Forensic DNA: STR Analysis, Mixture Interpretation and CODIS

This mock takes the foundational forensic-DNA layer (covered in our DNA Foundations mock) and pushes it into the casework-grade material that real STR analysts work with every day. Thirty medium-difficulty questions across STR fundamentals (tetranucleotide vs pentanucleotide repeat units, the Bär 1997 ISFG nomenclature with decimal alleles like 9.3 at TH01, the role of the allelic ladder, off-ladder microvariants, and tri-allelic patterns), the CODIS core loci (the original 13-locus panel adopted in 1998 and the expansion to 20 loci on 1 January 2017, the seven added markers, and the published rationale of discrimination power, Rapid-DNA compatibility, and international ENFSI alignment), PCR amplification (the typical 28-30 cycle range and why kit-specified protocols matter, multiplex design with matched Tm and dye channels, and the major kit families — GlobalFiler, PowerPlex Fusion, Investigator), capillary electrophoresis (POP-4 polymer on the ABI 3500 / 3130, the LIZ / ILS internal size standards, and the analytical and stochastic thresholds in RFU), STR artefacts (n-4 stutter, pull-up across dye channels, -A peaks from incomplete adenylation, and the drop-out / drop-in distinction in low-template work), and the modern mixture-interpretation paradigm (random match probability vs likelihood ratio vs combined probability of inclusion, the SWGDAM 2017 Interpretation Guidelines, the move from subjective interpretation to validated probabilistic genotyping software — STRmix, TrueAllele, EuroForMix, LRmix Studio — driven by the PCAST 2016 critique). The mock closes on the database and Indian-law layer: the three-tier CODIS architecture (NDIS / SDIS / LDIS), Y-STR analysis for sexual-assault deconvolution and paternal lineage, mtDNA HV1/HV2 work for degraded samples, the *Selvi v. State of Karnataka* testimonial / non-testimonial distinction as it applies to compelled DNA sampling under CrPC s.53/s.53A and now BNSS s.51-52, the *Ashok Kumar v. Raj Gupta* (2021) cautious-approach rule for paternity DNA testing, and the lapsed status of the DNA Technology (Use and Application) Regulation Bill 2019.

It is pitched at MSc Forensic Science students at NFSU, LNJN-NICFS, AIIMS Forensic Medicine departments, and other Indian universities; FACT and FACT Plus aspirants attempting the Forensic Biology / DNA paper; and UGC-NET candidates whose Life Science paper increasingly reaches into forensic DNA topics. The questions assume you already have the DNA Foundations layer; the medium-difficulty bar is set so that a careful read of the explanation closes the gap if you got the question wrong.

Topics covered:

• STR fundamentals: repeat-unit length, Bär 1997 ISFG nomenclature, ladders, microvariants, tri-allelic patterns
• CODIS core loci: original 13 (1998), expanded 20 (2017), drivers (discrimination, Rapid-DNA, ENFSI alignment)
• PCR amplification: ~28-30 cycle window, multiplex design, kit families (GlobalFiler, PowerPlex Fusion, Investigator)
• Capillary electrophoresis: POP-4 polymer, LIZ / ILS internal size standard, analytical and stochastic thresholds
• STR artefacts: n-4 stutter, pull-up, -A peaks (incomplete adenylation), drop-out vs drop-in
• Mixture interpretation: RMP vs LR vs CPI / RMNE, SWGDAM 2017, probabilistic genotyping (STRmix, TrueAllele, EuroForMix, LRmix Studio), PCAST 2016
• DNA databases: CODIS three-tier architecture (NDIS / SDIS / LDIS), Y-STR, mtDNA HV1/HV2
• Indian context: *Selvi v. State of Karnataka* (testimonial / non-testimonial), CrPC s.53/s.53A and BNSS s.51-52, *Ashok Kumar v. Raj Gupta* (2021), lapsed DNA Bill 2019

Each question carries a detailed 220+ word explanation citing primary sources — Butler's *Advanced Topics in Forensic DNA Typing: Methodology* (2012) and *Interpretation* (2015), Goodwin / Linacre / Hadi's *Introduction to Forensic Genetics*, the SWGDAM 2017 Interpretation Guidelines, the PCAST 2016 report, NRC II (1996), the ISFG nomenclature paper of Bär et al. (1997), the FBI CODIS / NDIS public documentation, and the Indian Supreme Court judgments and PRS Legislative Research Bill summaries. Allow 30 minutes; the explanations are long enough to use as study notes by themselves. This is a premium mock, intended as serious revision before viva and written examinations on forensic DNA typing.