Chapter 7 — Exercises

These exercises move from recall to applied reasoning to ethics and evidence interpretation. Items marked with a dagger (†) have worked solutions in the answers appendix. The final items extend the running Cold Case. There are no answers in this file — work them honestly first.

Legend: † = worked solution provided in the answers appendix · (CSI / Lab / Law / Juror) tags flag which learning path an item most serves (all readers benefit from all of them).

A. Recall and definitions

1. Define each of the following in one precise sentence: DNA, STR, locus, allele, PCR, electropherogram, DNA profile, CODIS, random match probability.

2. State the load-bearing biological fact that lets a cheek swab from a suspect be compared to a bloodstain from a scene. (Juror)

3. Roughly what percentage of the human genome is identical from person to person, and why does forensic typing deliberately target the varying, non-coding regions rather than genes? (Lab)

4. † List the four major stages of the standard STR workflow in order, and give one failure mode for each. (Lab)

5. How many alleles does a person carry at a single autosomal locus, and why? What does it mean if only one allele value shows up at a locus?

6. What does it mean to say two profiles "match"? What does it mean to say they are "excluded"?

7. Name the three things a CODIS database can hold in separate indexes, and state in one sentence what an offender hit and a forensic hit each tell you.

8. † Express this random match probability two ways — as a fraction and in plain human terms: "1 in 1,200,000,000." Why does the book insist on stating RMPs both ways? (Juror)

B. Comprehension and explanation

9. Explain why PCR's exponential copying is described in the chapter as "its gift and its standing risk." (Lab)

10. In capillary electrophoresis, smaller DNA fragments arrive at the detector first. Explain the physics in one or two sentences (charge, field, polymer).

11. † Why is DNA placed at the top of the validity spectrum? Give the three properties from §7.1 that the weaker pattern methods lack, and explain what each one buys. (Law)

12. The chapter argues that "every gain in sensitivity comes with a cost." State the cost, and explain why it grows as the method detects smaller and smaller amounts of DNA. (CSI)

13. Why is a negative control (a reagent blank carried through the whole process) run alongside the evidence sample? What specific error is it designed to catch? (Lab)

14. Explain why the RMP is calculated for unrelated people, and why a close relative — say a sibling — is not covered by the impressive one-in-a-billion figure.

15. † Distinguish, with an example, between the coincidental-match risk (which the RMP describes) and the laboratory-error risk (which it does not). Which is realistically larger, and why doesn't typing more loci shrink the second one? (Law)

C. Applied reasoning and evidence interpretation

16. An evidence profile shows genotype (15, 16) at locus D3. A suspect's reference profile shows (14, 17) at D3 and matches the evidence at every other locus. Is the suspect included or excluded, and how confident can you be on the strength of that one locus? Explain the asymmetry. †

17. At five hypothetical loci, the chance a random person matches the evidence genotype is about 1 in 6 at each. Treating the loci as independent, estimate the chance of matching at all five. Show the arithmetic and label your numbers as illustrative. (Lab)

18. † A detective says: "The lab got a one-in-a-billion match, so there's only a one-in-a-billion chance our guy is innocent." Name the fallacy, then write one sentence the expert could say instead that is statistically honest. (Law)

19. Read Figure 7.1 (the single-locus electropherogram). Write your own six-field "Read the Evidence" block for a hypothetical electropherogram at one locus showing a single tall peak at allele 12 and a much shorter peak at allele 9 from a clean reference sample. What does the height difference make you want to ask before calling the short peak a real allele? (CSI/Lab)

20. A CODIS search returns a candidate. Walk through, step by step, what must happen between that database hit and DNA evidence being presented at trial. Why is the hit itself not the evidence? (Law)

21. † A profile recovered from a doorknob matches a suspect at all 20 loci, RMP one in a trillion. List three distinct questions the chapter says this match cannot answer, and explain why each matters to whether the suspect is guilty. (Juror)

22. Secondary transfer: describe a realistic, entirely innocent chain of events by which a person's DNA could end up on an object at a scene they never visited. Then state what investigators and the lab should do with that possibility (not ignore it; account for it). (CSI)

D. Spot the overstatement

For each statement, decide whether it is defensible as written or an overstatement, and if it overstates, rewrite it honestly.

23. "The DNA proves the defendant was at the scene." †

24. "The random match probability is one in 50 billion, so the defendant is almost certainly guilty."

25. "We found a complete, single-source profile; the suspect cannot be excluded and is consistent with being the source, with a random match probability of one in 900 million among unrelated individuals." (Law)

26. † "A CODIS hit identified the perpetrator." (Law)

27. "Because the loci are independent, we multiplied the per-locus frequencies; the resulting profile frequency is approximately one in 2 billion in the relevant population." (Lab)

E. Ethics and judgment

28. † Forensic STR typing is deliberately designed to read only non-coding regions and reveal nothing about health or traits. Argue why this design choice is an ethical feature, not just a technical one — and name one privacy risk that the rise of genealogy searching (Chapter 8 previews it) reintroduces.

29. An analyst is told, before interpreting a borderline mixture, exactly which suspect's profile the detective expects to find. Explain the specific risk this creates and name the safeguard. Which of the book's four themes does this implicate? (Lab)

30. You are an expert on the stand. The prosecutor asks you to agree that "a one-in-a-billion match means there's essentially no chance anyone else left this DNA." How do you answer truthfully without either overstating the science or letting the jury be misled? (Law)

31. A defense expert argues the jury should ignore a one-in-a-trillion match entirely because "lab error is possible." Is this argument fair, an overstatement, or partly both? Explain how an honest report handles lab-error risk without letting it erase a genuine result. †

F. Cold Case extensions

32. The cold case has produced a partial DNA profile from the gas-can handle. Write the two-to-three sentence Case File entry as a careful analyst would — stating exactly what the profile does and does not establish at this point. Do not include or exclude anyone. †

33. The gas can was recovered from a renovation site where many people had legitimate reason to handle tools and equipment. Using Locard's principle (Chapter 3) and §7.6, explain why the mere presence of someone's DNA on the can is weak associative evidence on its own — even if the match is statistically strong.

34. † The chapter flags that a touched surface near a fire is a prime candidate to yield a mixture of several partly degraded contributors rather than one clean profile. Without solving it (that is Chapter 8's job), explain why a mixture would lower the interpretive confidence relative to the clean single-source case in §7.3, and what question you would want the lab to answer first.

35. Imagine the CODIS search on the gas-can profile returns no hit (the contributor is not in the database). List two legitimate next investigative steps a lab/agency might consider, and name the one that Chapter 8 will develop in full. Why must whatever lead results still be confirmed by direct STR typing of a sample taken from the candidate?