STR Analysis: Short Tandem Repeat

STR analysis stands for short tandem repeat analysis. It is one of the most widely used methods of DNA testing, and it works by looking at specific small sections of a person's DNA where short sequences repeat themselves over and over. The number of repeats at each of these locations differs from person to person, and that variation is what makes the technique useful.

Although any two people share almost all of their DNA in common, the tiny fraction that varies is enough to tell us apart. STR analysis takes advantage of this. By measuring the repeat counts at several specific points in the genome, a scientist can build up a profile that is, for practical purposes, unique to one individual.

Why several regions are tested

In criminal investigations, the standard approach historically involved looking at thirteen specific regions of DNA, each known to vary widely between people. The UK National DNA Database now uses 17 loci, which improves accuracy and helps with international comparisons. The combined pattern across all the tested regions is what gives the method its power. The chance of two unrelated individuals matching across every one of those regions is very small, often quoted as being in the order of one in a billion.

National DNA databases are built around these short tandem repeat profiles. When a sample is taken from a crime scene or from a person in custody, the resulting profile can be compared against records already held.

How the analysis is carried out

The process begins with a sample. This might be blood, saliva, hair with the root attached, skin cells, or another tissue containing nuclear DNA. The DNA is extracted from the cells and separated from the proteins and other material around it.

Once the DNA has been isolated, the regions of interest are copied many times over using the polymerase chain reaction, usually shortened to PCR. This step is needed because the amount of DNA recovered from a real-world sample is often very small. PCR amplifies the chosen sections so there is enough material to work with.

After amplification, the fragments are separated by size. Capillary electrophoresis is the standard method today, though gel electrophoresis was used in the past. The number of repeats at each location determines the length of the fragment, and the length is what the scientist measures. Fluorescent dyes attached to the PCR primers make the fragments visible. The result is a readout showing the repeat count at each region tested, and that set of numbers is the person's STR profile.

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Where it is used

Forensic science is the best-known application. STR profiling is used to link suspects to crime scenes, to identify victims, and to rule people out of investigations. It is also used to identify human remains where other forms of identification are not possible, including in mass disasters and in historical cases.

The same techniques are used in paternity and other family relationship testing. Because children inherit half of their STR markers from each parent, comparing profiles can confirm or rule out a biological relationship with a high degree of confidence. Sibling tests, grandparent tests and similar checks all rely on the same underlying analysis, although the certainty of the result depends on which relatives are available for comparison.

STR analysis also has a role in immigration cases, where DNA evidence may be used to support a claimed family relationship. In the UK, this kind of testing is voluntary, and anyone giving a sample should be clear about what they are consenting to and how their genetic data will be stored or used. The Home Office has its own guidance on what it will accept, and tests are usually arranged through accredited laboratories.

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How reliable is it?

As a laboratory technique, STR analysis is well established and the underlying science is not seriously disputed. The figures quoted for the chance of a random match are based on population genetics studies and are generally accepted in court, though the interpretation in any given case can depend on the quality of the sample and the questions being asked.

The result of any DNA test is only as good as the sample and the handling. Contamination, mix-ups, degraded samples and partial profiles can all complicate matters. A partial profile may still be useful but will not carry the same statistical weight as a full one. Mixed samples, where DNA from more than one person is present, are harder to interpret and have been the subject of debate in some criminal cases.

For close relatives, particularly identical twins, standard STR analysis cannot tell people apart, because their repeat patterns are the same. More specialised methods are needed in those situations, and they are not part of routine testing.

Privacy, the database and private testing

The UK National DNA Database is run by the Home Office and is one of the largest of its kind. Rules on whose profile can be retained, and for how long, have changed over the years following legal challenges. The European Court of Human Rights ruling in S and Marper v United Kingdom found that the indefinite retention of profiles from people who had not been convicted was a breach of the right to private life. The Protection of Freedoms Act 2012 then reshaped the rules in England and Wales. Profiles from people who are not convicted are generally deleted or held for limited periods, often three to five years depending on the offence and the circumstances. Anyone wanting current information on retention rules should check GOV.UK or seek legal advice.

For people considering a private DNA test, whether for paternity, ancestry or another reason, it is worth checking that the laboratory is accredited, ideally by UKAS, and clear about what it does with samples and results. Tests bought online vary in quality, and the legal weight of the result will depend on how the sample was collected and handled. If a test is intended for use in court or in an immigration matter, the chain of custody matters as much as the science. Where a result is disputed, a solicitor or Citizens Advice can help work out the next steps.

STR analysis is, in the end, a fairly straightforward idea built on solid science. Count the repeats at a handful of carefully chosen spots in the genome, and you have a set of numbers that means very little on its own but, taken together, points to one person rather than anyone else.

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