Discipline
Forensic genetics
Forensic genetics uses DNA to identify people, establish family relationships and link biological traces to a source, with results that must stand up in court.
- DNA fingerprinting first publishedJeffreys, Nature
- 1985
- Core STR loci in the US CODIS standardSince 2017
- 20
- Of the genome shared by any two people
- 99.9%
- Base pairs in the human mitochondrial genome
- 16,569
From crime scene to court
How a DNA case is processed
- Step 01
Collection and chain of custody
Samples are collected with gloves and sterile swabs, sealed, labelled and logged so every hand-over is recorded.
- Step 02
Extraction
DNA is released from cells and separated from proteins and inhibitors.
- Step 03
Quantification
Real-time PCR measures how much human (and male) DNA is present and whether it is degraded.
- Step 04
Amplification
Multiplex PCR copies the marker regions millions of times in one reaction.
- Step 05
Detection
Capillary electrophoresis or sequencing reads the markers and produces a DNA profile.
- Step 06
Interpretation and reporting
Analysts compare profiles, resolve mixtures and express the weight of evidence, usually as a likelihood ratio.
Methods
Markers and methods
Each method reads a different part of the genome and answers a different question.
- Autosomal STR profilingSTR
- Counts repeats at short tandem repeat loci inherited from both parents; the standard for identification and DNA databases.
- Used forIdentification, paternity, databases
- Y-chromosome STRsY-STR
- Markers on the Y chromosome, passed unchanged from father to son.
- Used forMale DNA in sexual assault mixtures, paternal lineages
- Mitochondrial DNA sequencingmtDNA
- Sequences the many-copy mitochondrial genome, inherited through the maternal line.
- Used forHair shafts, old bones, degraded remains
- SNP typingSNP
- Reads single-letter differences on very short DNA fragments.
- Used forDegraded DNA, ancestry, appearance prediction
- Massively parallel sequencingMPS
- Sequences hundreds of markers at once, including variation hidden inside STR repeats.
- Used forComplex mixtures, kinship, missing persons
Glossary
Key terms
- Locus
- A specific location in the genome that is tested.
- Allele
- One version of a marker; each person has two at each autosomal locus.
- Mixture
- A sample containing DNA from more than one person.
- Likelihood ratio
- How much more probable the evidence is if one proposition is true than if an alternative is true.
- Chain of custody
- The documented record of who handled an exhibit, when and why.
Frequently asked questions
How long does a DNA test take?
The laboratory work can take a day, but casework usually takes days to weeks because of case queues, reviews and reporting. Rapid DNA instruments can profile reference samples in about two hours.
Can DNA identify a relative rather than the person?
Yes. Close relatives share part of their DNA, so kinship testing can identify missing persons or disaster victims through parents, children or siblings.
Is DNA evidence used in Pakistan’s courts?
Yes. DNA reports are presented as expert evidence, and courts weigh them together with how the samples were collected, stored and analysed.
References
- Jeffreys AJ, Wilson V, Thein SL (1985). Individual-specific “fingerprints” of human DNA. Nature. doi:10.1038/316076a0
- Butler JM (2015). Advanced Topics in Forensic DNA Typing: Interpretation. Academic Press.
Further reading
DNA testing and the law in Pakistan
For the practical and legal side of DNA testing, read our guides to the DNA test in Pakistan, DNA evidence in Pakistani courts and the chain of custody.