Free response · Unit 6 Beta

Read a gel: which suspect matches

Analyze Model or Visual Representation · 4 points

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The question

Name: ______________________   Date: ____________   Read a gel: which suspect matches (4 points)

Model

DNA profiles from a crime scene

DNA from blood found at a crime scene, from three suspects and from the victim was copied by PCR at two short tandem repeat (STR) sites, and the products were separated by gel electrophoresis beside a DNA ladder. Site 1 products are 100-200 base pairs long and site 2 products 250-350. At each site, a person carries two copies, one inherited from each parent.

DNA profiles at two STR sitesA gel with six lanes. Lane 1, a DNA ladder with bands at 350, 300, 250, 200, 150 and 100 base pairs. Lane 2, crime scene sample: bands at 300 and 270 (site 2) and at 160 and 140 (site 1). Lane 3, suspect 1: 300, 270, 170, 140. Lane 4, suspect 2: 300, 270, 160, 140. Lane 5, suspect 3: 300, 280, 160, 130. Lane 6, victim: 310, 260, and a single band at 150.LadderSceneSuspect 1Suspect 2Suspect 3Victim350 bp300 bp250 bp200 bp150 bp100 bpsite 2site 1

  1. (a) Describe why the bands in each lane end up at different distances from the wells. [1 point]

  2. (b) Identify the suspect whose DNA profile matches the crime scene sample, and support your answer with evidence from the gel. [1 point]

  3. (c) The victim's lane shows only one band at site 1. Explain this result. [1 point]

  4. (d) A defense lawyer says one matching site would be enough to identify the person. Explain why labs compare many STR sites instead. [1 point]