Chapter 8 Exercises — Advanced DNA

Work these in order; they climb from recall to applied reasoning to ethics and evidence interpretation. Items marked with a dagger (†) have worked solutions in the answers appendix. No answers appear in this file — wrestle with each before you check.

A reminder of the honest-verb discipline from Chapter 1: when an exercise asks "what can you say," it is asking which of exclude, consistent with, strongly supports, or inconclusive the evidence actually earns — never "proves," unless the evidence genuinely reaches that rare bar.

Part A — Recall and definitions

  1. Define touch DNA in one sentence, and explain how it differs from the broader term trace DNA.

  2. † Distinguish degradation from low template. Can a single sample suffer from both at once? Give an example.

  3. Name the three stochastic effects that appear when typing low-template DNA, and state in one line what each does to a profile.

  4. What is the characteristic "ski-slope" shape of a degraded single-source electropherogram, and which loci drop out first — and why?

  5. Define a DNA mixture. What is the minimum number of contributors you can be certain of from counting alleles, versus what the true number might be?

  6. Define deconvolution in your own words.

  7. † Explain why mtDNA can be recovered from samples that defeat nuclear STR typing. What single biological fact about mitochondria makes this possible?

  8. Why is an mtDNA "match" a statement about a lineage rather than an individual? Name the relatives who would share a given mtDNA type.

  9. What is a Y-STR, and what is the one casework situation it is most prized for?

  10. Define SNP and explain why investigative genetic genealogy uses hundreds of thousands of them rather than the ~20 STR loci of standard typing.

  11. List, in order, the four main steps of investigative genetic genealogy (IGG) from crime-scene DNA to confirmed suspect.

Part B — Applied reasoning

  1. A profile is recovered from a steering wheel and matches a suspect with a random match probability of about one in two billion. The suspect denies ever being in the vehicle. Explain, using the §8.1 concept, how both the DNA result and the suspect's denial could be true at the same time.

  2. † A lab reports a "partial profile" matching a suspect at 7 of the usual 20 loci, every typed locus agreeing. A detective says, "Seven for seven — that's a perfect match." Identify what is and isn't right about that statement, and restate the result honestly.

  3. An analyst typing a few skin cells sees one allele at a locus where the suspect's reference shows two. List the possible explanations and say which stochastic effect is most relevant to whether this should be called an exclusion.

  4. You are handed an electropherogram from a doorknob in a shared household. At three loci you count four alleles; at others you count two or three. How many contributors are you certain of? How many might there actually be, and why can't you be sure?

  5. † A sexual-assault sample contains a large amount of female DNA and a trace of male DNA; standard autosomal typing yields essentially the victim's profile and nothing usable from the assailant. Which specialized method would you request, why, and what is the major limitation of the result it produces?

  6. A bone exhumed after fifteen years yields no nuclear STR profile. Walk through which DNA method you would try next, what it could establish, and what it could not.

  7. Explain why a minor contributor in a mixture is usually harder to resolve than the major contributor, connecting your answer to the low-template problems of §8.2.

  8. † Two crime-scene samples both "match" a suspect: (A) a clean single-source bloodstain at 20 loci; (B) a low-template touch deposit that is a two-person mixture with the suspect as a possible minor contributor. Rank the strength of the two results and justify the ranking in terms of what each can support.

  9. A defense expert argues a complex five-person, low-template mixture is "inconclusive." The prosecutor calls this "the lab giving up." Defend or rebut the defense expert's position using this chapter's reasoning.

Part C — Spot the overstatement

For each statement, identify the overstatement and rewrite it as an honest forensic claim.

  1. "The mtDNA on the hair matches the defendant, so the hair is his."

  2. † "We found the suspect's DNA on the murder weapon, which proves he was the one who used it."

  3. "Genetic genealogy identified the killer."

  4. "The Y-STR profile matches the defendant — only he could have left it."

  5. "It's a DNA mixture, but the suspect's alleles are all in there, so he's a contributor."

  6. † "We ran the touch sample at extra cycles to boost sensitivity; it's the same reliable DNA test, just more of it."

Part D — Ethics and the courtroom

  1. A consumer-genealogy user uploaded her DNA two years ago to find a half-sibling. Her data later helps law enforcement identify a distant cousin as a murder suspect. Identify at least two distinct consent problems this raises (§8.6).

  2. † Investigative genetic genealogy is defended as a tool for "the worst cases." Construct the strongest argument for limiting it to violent crimes, and the strongest argument against any such limit. Where do you come down, and why?

  3. You are an expert preparing to testify about a touch-DNA profile on an object the defendant is alleged to have handled. Draft two sentences you could say truthfully on direct examination, and one sentence you must refuse to say even if the prosecutor invites it.

  4. Why has IGG faced relatively limited challenge on scientific reliability grounds in court, even as it is fiercely contested on other grounds? Name the kind of legal challenge it does tend to face. (§8.6, In the Courtroom)

  5. † A lab's old mixture-interpretation protocol is found to have overstated some past inclusions, and hundreds of cases are flagged for review. Connect this to the second theme of the book (the validity spectrum) and to the idea that "a mixture statistic is only as good as its interpretation protocol."

Part E — Evidence interpretation (Read the Evidence)

  1. Re-read Figure 8.3 ("The gas-can handle profile"). In your own words, fill out the THE INFERENCE field as if testifying — at its true strength, no more.

  2. † Sketch (in ASCII or describe in words) the electropherogram you would expect from a single-source, undegraded reference sample of the victim, and contrast it with the degraded mixture of Figure 8.3. Name two visible differences.

Part F — Cold-case extension

  1. † Cold case. The state lab's report says the gas-can handle yielded a heat-degraded mixture: a major component consistent with the victim and a minor, partial, unknown contributor not in CODIS. (a) State precisely what this does and does not establish about who handled the can. (b) Explain why the investigators turn to genetic genealogy for the minor contributor specifically. (c) A detective says, "The DNA puts our suspect at the cabin." Why is that statement premature on the basis of this chapter alone, and which later chapter actually does the interpreting?

  2. Cold case. Suppose, hypothetically, the minor contributor's genealogy lead pointed to a candidate. Describe the indispensable confirmatory step that would still be required before that name could mean anything in court, and why the genealogy alone is not enough. (Keep your answer to the method; do not invent a solution to the case.)

  3. Cold case. Argue both sides of a courtroom exchange about the gas-can mixture: write the prosecutor's strongest honest characterization of it and the defense's strongest honest counter, with neither side overstating. (Connect the defense's move to §8.1.)

Part G — Synthesis and transfer

  1. The chapter claims DNA — the book's strongest method — still contains a validity gradient inside itself. Defend that claim with three examples from this chapter that sit at different points on it.

  2. † Explain how the Golden State Killer case is "honest progress" rather than merely "impressive progress," using the distinction between a lead and an identification. Why does that distinction protect against the failure mode that ruined bite-mark evidence (preview Ch. 16)?

  3. Cognitive bias (theme three) re-enters wherever interpretation does. Identify the three places in this chapter where interpretation creeps back into DNA, and name the safeguard the book prescribes for each.

  4. † A journalist writes, "Modern DNA is so sensitive it can solve any case." Using at least three specific concepts from this chapter, write a one-paragraph correction that is neither dismissive of DNA's power nor inflated about its reach.