Can Two DNA Profiles Replace a DNA Infidelity Test?

It may appear possible to bypass a conventional DNA infidelity test by ordering two separate analyses. The first would establish a reference DNA profile from a cheek swab. The second would attempt to obtain another profile from a biological trace found on clothing, bedding, fabric or a personal object.

From a purely technical perspective, this approach can sometimes produce two DNA profiles that a laboratory can compare. However, it does not provide a reliable way to prove infidelity. Nor does it guarantee that the trace sample will produce a usable genetic profile.

Technical feasibility must also be distinguished from legality. In the UK, analysing another person’s DNA without the required consent can constitute a criminal offence. Before submitting any clothing, personal item or biological material, it is therefore essential to consider both the scientific limitations and the consent requirements.

Can Two DNA Profiles Replace an Infidelity Test?

The answer is not a simple yes or no.

A laboratory may be able to compare two genetic profiles when it has:

• a sufficiently complete reference DNA profile;

• a usable profile extracted from the biological trace;

• compatible genetic markers in both analyses;

• an appropriate statistical method for evaluating the comparison.

This process reproduces some of the stages normally included in a DNA infidelity test. The difference is that the work is divided into separate services: creating the first profile, extracting the second profile and then comparing the genetic markers.

Ordering two profiles separately does not remove the limitations of the underlying analysis.

If the trace contains very little DNA, has deteriorated or includes cells from several people, the result may remain partial or impossible to interpret. A DNA analysis also cannot reconstruct the circumstances in which the biological material was deposited.

What Is a DNA Infidelity Test?

A DNA infidelity test does not directly demonstrate that someone has been unfaithful. It examines biological material found on an item submitted to a laboratory.

Possible items include:

• underwear;

• other clothing;

• sheets or fabric;

• tissues;

• personal objects;

• stains that may contain a biological fluid.

Depending on the service requested, a laboratory may:

• look for a particular biological fluid;

• check whether human DNA is present;

• attempt to produce a genetic profile;

• compare the profile with a reference sample.

These analyses answer different questions.

For example, a semen detection test looks for components associated with seminal fluid. A DNA profiling test instead examines genetic markers that may allow a person to be included in or excluded from a comparison.

The presence of semen and the identification of a DNA profile are therefore not the same finding. A fluid may be detected without sufficient DNA being available for profiling. Conversely, human DNA may be recovered without the laboratory being able to determine which biological fluid deposited it.

What Can a Laboratory Determine?

When a sample contains enough usable DNA, the laboratory may be able to report:

• that a genetic profile was obtained;

• whether the profile is complete or partial;

• whether the sample appears to contain one contributor;

• whether DNA from several contributors is present;

• whether a reference profile can be included or excluded;

• whether the result is inconclusive.

These are strictly genetic findings.

They do not normally establish:

• when the DNA was deposited;

• where the contact occurred;

• how the material reached the item;

• whether the contact was direct or indirect;

• whether the circumstances were intimate.

A laboratory can analyse biological material. It cannot reconstruct a relationship or determine the behaviour that led to the trace being present.

What Can a DNA Infidelity Test Not Prove?

Finding someone’s DNA on clothing or an object is not sufficient to prove infidelity.

Biological material can be transferred in many ordinary situations. A trace may result from direct contact, shared use of an item, previous handling or indirect transfer through another person or surface. It may also have been present for a considerable period before the suspected incident.

In most cases, the laboratory cannot determine with certainty:

• the age of the trace;

• the exact method of transfer;

• the context in which contact occurred;

• whether the deposit resulted from sexual activity;

• whether the trace was already present before the relevant event.

A positive comparison must therefore be interpreted as limited biological information. It is not a reconstruction of what happened.

How Does the Comparison of Two DNA Profiles Work?

The process involves three separate stages.

1. Creating a Reference DNA Profile

The reference sample is normally collected with a buccal swab. The swab is rubbed against the inside of the cheek to collect cells containing the participant’s DNA.

Cheek swabs are generally preferred because they are:

• straightforward to collect;

• non-invasive;

• normally rich in cells;

• easy to dry and transport;

• less likely to contain mixed DNA when collected correctly.

The laboratory extracts DNA from the cells and analyses a set of genetic markers. These usually include short tandem repeats, commonly known as STRs.

STRs are short DNA sequences that repeat a different number of times in different people. By examining several STR locations, or loci, a laboratory can create a genetic profile that can be used for comparison.

Readers who need a broader explanation of the available analyses can consult the InfoTestADN overview of online DNA testing options.

2. Extracting DNA from a Non-Standard Sample

The second analysis concerns the clothing, fabric or object believed to carry biological material.

This is known as a non-standard, alternative or trace sample. Unlike a controlled cheek swab, its quality cannot be confirmed before laboratory testing.

The laboratory must first identify or select the area most likely to contain biological material. It then attempts to recover enough DNA to generate readable genetic data.

The likelihood of success depends on several factors:

• the material from which the object is made;

• the type of biological trace;

• the number of cells present;

• the age of the sample;

• storage conditions;

• previous handling;

• exposure to moisture or heat;

• the presence of cleaning products or other chemicals.

A garment may contain many contact areas, but analysing a large or poorly selected area can increase the risk of recovering DNA from several people. Targeted sampling is therefore important.

The InfoTestADN guide to DNA sample types and collection methods explains the differences between buccal swabs, clothing, hair, personal items and other non-standard samples.

3. Comparing the Genetic Markers

If both analyses produce sufficiently complete and compatible profiles, the laboratory can compare the STR markers.

Three broad outcomes are possible.

Inclusion or compatibility

The alleles in the reference profile are compatible with those detected in the trace sample.

This does not mean that the comparison should simply be described as a “match”. The laboratory should also assess how likely it would be to observe the genetic findings if the trace originated from the reference person rather than from another unrelated person.

Exclusion

The profiles contain sufficient unexplained genetic differences to exclude the reference person as the source of the usable trace profile.

An exclusion is generally clearer when the trace profile is complete and originates from a single contributor.

Inconclusive result

The available information is insufficient to support either a reliable inclusion or a reliable exclusion.

This may occur when:

• only part of the profile was recovered;

• the DNA was degraded;

• some alleles failed to appear;

• several contributors are present;

• the amount of DNA is extremely low;

• the two reports analyse different markers.

An inclusion does not necessarily mean that the entire biological trace came from one individual. It also does not show how that person’s DNA reached the item.

Why Are Non-Standard Samples Difficult to Analyse?

The main weakness in this type of comparison is rarely the reference cheek swab. It is usually the biological trace recovered from the object.

The Sample May Contain Too Little DNA

An object may hold only a very small number of human cells.

Modern laboratory techniques can detect low quantities of genetic material, but detection does not always produce a complete DNA profile. Some markers may appear while others remain absent.

This is known as a partial profile.

A partial profile may sometimes provide useful exclusionary information. However, it generally provides less support for a strong inclusion because fewer genetic markers are available for the statistical assessment.

The smaller the quantity of DNA, the greater the risk of inconsistent results such as allelic drop-out, where an allele that is genuinely present fails to appear in the final profile.

The DNA May Be Degraded

DNA deteriorates when exposed to environmental conditions such as:

• heat;

• humidity;

• ultraviolet light;

• washing;

• chemicals;

• prolonged unsuitable storage.

When DNA molecules are damaged, the laboratory may struggle to amplify the genetic markers required for profiling.

The result may be:

• a partial profile;

• an unbalanced profile;

• inconsistent marker detection;

• no interpretable profile at all.

A visible stain does not guarantee that intact DNA remains available. Equally, the absence of a visible stain does not mean that no cells are present.

The Sample May Be Contaminated

Anyone who handles an item can deposit additional DNA on it.

Contamination may occur when the object comes into contact with:

• bare hands;

• another garment;

• a table or work surface;

• a bag or container;

• collection tools;

• unsuitable packaging.

The more frequently an object is handled before analysis, the greater the risk of additional contributors being introduced.

Gloves can reduce direct handling but do not eliminate all contamination risks. The gloves themselves, the packaging or the work surface may already carry biological material.

Several People May Have Contributed DNA

Everyday clothing and personal objects commonly carry DNA from more than one person.

The laboratory may then obtain a mixed DNA profile. This means that the detected STR markers originate from several contributors rather than from a single individual.

The interpretation becomes particularly difficult when:

• one contributor has deposited much more DNA than another;

• a minor contributor is present only at very low levels;

• several contributors share common alleles;

• alleles have dropped out;

• contamination cannot be separated from the relevant trace.

Scientific research on the evaluation of low-template mixed DNA profiles shows why these samples require statistical modelling that accounts for different contributors, allelic drop-out and occasional additional alleles.

A mixed profile cannot always be divided into separate individual profiles.

In some cases, the laboratory may only conclude that the reference person cannot be excluded as a possible contributor to the mixture. This is not equivalent to showing that the entire trace belongs to that person.

Are Two Separate DNA Profile Reports Enough?

Not necessarily.

Two reports may contain tables of genetic markers that appear similar or different. However, a visual comparison by a non-specialist is not an adequate substitute for laboratory interpretation.

Several problems may arise:

• the laboratories may analyse different sets of loci;

• some markers may be missing from one profile;

• the reporting thresholds may differ;

• one profile may be partial;

• one sample may contain several contributors;

• allelic drop-out may hide a genuine allele;

• artefacts may be mistaken for true alleles;

• the match requires statistical evaluation.

Whenever possible, the laboratory should know from the outset that the profiles are intended to be compared. It can then confirm that the same or compatible marker systems will be used and prepare a report that directly addresses the comparison.

Ordering two independent profiles without a comparison service may therefore produce two technically valid reports that are difficult to interpret together.

How Are the Loci and Alleles Compared?

Each STR locus is normally represented by one or two numbers. These numbers correspond to the observed alleles at that genetic location.

A methodical comparison involves examining every shared locus and checking whether the allele values are compatible between the two profiles.

When two complete, single-source profiles show the same alleles at every tested locus, this provides strong support for the conclusion that they originated from the same person. The strength of that conclusion still depends on the number and type of markers tested and the appropriate statistical calculation.

A genuine unexplained difference requires further assessment. It may indicate:

• that the profiles originate from different people;

• that the trace is a mixture;

• that one profile is partial;

• that an allele has dropped out;

• that the laboratories used different analytical systems;

• that an analytical artefact is present.

For this reason, one apparent discrepancy should not be interpreted without examining the complete laboratory data.

The InfoTestADN guide to reading DNA test results provides a more detailed explanation of loci, alleles and genetic report tables.

Does the Absence of a Profile Mean That No DNA Was Present?

No.

The absence of an interpretable profile only means that the laboratory did not recover enough reliable genetic information from the submitted sample.

Possible explanations include:

• no detectable human DNA was present;

• the amount of DNA was below the laboratory’s threshold;

• the DNA was severely degraded;

• extraction was unsuccessful;

• substances on the object inhibited the analysis;

• the profile contained a mixture that was too complex to interpret.

A non-conclusive result does not establish that the item never contained biological material.

It also does not prove that no contact occurred between two people.

Likewise, a negative or inconclusive result cannot be converted into proof that no infidelity took place. It only describes what could or could not be recovered from the particular area and item analysed.

Is a DNA Match Absolute Proof?

A DNA comparison may show that the genetic markers in a reference profile are compatible with those recovered from a biological trace.

The strength of that conclusion is generally greater when:

• the profile is complete;

• the trace comes from one contributor;

• all relevant markers produced results;

• the DNA is not degraded;

• the collection conditions are documented;

• contamination controls are available;

• a statistical evaluation accompanies the finding.

Even under favourable conditions, the analysis primarily answers a biological source question:

Could the DNA recovered from the item have originated from the same person as the reference sample?

It does not answer the behavioural question:

Was that person unfaithful?

These are fundamentally different propositions.

A strong DNA comparison can support the identification of a biological contributor. It cannot establish the nature of the relationship, the time of contact or the circumstances in which the material was deposited.

Is It Legal to Compare Two DNA Profiles in the UK?

The legality of the analysis depends primarily on consent and the intended use of the results.

Section 45 of the Human Tissue Act 2004 creates an offence involving the possession of bodily material with the intention of having its DNA analysed without qualifying consent, where the results are to be used for a purpose that is not legally excepted.

The provision governing non-consensual DNA analysis applies throughout the United Kingdom, including Scotland.

For the purpose of the Act, bodily material includes material that:

• has come from a human body; and

• contains human cells.

This can include saliva, hair containing cells, semen, blood, tissue and cells deposited on clothing or personal objects.

A private attempt to identify another person from a secretly collected trace for the purpose of investigating suspected infidelity would not normally fall within the recognised exceptions. Submitting someone else’s biological material without their knowledge or consent may therefore constitute an offence.

The fact that a laboratory is technically capable of analysing the sample does not make the analysis lawful.

Consent Must Cover the Relevant Person

When a reference cheek swab is provided voluntarily, the person giving the sample can consent to the analysis of their own DNA.

The trace sample raises a different issue. If it may contain another identifiable person’s cells, that person’s consent may also be required.

The Human Tissue Authority specifically advises that when a testing company knows that a third person’s DNA may be present in material submitted for analysis, consent from that person should also be obtained.

Dividing the analysis into two separate orders does not avoid this requirement. The legal question concerns the possession and intended analysis of the biological material, not merely the commercial name given to the service.

Genetic Data Requires Enhanced Protection

Once a laboratory analyses a biological sample and links the resulting information to an identifiable person, the result constitutes genetic data.

Under the UK GDPR, genetic data is treated as special category personal data. Organisations processing it must therefore establish an appropriate lawful basis and satisfy an additional condition for processing sensitive data.

This has practical implications for:

• informed consent;

• privacy notices;

• data security;

• access to results;

• retention periods;

• sharing with third parties;

• deletion of samples and genetic data.

A laboratory should clearly explain how samples and results are stored, used and destroyed before accepting an order.

This section provides general information and should not be treated as individual legal advice.

Do These Results Have Legal Value?

A privately submitted or anonymously collected sample will rarely have the traceability required for formal legal proceedings.

For a DNA result to have potential evidential value, it must normally be possible to establish:

• the identity of every tested participant;

• valid consent;

• the precise origin of each sample;

• who collected each sample;

• how the samples were labelled;

• how they were transported and stored;

• that substitution was prevented;

• an uninterrupted chain of custody;

• the competence and validated procedures of the laboratory.

A garment sent directly by a private individual will usually not satisfy these requirements.

Even if the laboratory obtains a technically strong profile comparison, the result should not be presented as judicial proof of infidelity.

A chain of custody may establish that a particular sample was collected from a particular item. It still cannot prove the circumstances in which the DNA originally reached that item.

Key Points to Remember

Creating a reference profile from a cheek swab and a second profile from a biological trace may technically allow a DNA comparison.

However, this is not a genuine way to bypass a DNA infidelity test. It simply separates some of the same laboratory stages into different services.

The usefulness of the result depends mainly on the non-standard sample. Low quantities of DNA, degradation, contamination and mixtures from several contributors can make the analysis partial, complex or impossible.

Even when the two profiles are compatible, the comparison does not establish when the trace was deposited or how it reached the item. It therefore cannot directly prove infidelity.

Finally, technical possibility must not be confused with legal permission. UK law regulates non-consensual DNA analysis, and secretly submitting another person’s biological material may constitute an offence.

Can two DNA profiles prove infidelity?

No. A comparison may show that a reference profile is compatible with a biological trace. It cannot establish when the trace was deposited, how it reached the item or whether it resulted from an intimate relationship.

Can a DNA profile be obtained from clothing?

Yes, provided the clothing contains enough usable human cells. Success is not guaranteed because the DNA may be absent, degraded, contaminated or mixed with DNA from other people.

Can I compare two DNA reports myself?

You can compare the allele tables at a basic level, but this may be misleading. The reports may use different markers, contain partial profiles or include several contributors. A laboratory comparison with statistical interpretation is preferable.

Does the absence of a DNA profile mean that the item never contained biological material?

No. It only means that the laboratory did not recover enough interpretable DNA from the area and sample tested.

Is it legal to test another person’s DNA secretly in the UK?

It can constitute a criminal offence. Section 45 of the Human Tissue Act 2004 regulates non-consensual DNA analysis throughout the UK. A person should not submit another individual’s biological material for private identification testing without obtaining the required consent.