Unit 06 · DNA & Biological Evidence
DNA is the most powerful individualizing evidence forensic science has — and the easiest to overstate. This unit covers why the structure of DNA lets short tandem repeat (STR) profiling distinguish one person from nearly everyone else, the extraction, PCR amplification, and gel electrophoresis that turn a trace into a readable profile, how a profile is compared, and the statistics that give a match its weight. Mastery means you can read a profile, weigh it with a random-match probability, and report it as a very strong association — a statistic about the DNA, never a statement of guilt.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| DNA structure & STR profiling | Cannot explain why DNA differs between people. | Describes DNA structure but not why STR regions individualize. | Explains how variation at STR loci lets a profile distinguish one person from nearly everyone else. |
| Extraction, PCR & electrophoresis | Cannot outline how a sample becomes a profile. | Names the steps but confuses amplification with separation. | Explains extraction, PCR amplification, and gel electrophoresis as concepts and what each contributes to a profile. |
| Reading a profile & comparison | Cannot read the peaks or lanes on a profile. | Reads a profile but compares it loosely or over-reads a partial. | Reads a profile and makes a disciplined comparison, stating clearly whether loci are included, excluded, or inconclusive. |
| Match probability & statistics | Calls a match a certainty or commits the prosecutor's fallacy. | Cites a random-match probability but slips into stating it proves identity. | Interprets the random-match probability correctly, avoids the prosecutor's fallacy, and reports a match as a statistic — never as 100% or as proof of guilt. |
| Sample integrity | Ignores contamination, degradation, or partial profiles. | Notices a problem sample but draws firm conclusions anyway. | Recognizes contamination, degradation, and partial or absent profiles, and limits conclusions to what the sample supports. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“The random-match probability is about one in a billion for this profile, so I’d report it as a very strong association — but that’s a statistic about the DNA, not a statement that he’s guilty. That’s for the jury.”
“The DNA matches, so it’s a 100% match — it proves he did it.”
You demonstrate this unit by reading a worked STR profile, making a comparison, and reporting a match with its random-match probability aloud — not a multiple-choice test. A criterion counts as mastered only when you weigh the statistic honestly, avoid the prosecutor's fallacy, and state that the DNA evidence is for the court to weigh. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.