Q5 LONG ANSWER 10M ★ High-yield

Describe DNA profiling with its principles, sources, applications and limitations.

Forensic Medicine · 2nd Professional — Para-Clinical

Principles

DNA profiling (DNA fingerprinting, DNA typing) is the identification of an individual from the pattern of highly variable regions of their DNA. It is the most conclusive method of identification available, and is applicable to almost any biological material.

The basis is that 99.9% of human DNA is identical between individuals; identification depends on the remaining fraction, and specifically on regions where the variation between people is greatest
STRs (short tandem repeats) are the markers used in modern practice — short sequences repeated a variable number of times at defined loci. The number of repeats at each locus differs between individuals, and analysing 13 to 24 loci gives a probability of coincidental match of the order of one in many billions
Everyone inherits one allele at each locus from each parent, which is what makes the technique equally applicable to parentage testing
The older method was RFLP (restriction fragment length polymorphism) with VNTR probes, developed by Sir Alec jeffreys in 1984; it required large quantities of good-quality DNA and has been replaced by PCR-based STR analysis, which needs only nanogram quantities and works on degraded material
Mitochondrial DNA is inherited exclusively from the mother and is present in hundreds to thousands of copies per cell. It is therefore invaluable in old bones, teeth, hair shafts without roots and badly degraded remains where nuclear DNA has been lost — but it identifies a maternal lineage, not an individual, since all maternal relatives share it
Y-chromosome STRs are transmitted through the male line and identify a paternal lineage; useful in sexual assault where a small amount of male DNA must be found among a large excess of female DNA

CLINICAL PEARL

Mitochondrial DNA identifies a family, not a person — and that is often exactly what is needed. It cannot distinguish a man from his mother, his siblings or his maternal cousins. But when a fifty-year-old skeleton is recovered and the only surviving relative is a sister’s daughter, nuclear DNA has long since degraded and mitochondrial DNA is the only thing left that still works. Its weakness in specificity is the price of its durability and abundance.

Sources and Collection

Source	Notes
Blood	The reference sample; liquid blood in EDTA, or dried on FTA card or gauze. Note that mature red cells have NO nucleus — the DNA comes from the leucocytes
Buccal swab	Painless and non-invasive; the routine reference sample from a living person
Semen and vaginal swabs	Central in sexual assault; differential extraction separates sperm DNA from the victim's epithelial cell DNA, so a male profile can be obtained from a mixture
Hair	The root (with follicular tissue) gives nuclear DNA; a plucked hair is therefore far more valuable than a cut or shed one. The shaft alone gives only mitochondrial DNA
Bone and teeth	The best source in decomposed and skeletal remains; the femur and the tooth pulp are preferred, being dense and protected
Other	Saliva on cigarette ends, glasses, stamps and bite marks; nail clippings and nail scrapings; skin cells on a weapon handle or clothing ("touch DNA"); urine and faeces (poor yield); and tissue at autopsy

Collection principles — wear gloves and change them between samples; air-dry before packing; pack in paper, never in plastic, because plastic retains moisture and promotes bacterial degradation; label, seal and maintain the chain OF custody; and collect a control sample from an unstained area alongside every stain
Contamination is the principal danger, and PCR amplifies a contaminant as readily as the sample. The examiner's own DNA is the commonest contaminant, which is why gloves, masks and separated work areas are required, and why elimination samples from investigating personnel are taken

Applications and Legal Position

Application	Use
Criminal identification	Linking a suspect to a scene, weapon or victim — and equally, excluding a suspect, which is the more certain conclusion. DNA evidence has exonerated many wrongly convicted persons
Sexual assault	Semen and other material from the victim, and the victim's material from the accused
Paternity and maternity disputes	Maintenance, inheritance, legitimacy, and immigration. Exclusion is absolute; inclusion is expressed as a probability of paternity, conventionally above 99.9%
Identification of the dead	Mutilated, burnt, decomposed and skeletal remains; and mass disasters, where DNA is matched against relatives or against antemortem personal effects
Interchange of newborns	And disputed adoption or surrogacy
Transplantation and transfusion	Engraftment monitoring; and detection of the origin of a sample
Wildlife and other	Species identification in poaching; food adulteration; and anthropological and lineage studies

The legal framework in India — DNA evidence is admissible as expert opinion under BSA 39 (formerly IEA 45) and as electronic record; BNSS 51 and 52 (formerly CrPC 53 and 53A) permit examination of an accused including the collection of blood and other samples using reasonable force; and BNSS 184 (CrPC 164A) governs the examination of a rape survivor, which requires consent
Section 112 of the Evidence Act (now BSA 116) creates a strong presumption OF legitimacy for a child born during a valid marriage, rebuttable only by proof of non-access. The Supreme Court has held that DNA testing cannot be ordered routinely to displace this presumption, and that a court must weigh the child's right to dignity and legitimacy against the search for truth — so a party cannot demand a paternity test at will
Article 20(3) (protection against self-incrimination) has been held not to bar the compulsory taking of blood, hair and other physical samples, which are not "testimony"; but narcoanalysis, polygraph and brain mapping without consent were held unconstitutional in Selvi v State of Karnataka (2010), since they extract the contents of the mind
The DNA Technology (Use and Application) Regulation Bill has been debated for years and was withdrawn in 2023; India therefore has no dedicated DNA legislation, and the position should be checked against the current law

Interpreting a DNA Report

Conclusion	What it means
Exclusion (no match)	Absolute and certain — the person is not the source. A single genuine mismatch at one locus excludes, and this is the strongest conclusion DNA evidence can give
Inclusion (match)	The person cannot BE excluded, and the finding is expressed as a random match probability — the chance that an unrelated person would match by coincidence, commonly one in many billions
Likelihood ratio	How much more probable the evidence is if the accused is the source than if an unrelated person is — the statistically preferable way of expressing the result
Paternity	Exclusion is absolute; inclusion is given as a probability OF paternity, conventionally required to exceed 99.9%
Inconclusive	Insufficient, degraded or mixed material; a common and honest outcome that should not be presented as a weak inclusion
The prosecutor’S fallacy	Confusing the probability of a match given innocence with the probability of innocence given a match. A random match probability of one in a billion does not mean the accused is a billion times more likely to be guilty; other evidence is still required

Limitations and Applied Aspects

Identical twins cannot be distinguished by standard STR profiling, which is why fingerprints remain necessary
DNA proves presence, not action — a profile on a weapon shows the person touched it, not that they used it, and not when. Secondary transfer occurs, and the significance of a finding depends entirely on where it was found and in what quantity
Degradation, inhibitors and small quantity limit the technique; and a mixed profile from more than one contributor may be difficult or impossible to interpret
Contamination and mislabelling are the real-world failure modes, not the chemistry — and both are matters of collection and chain of custody rather than of the laboratory
Collect and dry properly, and pack in paper; sealing a wet swab in a plastic container destroys the sample within a day and is the commonest avoidable error
Take reference samples from relatives early in a mass disaster; families disperse, and a match cannot be made without something to match against
Preserve tissue for DNA at every unidentified autopsy — a piece of muscle, a femur and a tooth — even where identification seems certain, since the body will be released and cannot be recovered
Beware the prosecutor’s fallacy; a random match probability of one in a billion says nothing directly about the probability of guilt, and expressing it that way in court is wrong
Exclusion is the strongest conclusion DNA gives, and a report should say so plainly; it has exonerated many people, and its certainty in that direction exceeds its certainty in the other
The presumption of legitimacy limits paternity testing in India; a court will not order a test merely because a party asks, and the child’s interests are weighed against the search for truth
Physical samples may be taken by compulsion; the contents of the mind may not — blood and hair are outside Article 20(3), whereas narcoanalysis and polygraph without consent were held unconstitutional in Selvi
Take a control sample alongside every stain, from an unstained area of the same surface; without it the laboratory cannot exclude contamination of the substrate
Pluck hairs for reference, do not cut them; a cut hair has no root, gives no nuclear DNA, and cannot be compared for mode of separation
Consent is required to examine a rape survivor and is not required to examine an accused; the difference is statutory and absolute
India has no dedicated DNA legislation; the Bill was withdrawn in 2023, so admissibility rests on the general law of expert evidence and the position should be checked before it is stated in court

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