Forensic Science Basics: DNA Fingerprints and Chromatography Worksheet

Question 1

Explain what a DNA fingerprint is and why it can be useful in a forensic investigation.

Accepted Answer

A DNA fingerprint is a pattern made from specific regions of a person's DNA, often regions with repeated sequences. It can be useful because most people have different DNA patterns, so investigators can compare DNA from evidence to DNA from known samples.

Question 2

A gel electrophoresis result shows evidence DNA bands at 120, 200, and 350 base pairs. Suspect A has bands at 120, 200, and 350 base pairs. Suspect B has bands at 100, 200, and 360 base pairs. Which suspect matches the evidence most closely, and why?

Accepted Answer

Suspect A matches the evidence most closely because all three of Suspect A's DNA bands are the same sizes as the evidence bands. Suspect B has only one matching band, so Suspect B is not as strong a match.

Question 3

In gel electrophoresis, DNA samples are placed near the negative end of the gel. Explain why DNA moves toward the positive end.

Accepted Answer

DNA moves toward the positive end because DNA has a negative charge due to its phosphate backbone. Opposite charges attract, so the negatively charged DNA fragments move toward the positive electrode.

Question 4

During gel electrophoresis, a 100 base pair DNA fragment and a 900 base pair DNA fragment are placed in the same gel. Which fragment is expected to travel farther, and why?

Accepted Answer

The 100 base pair fragment is expected to travel farther because smaller DNA fragments move more easily through the pores of the gel than larger fragments.

Question 5

A chromatography strip has a solvent front that moved 8.0 cm from the starting line. A dye spot moved 3.2 cm from the starting line. Calculate the Rf value for the dye.

Accepted Answer

The Rf value is 0.40 because Rf equals the distance traveled by the dye divided by the distance traveled by the solvent front, so 3.2 cm divided by 8.0 cm equals 0.40.

Question 6

Two black pens are compared with ink found on a ransom note. The note ink separates into blue, purple, and yellow spots. Pen 1 separates into blue, purple, and yellow spots. Pen 2 separates into blue and green spots. Which pen is more consistent with the note ink?

Accepted Answer

Pen 1 is more consistent with the note ink because it separates into the same three colors as the ink from the note. This does not prove Pen 1 wrote the note, but it makes Pen 1 a better match than Pen 2.

Question 7

Describe two reasons forensic scientists use control samples or reference samples when analyzing DNA evidence.

Accepted Answer

Forensic scientists use control or reference samples to compare unknown evidence with known DNA profiles and to check whether the testing process worked correctly. Controls can also help detect contamination or errors.

Question 8

A technician touches a swab from a crime scene without changing gloves after handling a suspect's sample. Explain why this is a problem.

Accepted Answer

This is a problem because DNA from the suspect's sample could be transferred to the crime scene swab. That contamination could create a misleading result and weaken the reliability of the evidence.

Question 9

Explain the purpose of a chain of custody in a forensic investigation.

Accepted Answer

A chain of custody records who collected, handled, stored, and transferred evidence. It helps show that the evidence was protected from tampering, loss, or mix-ups.

Question 10

A paper chromatography test separates a marker ink into several colors. Explain what causes the colors to separate.

Accepted Answer

The colors separate because different dyes have different levels of attraction to the paper and different solubilities in the solvent. Dyes that dissolve more easily in the solvent travel farther, while dyes that stick more strongly to the paper travel less far.

Question 11

At three independent DNA loci, the chance of a random person matching the evidence is 1 in 10, 1 in 20, and 1 in 5. Calculate the combined random match probability.

Accepted Answer

The combined random match probability is 1 in 1,000 because 10 times 20 times 5 equals 1,000. This means about 1 person in 1,000 would be expected to match at all three loci by chance, assuming the loci are independent.

Question 12

A DNA profile from a crime scene is partial because the sample was degraded. It matches a suspect at two tested locations, but several other locations could not be read. Explain why investigators should be cautious.

Accepted Answer

Investigators should be cautious because a partial DNA profile contains less information than a full profile. A match at only two locations may not be rare enough to identify one person, so the result should be combined with other evidence.

Question 13

List the correct basic steps for paper chromatography using ink: place the ink spot, place the strip in solvent, mark the solvent front, and compare the separated spots. Explain why the ink spot must start above the solvent level.

Accepted Answer

The ink spot must start above the solvent level so the ink does not dissolve directly into the solvent pool. The solvent should move up the paper and carry the ink components at different rates.

Question 14

A lab report says that a suspect's DNA profile is consistent with evidence from a crime scene. Explain the difference between saying the suspect is consistent with the evidence and saying the suspect is definitely guilty.

Accepted Answer

Saying the suspect is consistent with the evidence means the DNA results do not exclude the suspect and may support a connection. It does not prove the suspect is definitely guilty because DNA evidence must be considered with other evidence, possible transfer, contamination, and the details of the case.

Question 15

A crime scene has a note written in black ink and a small blood stain. Describe one forensic test that could be used for each type of evidence and what each test could show.

Accepted Answer

Paper chromatography could be used on the black ink to compare its dye pattern with inks from known pens. DNA fingerprinting could be used on the blood stain to compare the DNA profile with known samples from a victim or suspect.

Forensic Science Basics: DNA Fingerprints and Chromatography

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