How forensic DNA is collected, extracted, and profiled

DNA forensic analysis has come a long way since it was first developed in the early 1980s and commercially available in the late 1980s.  Improvements in collection, quantification, and amplification techniques have meant that forensic technicians – the men and women employed by places like the State Bureau of Investigation – are now able to develop DNA profiles from relatively small amounts of biological evidence collected from a crime scene.

While in the past, a forensic technician might need a quarter-sized droplet of blood from a crime scene in order to produce a DNA profile, these days, forensic technicians can develop DNA profiles from blood left at a crime scene that may be as small as the head of a needle.  In addition, modern forensic DNA techniques allow forensic technicians to develop DNA profiles from touch DNA – biological residue from a person touching or handling something left behind at a crime scene.

What is DNA? DNA is the genetic code that makes us biologically unique.  And none of us shares a DNA profile with anyone else, unless we are identical twins.  (Identical twins have the same DNA.)  Because developing a DNA profile of each person would take years and cost incredible amounts of money, forensic techniques take a very small portion of our DNA – far less than 1 percent – and then compare those DNA markers against the DNA markers found in DNA left behind at a crime scene.

How is DNA from a crime scene collected?  DNA from a crime scene is typically collected as part of a very straightforward process.  A police officer, following a crime, might put an item thought to have been touched by a suspect into a plastic bag to be sent to the crime lab.  Or a forensic technician from the crime lab might come to a crime scene to “swab” (collect DNA) from the walls, furniture, floor, and so forth.  Those swabs are wet with purified water, rubbed over the area of the crime scene, dried, and then placed into bags and labled with information about who collected the DNA or item, and where it was collected.  DNA may be collected from a Defendant by swabbing the inside of the defendant’s mouth or cheek with a sterilized q-tip.

Once the swabs or items are received at the lab, the swabs are wet with purified water (or the items are swabbed at the lab) and then placed into small test tubes.

The first technique used in the forensic analysis is called “quantification.”  The idea behind quantification is to find out whether enough DNA has been collected so that it can be tested.  If not enough DNA has been collected, then no further tests are typically run, and no results are reported.  “Quantification” is used in order to cheaply and efficiently find out whether enough DNA was collected before going forward with the more expensive and time-consuming parts of the forensic analysis.

If enough DNA has been collected, then the sample is “amplified” through a process called polymerase chain reaction (PCR).  PCR uses a primer which essentially causes the rapid and dramatic reproduction of specific parts of the DNA collected.  Think of this process as the equivalent of taking one copy and xeroxing it 100,000 or 1 million times.

Amplification is required because the DNA originally collected is so tiny that analyzing it through scientific instruments is impossible.  Amplification reproduces specific segments called loci (plural of “locus”) of the DNA so that it can be further analyzed to determine its profile.

The next step in the process is called “capillary gel electrophresis”.  The purpose of this process is to make the amplified parts of the DNA (the “loci”) visible to human beings through computerized techniques. The various molecules are stained in the amplified DNA so that each locus has a different color that can be distinguished from other loci.

A forensic analyst uses a syringe (needle) to insert some of the amplified DNA from the test tubes into a gel.  An electric current is run through the gel, which causes various molecules from different loci to move at different speeds.  These molecules move at different speeds because some are larger, and others are smaller.  The smaller molecules will move more quickly.

At the same time, a computer is used to identify the Short Tandem Repeats (STR) at each locus. Most forensic laboratories test 15 separate loci.  For each individual, the result is a STR for each of two alleles at each of the 15 loci.

At any given locus of the 15 tested, you and I might share the same STR for both of our alleles.  Maybe we even share the same STR for our alleles at two loci.  However, as we look at 3, 4, 5, 6 and up to 15 loci, differences will appear so that my DNA profile will be different from yours.

In a common criminal situation, evidence left behind at a crime scene may produce a partial profile.  Perhaps forensic analysts aren’t able to develop a full 15-loci profile because not enough DNA is left behind.  That may reduce the scientific certainty of the conclusions the forensic analyst can give.

However, forensic analysts can frequently give a very conclusive result with as little as 6 loci reporting from a DNA profile. If a defendant’s profile matches 7 loci of a partial DNA profile produced from a piece of evidence at a crime scene, a forensic analyst may be able to say there is 1 in 10,000,000,000 (or greater) chance that the person is not the defendant.

How to explain away forensic DNA at a crime scene

Let’s assume that a lab has come back with a report that a person’s DNA was found at a crime scene, and let’s assume that the technician at the lab hasn’t committed some kind of fraud.  For instance, let’s assume that the technician hasn’t falsified the results.  What could explain a person’s DNA at a crime scene if that person was really and truly not the person who committed the crime.

Contamination.  If police suspect that a person is involved in a crime and want to test that person’s DNA against DNA found at the crime scene, contamination may occur which could seem to show the person was at the crime scene who he or she was no where near the crime.  Police will typically collect “knowns” from suspects either through that person’s consent or by getting a search warrant. They will use a buccal swab of the person’s inner cheek to collect DNA, which they will try to match against evidence collected from the crime scene.  If, however, in the process of doing this match, the lab technician inadvertently or mistakenly mixes the “buccal swab” with the unknown DNA on the evidence, the person’s DNA could end up on the evidence, having been mistakenly been put there by the lab technician. Then further testing would show that the person’s DNA was present on the evidence which would obviously be a result produced by contamination, and not accurate testing of the evidence.

Contact DNA.  DNA can be transferred from person to person.  Let’s assume that the real criminal knows the innocent person and that the two of them, for instance, shook hands.  Or maybe they’re close friends or family and share clothes.  It would be possible for DNA to be transferred from the innocent person, to the criminal, and then later left on evidence at the scene of the crime.  Or the innocent person could know one of the victims, and have gotten DNA, through a hug, or handshake, to the victim who was later victimized in the crime.

Residual DNA.  Let’s assume the person visited the home, business, car, or other place where the crime occurred.  Perhaps this visit happened months ago.  Maybe the person visited several times, or only once.  Whatever the case, it’s possible that the person left behind DNA from a purely innocent visit.  Later when the crime occurred and police conducted evidence collection, they collected items that have the person’s DNA on it even though the person was not involved in the crime at all.

Fake DNA. DNA could purposefully be planted.  For instance, a criminal, hoping to blame someone else for the crime, might intentionally collect DNA from someone who is entirely innocent of the crime.  The real criminal could commit the crime, and then leave DNA behind to try to get the innocent person accused and arrested of the crime.  This is obviously rare.

Mixed DNA.  Many DNA samples collected from a crime scene contain multiple DNA profiles from several people.  Some of those people may be victims, perpetrators, or even investigators or police officers.  It’s possible to distinguish a mixed DNA sample so that two partial profiles may be produced.  This can be done if one of the contributors to the DNA is identified.  For instance, if police obtain a swab from one of the victims and match it against DNA obtained from evidence, they will be able to get a partial profile for the other person (assuming only two DNA profiles are mixed on the evidence).  Mixed DNA samples make the mathematical computation of the final statistics somewhat less certain.  But even mixed DNA samples can yield results that would appear to be very conclusive to a potential jury.

These are some of the possible explanations for why an innocent person’s DNA might end up at a crime scene.

Basic overview of DNA and its use in Forensic Analysis

Deoxyribonucleic acid or DNA is a substance that contains the genetic instructions in all human beings (and nearly all life, with the exception of certain viruses).  Think of DNA has a ladder that spirals.  The two long spokes of the ladder are called nucleotides.  The rungs of the ladder contain the molecules, called bases, that form the actual genetic code.

There are four types of bases – Guanine, Cytosine, Adenine, and Thymine – which form pairs in the form of A-T and C-G.  These pairs form strings along the run of the ladder.  But because the bases appear in different orders – TTAAGGG or TAGAAG – and so forth, it’s the patterns that carry the instructions that enable the formation of cells that in turn form life, including human life.

DNA is contained as Chromosomes in human cells.  Humans typically have 46 chromosones (unless there’s a genetic abnormality).  Two of those chromosones indicate the sex of the human being – X-Y for male, and X-X for female.

There are roughly 3 billion base pairs – within those 46 chromosones.  More than 99 percent of the DNA in human beings is identical to other human beings.  What makes us different is determined by approximately 3 million base pairs, which indicate the different genetic characteristics – from hair color, to height, to other traits – that creates the differences between individual human beings.

No two human beings – with the exception of identical twins – share the same DNA.  We each get half of our DNA from our father, and half of our DNA from our mother.  But siblings – who are not identical twins – get different halves, and so brothers and sisters, while the share more DNA than strangers, do not have identical DNA patterns.

DNA was first identified in the 1950s.  But it wasn’t until the 1980s that scientists identified ways of easily and cheaply distinguishing between different DNA patterns.

The reason this is important is that people leave little bits of DNA behind whereever they go.  This is called “sloughing”.  Some people slough more than others, thereby leaving more DNA behind them. DNA might be carried in sweat, hair particles, skin particles, or saliva.

Unlike fingerprint analysis – which is always open to interpretation because it is relatively imprecise – DNA analysis is incredibly precise.  Assuming that the forensic analyst or technician has done his job properly, and assuming there has been no cross-contamination, the DNA profile generated by forensic analysts will reflect the DNA profile of people who left their DNA behind at a particular place.

And while there are more than 3 million differences between in the DNA between various human beings, by using statistical analysis, scientists can match profiles with very high degrees of precision based on just 15 commonly used loci (or spots) on a DNA strand.

Today the whole process is so automated and standardized, that forensic analysis of DNA can be accomplished relatively cheaply by easily-trained forensic analysts.  These analysts are not scientists.  Rather they are people who know how to use several machines and computers that help extract, quantify, amplify, and then analyze DNA strands.  They then use this information to “exclude” possible contributors to the DNA.  If, however, they cannot exclude a contributor, then they will sometimes conclude that the there is a 1 in 1,000,000 or 10,000,000 or even higher chance that the person’s DNA is not present.  The higher the number, the more likely it is that the person was a contributor.

DNA Database Expanded by the North Carolina General Assembly

The North Carolina General Assembly passed a new law this week that would expand the state’s DNA database by allowing police agencies to collect DNA samples from suspects when they are charged with certain crimes.

The bill authorizes law enforcement to hold people who refuse to give a sample. If charges are dropped or the suspect is acquitted, the individual’s DNA sample must be removed from the database.

“It’s a win for North Carolina citizens,” Cooper told WRAL News.

However, some lawmakers say requiring the sample upon arrest amounts to unreasonable search and seizure.

Rep. Rosa Gill, D-Wake, voted against the bill. She agrees that DNA is a useful tool in helping law enforcement solve crimes, but argues that obtaining samples from suspects who have not been convicted goes too far.

“I feel like it’s unfair. I think it may be a type of targeting that we don’t intend for it to be,” Gill said.

The problem with the law, as Rep. Gill argues, is that probable cause is a very low standard, and that many people are arrested who are, in fact, innocent of the crimes they have been alleged to have committed. By collecting DNA samples, the state engages in an invasive practice of creating a police state that contains detailed and personal information about people.



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