What Is DNA Profiling?

Almost every cell in your body contains:

DNA.

DNA contains the genetic information used in the development and functioning of an organism.

Humans share the vast majority of their DNA sequences with one another, but some regions of DNA vary considerably between:

individuals.

Scientists can examine these variable regions to produce a:

DNA profile.

A DNA profile is a pattern of genetic markers that can be compared between biological samples.

DNA profiling is sometimes called:

DNA fingerprinting.

However, a DNA profile is not a complete copy or sequence of all of a person's DNA.

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Why Can DNA Be Used to Identify Individuals?

People inherit DNA from their biological:

parents.

However, except for identical twins, individuals normally have different combinations of genetic:

variants.

DNA profiling examines regions where these differences are common.

By examining several variable regions together, scientists can produce a profile that can be highly:

discriminating.

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DNA Profiles and Identical Twins

Identical twins develop from the same fertilized:

egg.

They therefore begin development with nearly identical DNA.

Standard forensic DNA profiling generally cannot distinguish between identical twins using the usual set of genetic markers.

This is one important limitation of:

DNA profiling.

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What DNA Is Used for Profiling?

Modern forensic DNA profiling commonly examines regions called:

short tandem repeats, or STRs.

STRs contain short DNA sequences that are repeated several:

times.

For example:

GATA GATA GATA GATA

contains four repeats of the sequence:

GATA.

Different people may have different numbers of these repeats.

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Short Tandem Repeats

Consider one STR location.

Person A might have:

7 repeats.

Person B might have:

10 repeats.

Person C might have:

12 repeats.

These differences change the length of the DNA region.

Scientists can measure these differences and compare them between:

samples.

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DNA Markers

A genetic marker is an identifiable region of DNA that can be used for:

comparison.

Forensic DNA profiling examines multiple markers rather than relying on just:

one.

Why?

Because two unrelated people might happen to share the same allele at one marker.

Matching at many independently inherited markers provides much stronger:

evidence.

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Where Can DNA Samples Come From?

DNA can potentially be obtained from biological material containing cells or DNA.

Examples include:

• blood
• saliva
• semen
• hair with suitable tissue attached
• skin cells
• bone
• teeth
• other biological tissues

The quality and amount of DNA recovered can vary greatly.

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The Basic DNA Profiling Process

A simplified DNA profiling process can be represented as:

Collect sample → Extract DNA → Amplify selected regions → Separate or detect DNA fragments → Produce profile → Compare profiles → Interpret evidence

Each stage must be performed carefully to reduce errors and:

contamination.

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Step 1: Collect the Sample

The first step is to collect biological material containing:

DNA.

In forensic investigations, this might involve biological evidence collected from a:

scene.

Scientists must avoid contaminating the sample with DNA from:

• investigators
• laboratory workers
• other evidence samples
• other people at the scene

Careful collection procedures are therefore extremely important.

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Step 2: Extract the DNA

DNA must then be separated from other materials in the:

sample.

This process is called:

DNA extraction.

Cells are broken open and the DNA is isolated so that it can be:

analyzed.

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Step 3: Measure the DNA

Forensic laboratories may determine how much usable human DNA is present.

This is useful because some samples contain:

very little DNA.

Others may contain:

degraded DNA

or DNA from more than one:

person.

The condition of the sample affects how easily it can be analyzed.

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Step 4: Amplify the DNA

A forensic sample may contain only a tiny quantity of:

DNA.

Scientists can make many copies of selected DNA regions using:

PCR.

PCR stands for:

polymerase chain reaction.

PCR allows scientists to amplify specific STR regions so that they can be detected and compared.

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What Does PCR Do?

PCR repeatedly copies selected regions of:

DNA.

A simplified PCR cycle involves:

1. Denaturation – the two DNA strands separate.

2. Annealing – primers bind to target DNA sequences.

3. Extension – DNA polymerase builds new DNA strands.

Repeating these cycles produces many copies of the target:

DNA.

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Why Is PCR Important in Forensics?

Imagine that only a tiny amount of DNA is recovered.

There may not initially be enough DNA to analyze easily.

PCR can turn:

a small amount of target DNA

into:

millions of copies of selected regions.

This makes analysis possible even when the original sample is relatively small.

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Step 5: Separate and Detect DNA Fragments

Different STR alleles contain different numbers of repeats.

Therefore, the amplified DNA fragments can have different:

lengths.

Modern forensic laboratories commonly separate and detect these fragments using:

capillary electrophoresis.

DNA fragments move through a thin capillary under an electric field.

Smaller fragments generally move differently from larger:

fragments.

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Gel Electrophoresis

Simplified educational DNA profiling is often represented using:

gel electrophoresis.

DNA fragments are placed into wells in a gel.

An electric current causes DNA fragments to move through the:

gel.

Because DNA is negatively charged, it moves toward the:

positive electrode.

Smaller DNA fragments generally travel farther through the gel than larger fragments.

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Reading a Simplified DNA Gel

A simplified gel might contain several:

lanes.

Each lane represents a different sample.

For example:

Lane 1 – DNA from evidence

Lane 2 – Person A

Lane 3 – Person B

Lane 4 – Person C

Scientists compare the positions of DNA markers between:

lanes.

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What Do the Bands Represent?

In a simplified educational DNA profile, each visible band represents DNA fragments of a particular:

size.

Bands appearing at the same position indicate fragments with similar:

lengths.

If two profiles differ at multiple positions, the samples did not come from the same individual under the simple comparison model.

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Simple Profile Comparison

Imagine the evidence sample has bands at:

100, 180, 250, and 320 units.

Person A:

100, 180, 250, 320

Person B:

100, 160, 250, 300

Person C:

120, 180, 270, 320.

Person A matches the evidence profile at all four:

positions.

Persons B and C do not.

Therefore, under this simplified example:

Person B and Person C can be excluded as the source.

Person A:

cannot be excluded by this DNA comparison.

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A Match Does Not Automatically Mean "Guilty"

This is extremely important.

Suppose a person's DNA profile is consistent with DNA recovered from a:

scene.

The DNA evidence may support the conclusion that the biological material could have originated from that person.

It does not automatically explain:

• when the DNA was deposited
• why the DNA was present
• how it arrived there
• what the person was doing
• whether a crime was committed by that person

DNA evidence must therefore be interpreted together with other:

evidence.

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Exclusion vs Inclusion

DNA profiling is particularly powerful for:

excluding individuals.

If an evidence profile contains genetic markers that do not match a person's profile, that person may be excluded as the source of that sample.

If the profiles match, the interpretation is different.

The scientist asks how strongly the DNA evidence supports competing explanations about the source of the:

sample.

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Why Examine Many STR Markers?

Imagine that one STR allele is common in the:

population.

Many people may share it.

Therefore, matching at only one location provides limited identifying:

information.

But if two samples match across many highly variable markers, the combination can be much less common.

This greatly increases the power of:

DNA profiling.

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DNA Profiles Are Based on Probability

Forensic DNA profiling involves:

statistics.

Scientists may calculate how common a particular DNA profile would be within a relevant:

population.

For example, they may estimate how likely an unrelated person would be to share the observed genetic profile.

The exact calculation depends on the markers, population data, sample quality, and circumstances.

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Random Match Probability

One type of statistic used in forensic genetics is the:

random match probability.

This estimates the probability that a randomly selected unrelated person from a specified population would coincidentally have the same DNA profile.

A very small probability means the profile combination is:

rare.

However, the statistic must be explained carefully.

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A Common Statistical Mistake

Suppose the probability of a random unrelated person matching a profile is extremely:

small.

That does NOT directly mean:

"There is that same tiny probability that the suspect is innocent."

Those are different probabilities.

Forensic statistics describe the strength of the DNA evidence, not the person's overall guilt or innocence.

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DNA Profiling in Forensic Science

DNA profiling has become an important tool in:

forensic science.

It can be used to:

• compare biological evidence with known samples
• exclude individuals
• connect biological samples from different scenes
• assist in identifying unknown human remains
• re-examine evidence from older investigations

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DNA Profiling Can Exonerate People

DNA evidence is not only used to investigate possible involvement in a:

crime.

It can also demonstrate that biological evidence is inconsistent with a person's:

DNA profile.

Therefore, DNA profiling has been important in reviewing some previous convictions and excluding individuals who were incorrectly suspected or:

convicted.

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Identifying Human Remains

DNA profiling can help identify unknown human:

remains.

DNA obtained from remains can sometimes be compared with:

• personal biological samples
• relatives
• existing DNA records

This can be useful following:

• accidents
• natural disasters
• conflicts
• missing-person investigations

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Family Relationships

Because DNA is inherited, DNA profiling can also provide evidence about biological:

relationships.

A child inherits genetic material from both biological:

parents.

Therefore, genetic markers can be compared when investigating relationships such as:

parent and child.

More advanced methods can also investigate other biological relationships.

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DNA Profiles and Inheritance

At an STR location, a person normally inherits:

one allele from one biological parent

and:

one allele from the other biological parent.

Suppose a child has:

8 and 12 repeats.

If one parent has:

8 and 10,

then the child could have inherited the:

8 allele

from that parent.

The 12 allele must have come from the other biological parent under the simple model.

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Simple Parentage Example

Suppose:

Child: 10, 14

Parent 1: 10, 12

Possible Parent 2 candidates:

A: 8, 14

B: 8, 11

C: 9, 13.

The child could have inherited 10 from Parent 1.

The child therefore needs 14 from Parent 2.

Only Candidate A has:

14.

Therefore, Candidates B and C can be excluded at this marker.

However, real relationship testing examines:

many markers.

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Mixed DNA Samples

Sometimes biological evidence contains DNA from:

multiple people.

This creates a:

DNA mixture.

For example, a sample might contain DNA from two, three, or more contributors.

Mixtures are more difficult to interpret because several people's alleles can overlap.

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Example of a Mixture

Suppose one STR marker shows:

8, 10, 12, 14.

A single person normally has at most two alleles at that autosomal STR location.

Four alleles suggest that the sample may contain DNA from:

more than one person.

Scientists then need to determine which combinations of contributors could explain the evidence.

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DNA Degradation

DNA can break down over:

time.

This is called:

degradation.

Factors that can damage DNA include:

• heat
• moisture
• sunlight
• microorganisms
• environmental exposure

Highly degraded samples may produce incomplete:

profiles.

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Partial DNA Profiles

Sometimes only some genetic markers can be successfully:

analyzed.

This produces a:

partial profile.

A partial profile contains less identifying information than a complete profile.

Therefore, coincidental matches may be more likely than with a high-quality profile containing many:

markers.

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DNA Contamination

Contamination occurs when DNA from an unintended source enters a:

sample.

This might occur during:

• evidence collection
• transportation
• storage
• laboratory analysis

Even a small amount of foreign DNA can complicate interpretation.

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Preventing Contamination

Forensic investigators use procedures designed to reduce contamination.

These can include:

• gloves
• protective clothing
• sterile equipment
• separate packaging
• cleaning procedures
• negative controls
• careful laboratory workflows

Good forensic practice is essential because DNA profiling can be extremely:

sensitive.

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Secondary Transfer

DNA found on an object does not always come directly from the person who last:

used it.

DNA can sometimes be transferred indirectly.

For example:

Person A → object 1 → Person B → object 2

Small amounts of Person A's DNA might potentially reach object 2 through:

secondary transfer.

This means the presence of DNA does not always prove direct contact.

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DNA Can Persist

DNA may remain on an object after the original contact occurred.

Therefore, finding DNA does not necessarily reveal:

when it was deposited.

This is another reason DNA evidence must be interpreted within the context of the:

investigation.

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Laboratory Error

Forensic laboratories use quality-control procedures, but human and technical errors are still:

possible.

Potential problems include:

• sample mislabeling
• contamination
• incorrect interpretation
• equipment problems
• data-handling errors

Quality assurance and independent review help reduce these:

risks.

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DNA Databases

Some jurisdictions maintain databases containing DNA profiles from certain:

individuals or samples.

An evidence profile may be compared against profiles stored in a database.

A possible match can provide an investigative:

lead.

However, database searches also raise questions about:

privacy and data use.

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Privacy Concerns

DNA is biological information about an:

individual.

DNA data can also reveal information connected to biological:

relatives.

Important questions include:

• Who should have DNA profiles stored?
• How long should profiles be retained?
• Who should be allowed to access them?
• What purposes should DNA databases be used for?
• How should genetic privacy be protected?

These are scientific, legal, and ethical questions.

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DNA Profile vs Whole Genome

A forensic DNA profile does not normally contain a person's entire:

genome sequence.

Traditional forensic profiles examine a selected set of genetic:

markers.

Their primary purpose is:

identification and comparison.

This distinction is important when discussing the information contained in forensic DNA databases.

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DNA Profiling vs DNA Sequencing

These terms are related but not identical.

DNA profiling

examines selected variable genetic markers to compare individuals.

DNA sequencing

determines the order of nucleotide bases within DNA.

A profile asks:

"How do selected genetic markers compare?"

Sequencing asks:

"What is the DNA sequence?"

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DNA Profiling vs Fingerprints

Traditional fingerprints examine ridge patterns on:

fingers.

DNA profiling examines genetic markers in:

DNA.

Both can be used for identification, but they are completely different types of biological:

evidence.

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Analyzing a Simple DNA Profile

Suppose you are given this simplified profile:

Evidence:

A – C – F – H – K

Person 1:

A – C – F – H – K

Person 2:

A – D – F – H – K

Person 3:

A – C – E – H – J

Person 1 matches at all tested:

markers.

Persons 2 and 3 contain differences.

Therefore:

Persons 2 and 3 can be excluded.

Person 1:

cannot be excluded based on these markers.

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Why Say "Cannot Be Excluded"?

Scientific language matters.

Saying:

"This person definitely left the DNA"

may go beyond what the evidence alone establishes.

A more careful statement is:

"The person's DNA profile is consistent with the evidence profile."

or:

"The person cannot be excluded as a possible source."

Statistical analysis can then describe how strongly the profile supports that conclusion.

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Worked Example 1

Evidence profile:

6, 9, 12, 15

Person A:

6, 9, 12, 15

Person B:

6, 10, 12, 15

Who can be excluded?

Person B.

Person A matches all markers shown and therefore:

cannot be excluded.

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Worked Example 2

A DNA sample contains alleles from more than one person.

What type of sample is this?

A:

DNA mixture.

These samples are generally more difficult to interpret than single-source samples.

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Worked Example 3

A DNA sample has been exposed to heat, moisture, and microorganisms for several years.

What problem might occur?

The DNA may become:

degraded.

This may result in a:

partial or poor-quality profile.

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Worked Example 4

An evidence profile does not match a suspect at several reliable genetic markers.

What can scientists conclude?

The suspect can generally be:

excluded as the source of that DNA sample,

assuming the samples and analysis are valid.

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Worked Example 5

A person's DNA is found on a door handle.

Does this prove the person committed a crime involving that door?

No.

The DNA evidence may indicate that their biological material is present.

It does not necessarily establish:

when, why, or how it was deposited.

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Strengths of DNA Profiling

DNA profiling has several important strengths:

• highly discriminating when many markers are available
• can work with relatively small biological samples
• can exclude innocent individuals
• can connect related biological samples
• can assist with identification
• can provide quantitative statistical evidence

These features make DNA profiling a powerful forensic:

tool.

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Limitations of DNA Profiling

DNA profiling also has important limitations.

These include:

• contamination
• degradation
• partial profiles
• mixed samples
• identical twins
• interpretation errors
• secondary transfer
• statistical misunderstanding
• privacy concerns
• DNA evidence not showing when or why DNA was deposited

DNA evidence is powerful, but it is not:

infallible.

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Evidence Must Be Considered Together

Imagine DNA from a person is found at a location.

Investigators should also consider:

• witness evidence
• video evidence
• timelines
• physical evidence
• explanations for legitimate contact
• location of the DNA
• type and quality of the sample

Scientific evidence is strongest when interpreted within its proper:

context.

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Common Mistake: A DNA Match Proves Guilt

A DNA profile can provide evidence about the possible source of biological:

material.

It does not independently prove that a person committed a:

crime.

Source identification and activity are different questions.

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Common Mistake: Everyone Has Completely Different DNA

Humans share most of their DNA.

DNA profiling works because certain selected regions show enough variation to distinguish most:

individuals.

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Common Mistake: DNA Profiling Reads Every Gene

Standard forensic DNA profiling does not normally sequence every gene.

It examines selected genetic:

markers.

Therefore, a DNA profile is not the same thing as a complete genome sequence.

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Common Mistake: Matching One Marker Is Enough

Many people may share an allele at one:

marker.

Forensic profiling therefore compares:

multiple markers.

The combination provides much stronger identifying evidence.

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Common Mistake: DNA Cannot Be Transferred Indirectly

DNA can sometimes move through:

secondary transfer.

Therefore, DNA found on an object does not necessarily prove direct contact with that object.

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Common Mistake: DNA Evidence Is Always Perfect

Real samples may be:

• contaminated
• degraded
• mixed
• incomplete

Laboratory and interpretation errors are also possible.

Good forensic science recognizes and evaluates these limitations.

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Check Your Understanding

1. Define DNA profiling.

2. Why can DNA be used to distinguish most individuals?

3. Why are identical twins a challenge for standard DNA profiling?

4. What is an STR?

5. Why do STR regions vary between people?

6. What is a genetic marker?

7. Why are multiple markers examined?

8. Name four biological materials that may contain DNA.

9. Put these stages in the correct order: comparison, DNA extraction, sample collection, PCR, profile generation.

10. What happens during DNA extraction?

11. What does PCR stand for?

12. Why is PCR useful in forensic science?

13. Name the three basic stages of a PCR cycle.

14. What is electrophoresis used for?

15. Why do different STR alleles produce DNA fragments of different lengths?

16. In a simplified gel, what do matching band positions suggest?

17. Why can a person be excluded if reliable markers do not match?

18. Why does a matching DNA profile not automatically prove guilt?

19. What does "cannot be excluded" mean?

20. Why are statistics important when interpreting a DNA match?

21. What is random match probability?

22. Why is random match probability not the same as the probability that someone is innocent?

23. Explain one use of DNA profiling in criminal investigations.

24. Explain how DNA profiling can help exonerate someone.

25. How can DNA profiling help identify human remains?

26. How can DNA markers be used to investigate biological relationships?

27. What is a DNA mixture?

28. Why are mixed DNA samples difficult to interpret?

29. What is DNA degradation?

30. Name three environmental factors that can damage DNA.

31. What is a partial DNA profile?

32. Why is a partial profile less informative than a complete profile?

33. What is contamination?

34. Describe two ways investigators can reduce contamination.

35. What is secondary transfer?

36. Why can't DNA usually tell investigators exactly when it was deposited?

37. Explain the difference between DNA profiling and DNA sequencing.

38. Give two privacy concerns associated with DNA databases.

39. Give three limitations of DNA profiling.

40. Explain why DNA evidence should be considered alongside other evidence.

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Key Terms

• DNA profiling: Analysis of selected variable DNA markers to compare biological samples.
• DNA fingerprinting: Another term commonly used for DNA profiling.
• STR: Short tandem repeat; a short DNA sequence repeated a variable number of times.
• Genetic marker: Identifiable DNA region used for genetic comparison.
• DNA extraction: Isolation of DNA from biological material.
• PCR: Polymerase chain reaction; technique used to amplify selected DNA regions.
• Primer: Short DNA sequence used to identify the region to be copied during PCR.
• Electrophoresis: Technique used to separate DNA fragments.
• DNA profile: Pattern of genetic markers obtained from a DNA sample.
• Random match probability: Estimate of how likely an unrelated person from a specified population would be to share the observed profile.
• DNA mixture: Sample containing DNA from more than one individual.
• Degradation: Breakdown of DNA.
• Partial profile: DNA profile containing information from only some of the intended markers.
• Contamination: Introduction of DNA from an unintended source.
• Secondary transfer: Indirect movement of DNA between people or objects.
• Forensic science: Application of scientific methods to legal investigations.
• DNA database: Collection of stored DNA profiles used for comparison.
• Exclusion: Determination that a DNA profile is inconsistent with a particular sample.
• Capillary electrophoresis: Modern method commonly used to separate and detect DNA fragments during STR profiling.

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Key Takeaways

• DNA profiling compares variable regions of DNA.
• Modern forensic DNA profiling commonly examines short tandem repeats (STRs).
• STRs vary in repeat number between individuals.
• Except for identical twins, people normally have different combinations of DNA markers.
• A DNA profile is not a complete genome sequence.
• DNA can potentially be recovered from blood, saliva, semen, tissue, bone, teeth, and other biological material.
• The basic process involves collection, extraction, amplification, detection, comparison, and interpretation.
• PCR is used to make many copies of selected DNA regions.
• Electrophoresis can separate DNA fragments according to their properties, including size.
• Educational DNA profiles are often represented as patterns of bands.
• Modern forensic STR profiles are commonly analyzed using capillary electrophoresis.
• Matching at one genetic marker provides limited evidence.
• Comparing many markers greatly increases the discriminatory power of DNA profiling.
• A mismatch at reliable markers can exclude a person as the source of a DNA sample.
• A matching profile means a person may not be excluded as a possible source.
• A DNA match does not by itself prove guilt.
• DNA evidence does not necessarily reveal when, why, or how biological material was deposited.
• Statistical analysis is essential for interpreting the strength of DNA evidence.
• Random match probability is not the same as the probability that a suspect is guilty or innocent.
• DNA profiling can be used in criminal investigations.
• It can also help exclude wrongly suspected individuals.
• DNA profiling can assist in identifying human remains.
• Inherited DNA markers can help investigate biological relationships.
• Mixed samples contain DNA from multiple contributors and can be difficult to interpret.
• DNA can degrade because of environmental exposure.
• Poor-quality samples may produce partial profiles.
• Contamination can introduce unrelated DNA into evidence.
• Secondary transfer can move DNA indirectly between people and objects.
• Standard DNA profiling may not distinguish identical twins.
• DNA databases can aid investigations but also raise privacy and ethical questions.
• DNA evidence should always be interpreted together with the sample quality, circumstances, statistics, and other available evidence.