What you need to know
- 6.16–6.20Enrichment on the limits of time-of-death signs and of a DNA profilingComparing the lengths of non-coding repeated DNA sequences to identify individuals or show relationships.. Not extra case law to memorise.
No real cases, no invented match probabilities, no instruction to examine remains.
By the end, you should be able to…
- List the 6.20 signs and the main limit of each.
- Explain what a profile can add that a body temperature cannot.
- Keep both results short of a courtroom verdict.
Build your understanding
Five signs, five limits
Temperature depends on the scene. Rigor depends on ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. and later decay, and on warmth. Decomposition extent and the communityAll the populations of different species living in one habitat at the same time. of decomposers change with habitatThe place where an organism lives.. Insects date colonisationThe arrival and establishment of a species in an area where it was not found before., not always death. Writing all five without the limits looks thorough and is still weak.
A DNA profilingComparing the lengths of non-coding repeated DNA sequences to identify individuals or show relationships. answers a different question: whether a sample could have come from a particular plant, animal or person, or whether two individuals could be parent and offspring. It does not take a temperature. It does not replace the biological clock. It can fail if the DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. is degraded, mixed, or contaminated, which is why reference methods exist.
Do not collapse them into one sentence
“The maggots and the gel say it was this person at 6 pm” joins two different measurements and invents a precision neither has. Keep the sentences apart. Then say what extra evidence would be needed. That is the scientific habit, and it matches the idea that forensic science is a method applied to evidence.
Make your reasoning visible
Range language
“Consistent with” and “at least” are stronger than a false exact time.
No probability you were not given
Do not write one-in-a-billion unless the question supplies it.
Avoid this misconception
The claim: More scientific words make an exact time of death.
The correction: Each sign has a stated limit. Combining them narrows a range. It does not create a minute, and a DNA profilingComparing the lengths of non-coding repeated DNA sequences to identify individuals or show relationships. answers identity, not the clock.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Which 6.20 sign can still be used when the body is only bones and insects have long gone? What can it no longer tell you?
Show suggested answer
Little of the soft-tissue clock remains. Dry remains show that decay has gone a long way, but temperature, rigor and early insects are gone, so the time is a wide range.
Practice question
Retrieval and explanation · suggested answer
What question does a DNA profile answer that body temperature does not?
Show suggested answer
Whether the DNA could have come from a particular individual, or whether two individuals could be related. Temperature says nothing about identity.
Practice question
Retrieval and explanation · suggested answer
Why can identical twins not be separated by a standard profile?
Show suggested answer
They have the same DNA sequence, so the repeat lengths match.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A news headline says “Science proved the time and the person”. The only results are a core temperature with no ambient reading, and a profile at two loci that also matches the investigator’s contaminated control. Rewrite the headline as two cautious sentences.
Show a hint
Split the clock from the identity, and keep the failed control.
Explore the reasoning
The temperature cannot time the death because the surroundings were not measured and one reading has no fixed rate. The profile cannot identify anyone because the same bands appeared in the negative control, so contamination is a better explanation than a match. Two loci would have been too few even in a clean run.
Recall this lesson
Lesson recall questions
Question 1 of 5: List four components that must be in a PCR tube besides the template DNA.
PCR amplification
Practice question
Retrieval and explanation · suggested answer
List four components that must be in a PCR tube besides the template DNA.
Lesson: 6C 4: Polymerase Chain Reaction (PCR)
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Difficult DNA is a methods problem
6C · Thinking BiggerNIST’s forensic science programme includes reference materials for analysing difficult DNA, such as degraded samples or mixtures. That is the enrichment: a band is not self-explanatory when the…Specification 6.19Read & use the scienceClose article
Why this matters
NIST’s forensic science programme includes reference materials for analysing difficult DNA, such as degraded samples or mixtures. That is the enrichment: a band is not self-explanatory when the sample is poor. The page does not teach rigor mortis. Do not import a case. A damaged or mixed sample may produce missing or additional peaks. A forensic scientist has to state how uncertainty affects the inference, rather than silently treating every mark as a genuine allele.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.19 understand how DNA profiling is used for identification and determining genetic relationships between organisms (plants and animals)
Use the science
Why does a degraded sample make “the gel will be as clear as the textbook” unlikely?
Explore a possible response
Broken DNA may produce a partial pattern, and a mixture can add extra bands. NIST works on reference materials for those problems. A textbook lane is a clean illustration, not that sample.
Explore the source
A DNA band is not a verdict
6C · DNA ProfilingNIST explains why a forensic DNA mixture is harder to interpret than a clean textbook gel. Several people may contribute DNA, a small amount can produce weak signals,…Specification 6.18 · 6.19Read & use the scienceClose article
Why this matters
NIST explains why a forensic DNA mixture is harder to interpret than a clean textbook gel. Several people may contribute DNA, a small amount can produce weak signals, and some alleles can drop out during testing. Electrophoresis still separates fragments by length; interpretation then compares several genetic markers and considers uncertainty. A crime-drama image of one matching band cannot identify a person by itself. The analyst must also ask whether the sample is relevant, whether contamination is possible and how strong the comparison is. A profile can support an inference about identity or relationship, not a direct conclusion about what action occurred.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.18 know how gel electrophoresis can be used to separate DNA fragments of different length
- 6.19 understand how DNA profiling is used for identification and determining genetic relationships between organisms (plants and animals)
Use the science
Why is one band of matching length much weaker evidence than a well-controlled multi-locus profile?
Explore a possible response
Many people can share one fragment length. Multiple independent markers and controls reduce coincidence and reveal mixture or contamination issues.
Explore the source
PCR finds a species without seeing it
6C · Polymerase Chain Reaction (PCR)US Geological Survey scientists develop species-specific PCR assays for environmental DNA in water. An organism can leave cells or fragments behind, so a sample may reveal its DNA…Specification 6.17Read & use the scienceClose article
Why this matters
US Geological Survey scientists develop species-specific PCR assays for environmental DNA in water. An organism can leave cells or fragments behind, so a sample may reveal its DNA even when the animal itself is not seen. Primers select the sequence to amplify, and controls help detect contamination or failed reactions. A positive signal means target DNA was detected under the assay conditions; it does not directly count living animals or prove they are at that exact spot now. This conservation application uses the same amplification principle as the lesson while adding the problem of interpreting tiny traces in a changing environment.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.17 know how DNA can be amplified using the polymerase chain reaction (PCR)
Use the science
Why can a positive environmental-DNA PCR result not be converted directly into the number of fish in a lake?
Explore a possible response
DNA can persist, move and be shed at different rates; PCR detects a target sequence, not a one-to-one count of living fish.
Explore the source
- Species-specific quantitative PCR assays for eDNAUS Geological Survey