. Candidates were expected to understand that the skin cells could not have been from either the victim of the crime or the suspect, because the numbers of repeats were different. It was not enough for there to be the same number of repeats for one of the sequences in the skin cells and the suspect– all had to match, so answer A could be eliminated. The other answers all referred to the speed at which a band would move. Candidates were expected to know that a greater number of repeats will make a band move more slowly. The answer is therefore D – in the skin cells there is an STR sequence with the fewest number of repeats, which will move fastest. This question will have required most candidates to think for longer than the average time of 90 seconds that is available per question. A wise strategy is to return to questions such as this after answering all the others. Many candidates will have some spare time towards the end of the hour when they can think logically and calmly about problematic questions when they know that they are happy with all their other answers.

Syllabus sections

Core » Topic 3: Genetics » 3.5 Genetic modification and biotechnology

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Core » Topic 3: Genetics

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