MLT GlobeForensic Science: Forensic DNA & identification
40 Forensic Science MCQs on Forensic DNA & identification with answers and explanations.
In a whole-blood sample, which cells provide the nuclear DNA used for STR profiling?
Mature human red cells and platelets lack nuclei. Nucleated leukocytes supply the genomic DNA recovered from blood.
Most STR loci used in forensic kits have repeat units of what length?
Tetranucleotide repeats balance discrimination with low stutter and easy separation of alleles differing by one repeat in capillary electrophoresis.
Y-STR analysis is especially useful in which situation?
Y-STR primers amplify only male DNA, so male profiles can be obtained from mixtures such as intimate swabs where female DNA dominates.
Mitochondrial DNA is inherited from which parent?
Mitochondria in the zygote come from the egg, so mtDNA follows the maternal line; all maternal relatives share the same sequence barring mutation.
Why can mtDNA often be obtained from shed hair shafts or old bones when nuclear DNA fails?
High copy number increases the chance that intact templates survive degradation or keratinisation, though mtDNA has lower discrimination than STRs.
Which technique separates and detects fluorescently labelled STR amplicons in forensic laboratories?
Capillary electrophoresis separates fragments by size with single-base resolution, and laser-induced fluorescence detects dye-labelled amplicons.
Haem in bloodstain extracts is a well-known example of what?
Haem, indigo dye, humic acid and melanin can inhibit polymerase activity, reducing or preventing amplification.
An STR profile shows three or four alleles at several loci. What does this most likely indicate?
A person has at most two alleles per autosomal locus. More than two at multiple loci indicates at least two contributors; isolated tri-alleles are rare.
At each autosomal STR locus, how does a child inherit alleles?
Autosomal STRs follow Mendelian inheritance, so each parent passes one of their two alleles. This is the basis of paternity testing.
The term 'touch DNA' refers to DNA recovered mainly from what?
Handling transfers skin cells and sweat residues. Such samples are low-template, prone to stochastic effects, and can result from secondary transfer.
Why are FTA cards used for reference blood or buccal samples?
FTA paper contains chemicals that lyse cells, denature proteins and protect DNA from nucleases and microbes, allowing long-term ambient storage.
Which fingerprint pattern type is the most common in most populations?
Loops make up roughly 60–65% of fingerprints, whorls about 30–35%, and arches only about 5%.
Fingerprint individualisation relies mainly on comparing which features?
Ridge characteristics (second-level detail), in their type, position and relationship, are compared along with pattern and pore/edge detail.
How are unidentified human remains most commonly identified through dental evidence?
Restorations, missing teeth, root shapes and other features are compared with records, providing reliable identification when records exist.
Since January 2017, how many core STR loci are required for the US national DNA database (CODIS)?
The FBI expanded the core set from 13 to 20 loci to improve discrimination, reduce adventitious matches and increase compatibility with international databases.
What is the basis of sex determination with the amelogenin marker?
A 6-bp deletion in the X homolog makes its amplicon shorter than the Y product. Females show one peak (X); males usually show X and Y peaks.
What is a key limitation of a Y-STR haplotype match?
The Y chromosome passes largely unchanged from father to son, so brothers, fathers and paternal cousins cannot usually be distinguished.
Forensic mtDNA sequencing traditionally targets which region?
HV1 and HV2 in the non-coding control region show the greatest variation between unrelated people, making them informative for comparison.
A small peak appears one repeat unit shorter than a true STR allele. What is it most likely?
During PCR the strand can slip, producing a product one repeat smaller. Laboratories use validated stutter thresholds to separate stutter from minor alleles.
What electropherogram pattern is typical of degraded DNA?
Fragmented DNA provides fewer intact templates for longer amplicons, giving a 'ski-slope' profile with weaker or missing large loci.