Chapter 8 Self-Check Quiz — Advanced DNA

Twenty-four questions: multiple choice and short answer. Answer before opening the key at the bottom. The goal is calibration, not speed — for the multiple-choice items, be ready to say why the wrong options are wrong.

Multiple choice

1. "Touch DNA" most precisely refers to: - A) DNA from any biological fluid - B) DNA recovered from skin cells a person sheds onto an object by handling it - C) DNA that has been physically degraded - D) DNA from a database search

2. The biggest interpretive problem created by the high sensitivity of touch-DNA typing is: - A) The profile is always a mixture - B) Secondary transfer — DNA can arrive on an object indirectly - C) The random match probability becomes too small to calculate - D) CODIS rejects low-template profiles

3. A degraded single-source sample typically produces an electropherogram in which: - A) All loci show equal, tall peaks - B) The large STR loci drop out first while small loci survive - C) The small STR loci drop out first - D) Every locus shows four alleles

4. Allele dropout in a low-template sample can cause: - A) A heterozygote to appear homozygous - B) An extra contributor to appear - C) The random match probability to shrink - D) A mixture to look single-source only when the contributors are identical twins

5. The minimum number of contributors to a mixture, inferred by counting alleles, is: - A) Always the exact true number - B) A lower bound — the true number could be higher - C) An upper bound — the true number could be lower - D) Unknowable from the profile

6. mtDNA can be recovered from old bones and rootless hairs primarily because: - A) It is more chemically stable than any other molecule - B) There are many copies of mtDNA per cell, so some survive degradation - C) It is protected by the cell nucleus - D) It does not degrade

7. An mtDNA match is best described as a match to: - A) One specific individual, to the exclusion of all others - B) A maternal lineage (the person and their maternal relatives) - C) A paternal lineage - D) A random one in a billion

8. Y-STR typing is especially useful for: - A) Identifying females in a mixture - B) Isolating a male contributor from a female-heavy mixture - C) Producing a one-in-a-billion individualization - D) Replacing nuclear STR typing in all cases

9. In investigative genetic genealogy, the crime-scene DNA is re-typed across: - A) The same ~20 STR loci as CODIS - B) Hundreds of thousands of SNPs - C) Mitochondrial DNA only - D) The Y chromosome only

10. The output of the genealogy step of IGG is best described as: - A) A courtroom identification - B) An investigative lead, to be confirmed by conventional DNA - C) A random match probability - D) A guilty verdict

11. In the Golden State Killer case, the identification presented as evidence rested on: - A) The genealogy family tree alone - B) A conventional STR comparison between the crime-scene profile and the suspect's confirmed sample - C) An mtDNA match - D) A confession

12. CODIS failed to solve the Golden State Killer case for years because: - A) The crime-scene profile was too degraded - B) The offender's own profile was not in the offender database - C) CODIS does not store DNA - D) The profile was a mixture

13. "Low-template DNA" refers to a sample that is: - A) Physically broken into short fragments - B) Present in very small quantity, below standard validation levels - C) Contaminated with bacteria - D) From a maternal relative

14. The CPI/RMNE statistic for mixtures drew criticism mainly because, used carelessly, it: - A) Was impossible to compute - B) Overstated the strength of some inclusions - C) Only worked on single-source samples - D) Required the suspect's consent

15. Probabilistic genotyping software produces, for a proposed contributor, primarily: - A) A yes/no identification - B) A likelihood ratio (a strength-of-evidence statement) - C) An mtDNA sequence - D) A CODIS hit

16. Which statement about IGG and privacy is most accurate? - A) Only the suspect's privacy is implicated - B) A single relative's voluntary upload can make an entire extended family findable - C) Consumer databases were built by law enforcement - D) Everyone in every database consented to law-enforcement use

17. A "partial match" at fewer loci than the full panel is, compared to a full match: - A) Stronger, because the loci that survived are the reliable ones - B) Weaker, because fewer loci mean a larger random match probability - C) Exactly as strong - D) Always an exclusion

18. The honest verb for a touch-DNA profile recovered from an object is, at most: - A) "Proves the source handled the object" - B) "These cells are from this source" (whose cells, not how they arrived) - C) "Individualizes the act" - D) "Excludes everyone but the source from the scene"

Short answer

19. Explain in two or three sentences why secondary transfer means a perfect DNA profile on a weapon does not establish that its source handled the weapon.

20. A sample is both heat-degraded and low-template (the cold-case gas-can scenario). Name one consequence of the degradation and one consequence of the low template, and say why their combination is especially hard.

21. Why does mtDNA's high copy number give it robustness and its maternal inheritance give it weak discriminating power? Connect the two facts.

22. State the four IGG steps in order, and identify which step does the actual courtroom identification.

23. A detective says of a complex five-person mixture, "Just tell me if he's in it." Why might "inconclusive" be the only honest answer, and why is that a valid result rather than a failure?

24. Using one concept from this chapter, explain why investigative genetic genealogy is "powerful, valid, and contested" — all three at once.

Answer key (open after attempting) **Multiple choice:** 1‑B · 2‑B · 3‑B · 4‑A · 5‑B · 6‑B · 7‑B · 8‑B · 9‑B · 10‑B · 11‑B · 12‑B · 13‑B · 14‑B · 15‑B · 16‑B · 17‑B · 18‑B **19.** Touch DNA is the cells a person sheds; those cells can travel by secondary transfer (person → object → object, or via a handshake). So a profile establishes *whose cells* are on the weapon, but not that the source ever touched it — the cells may have arrived indirectly. The typing can be flawless and the inference "he handled it" still wrong. **20.** *Degradation* breaks the long molecules, so the large STR loci drop out (the ski-slope) and you get a partial profile with reduced rarity. *Low template* (too few cells) introduces stochastic effects — dropout, drop-in, peak-height imbalance — so individual allele calls become unreliable. Combined, you have faded loci *and* untrustworthy calls at the loci that remain, often layered on top of a mixture — every difficulty at once, which is why such a sample may be only weakly informative or inconclusive. **21.** Mitochondria number in the hundreds-to-thousands per cell, so even when most copies are degraded, *some* survive — that abundance is the robustness. But mtDNA is inherited maternally and is essentially identical down a maternal line, so a match is shared by all of a person's maternal relatives and cannot point to one individual — that shared inheritance is the weak discriminating power. The same biology gives both properties. **22.** (1) Generate a dense SNP profile from the crime-scene DNA; (2) upload to a genealogy database that permits law-enforcement matching and obtain distant-cousin matches; (3) build family trees from records and triangulate to candidate descendants; (4) narrow by demographics to a suspect and confirm with a conventional STR comparison on a confirmed/abandoned sample. **Step 4's STR confirmation** is the actual courtroom identification; the genealogy (steps 1–3) only generates the lead. **23.** Complex low-template mixtures of many contributors can exceed the limit of what any method can reliably resolve — too many overlapping alleles, too much dropout/drop-in, an uncertain contributor count. Forcing an include/exclude answer would overstate the evidence. "Inconclusive" honestly reports that the data cannot bear an interpretation; reporting it is ethical practice (a good lab reports it without embarrassment), not surrender. **24.** *Powerful:* it inverts the database problem and has solved decades-cold cases by finding relatives instead of the offender. *Valid:* its output is a lead confirmed by gold-standard STR typing, so the conclusion rests on the field's most rigorous method. *Contested:* it relies on relatives' DNA in consumer databases — consent at a distance — making innocent family members findable and outrunning the law meant to govern it. (Any one concept — secondary consent, the lead-vs-identification distinction, scope creep — earns full credit if it ties all three judgments together.)