What is forensic genetics, and what can DNA evidence actually tell us?

Forensic genetics applies molecular biology and genetic analysis to evidence that may be relevant to a legal, criminal, conservation, regulatory, or investigative question.

In human forensic genetics, DNA may be used to compare biological material from a scene with a known person, examine whether samples could share a common source, assist with missing-person identification, or evaluate biological relationships.

Wildlife forensic genetics may involve different questions. Investigators may need to determine:

The methods may overlap with those used in human forensic science, but the questions, reference databases, marker systems, population structures, and validation requirements can differ considerably.

DNA evidence can provide several different levels of information.

At one level, it may be used for species identification. This determines the animal species from which a biological sample most likely originated.

At another level, it may be used for individualization or individual-level comparison. This examines whether a trace and a reference sample could originate from the same individual animal or person.

DNA may also contribute to kinship analysis, sex determination, geographic assignment, population analysis, mixture interpretation, or the authentication of biological products.

These are not interchangeable conclusions.

A species-level result can support the conclusion that blesbok DNA, cattle DNA, or another species was detected. It does not by itself establish which individual animal the sample came from.

Similarly, a partial DNA profile may provide some comparison information without being strong enough to support a confident individual-level conclusion.

The value of the result depends on the genetic region examined and the question the method was designed to answer. It also depends on the sample, the reference material, the quality of the profile, the population data, and the way the result is interpreted.

This distinction is especially important in wildlife forensics.

Species identification asks: What animal species did this material originate from? Individualization asks: Can this material be associated with a particular individual animal?

Mitochondrial DNA is commonly used for species identification because there are many copies of it in each cell. This makes mitochondrial markers useful when samples are old, processed, trace-level, or degraded.

Regions such as cytochrome c oxidase subunit I, commonly referred to as COI, can be amplified and sequenced. The resulting sequence can then be compared with verified reference sequences in databases.

Nuclear markers, including STRs or microsatellites, may provide greater individual-level discrimination. However, nuclear DNA is usually present in fewer copies per cell and may become more difficult to recover as degradation increases.

The two approaches therefore answer different questions.

Mitochondrial DNA may still provide a species identification after the nuclear profile has become partial or uninformative. Nuclear markers may provide a stronger association with an individual, but only when enough reliable genetic information is recovered.

A DNA result is produced through a chain of decisions. Weakness at any stage can affect what follows.

1. 1

   Recognition and collection

   A suspected biological stain, tissue sample, hair, bone, meat product, or other biological item must first be recognised and collected. The collection method depends on the type of evidence and the surface on which it was found.

2. 2

   Packaging, storage, and chain of custody

   The sample must be packaged and stored in a way that limits further degradation or contamination. Its movement and handling must also be documented.

3. 3

   DNA extraction

   Cells and biological material are broken down so that DNA can be separated from the sample and other substances.

4. 4

   DNA screening or quantification

   The analyst may assess how much DNA is present and whether the extract contains substances that may interfere with downstream analysis. DNA concentration alone does not establish whether the target species or person is present.

5. 5

   Amplification

   Selected DNA regions are copied using PCR. The chosen primers determine which genetic region is targeted.

6. 6

   Detection or sequencing

   Depending on the method, the amplified DNA may be separated by fragment size, detected using fluorescence, or sequenced to determine the nucleotide order.

7. 7

   Comparison and interpretation

   The result is compared with a reference sample, genetic database, control profile, or expected pattern.

8. 8

   Reporting

   The conclusion must reflect the actual strength and limitations of the result. The report should state what was detected, what comparison was performed, and what the result does not prove.

The laboratory process is therefore not simply a machine producing an answer. Every stage involves choices that may influence the final result, including the collection method, extraction procedure, genetic marker, PCR conditions, database quality, thresholds used, and interpretation of incomplete data.

Biological evidence is not always recovered shortly after an event.

A bloodstain may remain on metal, wood, clothing, a vehicle surface, soil, or vegetation before investigators find it. Animal products may already have been dried, salted, seasoned, mixed, cooked, tanned, or chemically treated.

DNA can be affected by:

These factors may reduce the amount of amplifiable target DNA, fragment the DNA, interfere with PCR, or increase the contribution of environmental and non-target DNA.

A measurable DNA concentration is not automatically a useful forensic result. An extract may contain degraded DNA, microbial DNA, plant DNA, DNA from another animal, or other material that contributes to the measurement.

The important question is not only whether DNA was recovered.

It is whether the recovered DNA is amplifiable, interpretable, relevant to the case, and capable of answering the forensic question being asked.

Human and wildlife forensic genetics share many molecular principles, but wildlife cases often require the analyst to establish what species is involved before an individual-level comparison can even be considered.

DNA evidence can be highly informative, but it does not answer every question.

A DNA result may show that biological material matching a person, animal, or species was present on an item. It does not automatically explain:

Failure to detect a target does not always prove absence.

The DNA may have degraded. The sampled area may not have contained the target tissue. Inhibitors may have affected amplification. A more abundant contributor may have been preferentially amplified. The method may not have been sensitive to mixtures, or the target genetic region may not have survived.

This is why forensic conclusions should be stated carefully.

It is generally more defensible to report that a target was not detected using the method applied than to state that the target was absent from the entire item.

A DNA mixture contains genetic material from more than one contributor.

In human forensic genetics, this may involve DNA from several people. In wildlife or food-authentication work, it may involve several animal species within the same product.

A mixed meat product illustrates the problem clearly.

When universal primers are used, several species may contain DNA that can bind to those primers. However, the final sequence may mainly represent the contributor that was most abundant, best preserved, or most efficiently amplified.

Standard Sanger sequencing can therefore struggle with mixed products because overlapping sequence signals may become unreadable, or the dominant species may mask a minor contributor.

More targeted or mixture-sensitive methods may be required, including:

These methods are not automatically better in every case. They still require proper primer design, specificity testing, controls, sensitivity assessment, and forensic validation.

My ongoing MSc research focuses on the point where biological evidence becomes difficult.

Using blesbok as a model species, I investigated how wildlife DNA performs after blood is deposited on metal, wood, rubberized car mat material, cotton, soil, and grass.

The samples were exposed indoors, in a semi-covered outdoor environment, and under fully exposed outdoor conditions. This allowed me to examine how time, environmental exposure, and substrate type affected species identification and nuclear marker recovery.

I also examined processed products in the form of blesbok biltong and mixed dry sausage.

Mitochondrial COI barcoding was used for species identification, while microsatellite markers were used to assess whether individual-level information could still be recovered.

The biltong remained identifiable as blesbok after seven months. The mixed dry sausage produced sequences suitable for identification, but only cattle, Bos taurus, was detected through standard DNA barcoding.

A targeted qPCR assay was investigated to detect blesbok within the mixture. The primers amplified successfully during screening, but the probes lacked sufficient binding stability, preventing successful probe-based detection and complete multiplex validation.

DNA concentration and purity were assessed separately using a NanoDrop spectrophotometer.

The research therefore examines both the strength and the limits of current forensic genetic techniques. It asks not only whether DNA survives, but whether the DNA that remains can still support a scientifically defensible conclusion.

Read more about my MSc research

The laboratory result is only one part of the evidence.

Good forensic work requires the analyst to understand the biological sample, the method used, the comparison made, the quality of the data, the uncertainty involved, and the legal or investigative question being asked.

This is especially important where evidence is degraded, mixed, environmentally exposed, or obtained from a species with limited reference information.

A strong conclusion is not the most dramatic conclusion.

It is the conclusion that remains within the limits of the data.

DNA profile

A set of results obtained from selected genetic markers. A profile may be compared with a known reference sample or evaluated using relevant population data.

STR

A short tandem repeat. STR regions contain short DNA sequences repeated a variable number of times. The variation between individuals makes properly validated STR systems useful for comparison and individualization.

Microsatellite

A repeated nuclear DNA marker. The term is often used for STR-type markers in animal and wildlife genetic studies.

DNA barcoding

The use of a standardised genetic region, commonly a mitochondrial region such as COI in animals, to support species identification through comparison with reference sequences.

Species identification

The use of genetic information to determine which species a biological sample most likely originated from.

Individualization

The evaluation of genetic evidence to determine whether a biological sample can be associated with a particular individual, subject to the discriminatory power and limitations of the marker system.

Mitochondrial DNA

DNA located in the mitochondria. It occurs in multiple copies per cell and is therefore often useful for degraded or trace samples, particularly for species identification.

Nuclear DNA

DNA located in the cell nucleus. Nuclear markers can provide individual-level discrimination but may be more vulnerable when DNA is highly degraded or present in low quantities.

PCR

Polymerase chain reaction. A laboratory method used to produce many copies of a selected DNA region so that it can be detected or analysed.

qPCR

Quantitative or real-time PCR. A PCR method that measures fluorescence during amplification. Depending on the assay design, it may be used for DNA quantification or targeted detection. In my MSc research, the qPCR component was intended for targeted species detection in a mixture, not for measuring total extracted DNA.

Allele dropout

Failure to detect an allele that is present in the original sample, often because the DNA is degraded, present in low quantity, or affected by stochastic amplification.

Inhibitor

A substance that interferes with DNA extraction, PCR amplification, or another stage of molecular analysis.

Forensic genetics can associate biological material with a person, animal, species, or reference source. It can help identify unknown remains, expose product substitution, support wildlife investigations, and exclude incorrect sources.

However, the value of the science depends on more than the presence of DNA.

It depends on using the correct method, understanding the evidence, interpreting the result honestly, and stating clearly what the result can and cannot support.

That restraint does not weaken forensic genetics.

It is what makes the evidence defensible.