Frequently Asked Questions

DNA database FAQ’s


What is DNA?

DNA is a chemical that occurs inside every cell of a person’s body. The DNA is contained in 22 pairs of structures known as chromosomes, shaped like an X, plus an extra pair – the sex chromosomes – which determine whether someone is male or female. In this final pair, women have two X chromosomes, but men have one X and one Y chromosome. Each chromosome consists of two long strings of chemical letters, twisted together in the famous shape of the double-helix. The chemical letters occur in pairs as rungs on this twisted chemical ladder. The four chemical letters of the genetic code spell out instructions to the cell about how to make the proteins that allow the human body to grow and function normally. The parts of the DNA sequence that contain the instructions for making proteins are known as genes. The whole sequence is known as the genome.


How can DNA be used to identify a person?

DNA is useful to identify an individual because everyone’s genetic code (their genome) is thought to be unique, unless they have an identical twin. The string of chemical letters in a person’s DNA can therefore act like a unique bar code to identify them. DNA databases contain forensic DNA profiles which are not unique, but contain enough information about a person’s DNA sequence to be able to identify them.


Why is DNA different from a fingerprint?

DNA and fingerprints can be left wherever a person goes, so both can be used to track and identify individuals. But DNA can also identify relatives, including non-paternity, and DNA contains genetic information about a person’s health and other characteristics and the risks of genetic disorders in their children.


What is a DNA sample?

DNA can be obtained from any cell inside a person’s body. Often, blood or saliva is used. DNA can also be obtained from the roots of a person’s hair, although not from cut hair. The DNA inside the cells can be extracted from this tissue in a laboratory before it is analysed to obtain a DNA profile.


What is a forensic DNA profile?

A DNA profile is a string of numbers based on parts of a person’s DNA. Short strings of the chemical letters that make up a person’s DNA occur repeatedly in parts of the genome. These are known as short tandem repeats (STRs). The number of repeats differs between individuals and can be counted. The numbers of STRs that are used in a forensic DNA profile are different in different countries. For example, the US system (called CODIS) uses 13 STRs and the UK system (called SGM+) uses ten. Each STR is made up of two strands, one inherited from the mother and one from the father, so in the US 26 numbers are recorded and in the UK 20 numbers are recorded from the number of repeats at each STR. The DNA profile also includes a test of a gene called amelogenin, which can be used to test whether the DNA sample comes from a male (with an X and Y chromosome) or a female (with two X chromosomes). A DNA profile is not unique, but the probability of two full forensic DNA profiles matching wrongly by chance is thought to be very low, less than one in a billion. The probability of a false match depends on the profiling system used.


Can my DNA identify my relatives?

Because a person inherits half their DNA from their mother and half from their father, a person’s DNA can be used to identify their relatives. Close relatives have a DNA sequence that is more alike than distant relatives or than someone who is unrelated. Comparing two people’s DNA profiles can identify close genetic relatives such as parents, brothers, sisters and children. In some cases non-paternity can be identified if a child’s DNA profile contains chemical letters that are not in its mother’s or father’s DNA profile.


What can my DNA say about me?

Forensic DNA profiles contain very little information about a person’s characteristics, although they can be used to identify individuals and their relatives. However, DNA samples contain a person’s whole genome and can be analysed further to obtain other genetic information. For example, DNA can be analysed to find out whether a person has a genetic disorder, or is at risk of developing a genetic disorder in the future, or passing a genetic condition to their children. Mutations in a gene can also sometimes mean that a person is at high risk of developing a disease in the future, even though they don’t yet have symptoms. Information from hundreds of different genes can also be combined to try to work out a person’s risk of common diseases, although these predictions are much less certain and often unreliable.


When can the police take my DNA?

The rules about when the police can take your DNA are very different in different countries. In some countries you need to be suspected of committing a serious crime and a court order is needed. In other countries, DNA can be collected on arrest even for very minor crimes.


What happens when the police take a person’s DNA?

Depending on the country and the circumstances, a medical professional or a police officer might take a person’s DNA. Medical professionals can take blood samples and ‘intimate samples’ from private parts of a person’s body. The police usually take samples of saliva using a mouth swab. In some countries, such as the UK, the police are allowed to use “reasonable force” to take DNA from a suspect who has refused to give a sample. This usually means pulling out a few strands of their hair. Details such as a person’s name, suspected crime, date of birth, appearance and address, or ID number, will also be recorded, so the police have a record of whose DNA sample they have taken. A barcode is normally used to link the personal record to the sample.


What happens in a forensic laboratory?

Samples of blood, saliva or other tissue from crime scenes and individuals are sent to forensic laboratories for analysis. All crime scene samples and mouth swabs from individuals should be stored in tamper-proof bags and properly labelled because it is critical that the laboratory can trace where the evidence or individual sample came from. To protect privacy, individual samples can be barcoded to prevent laboratory staff knowing whose they are, although not all countries do this and often the personal details collected by the police will be sent to the laboratory. Usually two samples are sent from a given individual, so that one can be used for quality assurance. The DNA is extracted from the tissue sample being analysed and is amplified using a chemical process known as PCR, which creates multiple copies of the DNA. The amplified DNA is separated out into the parts that are being tested (the STRs being used to create the DNA profile, plus the sex marker). A laser is used to detect the presence of different DNA fragments distinguished by their size. The data is analysed by a computer which produces the string of numbers which makes up the DNA profile. DNA profiling machines are normally used which automate this process.


How long can the police keep DNA?

Different countries have different rules about how long DNA profiles and samples can be kept. In some cases, DNA profiles and samples have been kept indefinitely, even from innocent people. In other cases, there are strict rules about what information can be stored. For example, in the UK and Germany, the spare DNA samples collected for quality assurance purposes must be destroyed once the DNA profile has been loaded onto the database, which prevents them being re-analysed to reveal private information and saves the costs of storing them. But in the US, DNA samples are stored indefinitely. In the UK, DNA profiles from innocent people were being stored indefinitely until the European Court of Human Rights ruled this was a breach of human rights. Now, more than a million innocent people’s DNA profiles have been removed from the UK DNA database.


How can DNA help solve crimes?

People can leave traces of their DNA at a crime scene because it is inside every cell of their body. DNA can be extracted from blood, semen, saliva or hair roots left at a crime scene to obtain a forensic DNA profile. This can be compared with the DNA profiles of known suspects for the crime, or with the DNA profiles from other individuals stored on a DNA database. A match between the crime scene DNA profile and the individual’s DNA profile indicates that they were at the crime scene. The value of this evidence in solving the crime will vary: DNA on a cigarette butt could have been dropped earlier in the day or have been planted by someone who wanted to implicate an innocent person in the crime; in contrast, DNA in semen from a woman who has been raped can show that a particular man was or was not likely to have been involved. However, even a rape case may not be straightforward: for example, if the man argues that the woman agreed to have sex.


How is DNA evidence used in court?

An expert is usually required to explain the importance of the DNA evidence in court, including the statistics. This is more important if the DNA profile is not complete or comes from a very small or mixed sample. Often, there might be an innocent explanation of why the DNA was at the crime scene that has to be ruled out. In some countries, such as the UK, guidelines for prosecutors say that they should not rely on a DNA match alone, but in other countries, such as the USA, DNA matches are more often used without any corroborating evidence. In the UK, the defendant has a right to have a second sample tested to make sure there were no errors in their profile. However, crime scene evidence often cannot be retested and it is not always easy to check for possible mix-ups or contamination.


What is a DNA database?

A DNA database is a computer database containing records of DNA profiles. Usually there are two different sources of these DNA profiles: crime scene DNA samples and individuals’ DNA samples. DNA profiles from individuals must be stored with their names and sufficient other information to track them down if their DNA profile matches one from a crime scene. When a new crime scene DNA profile is added to the database it is searched against all the other DNA profiles stored on the database. The crime scene profile might match with stored DNA profiles from other crime scenes, indicating a link between these crimes. Or it might match with an individual’s DNA profile, suggesting that they could be a suspect for the crime. When a new DNA profile from an individual is added to the database it is searched against all the stored crime scene DNA profiles on the database. Again, a match may indicate that the individual may be a suspect for the crime. This process is known as ‘speculative searching’ and it results in reports of matches that can be sent back to the police for further investigation. The police will retrieve the name and other identifying information of the individual from the DNA database so they can interview, arrest and perhaps prosecute them for the crime. A DNA database of individuals’ DNA profiles is not needed in order to compare the DNA profiles of known suspects with DNA left at a crime scene. Its only purpose is to introduce new suspects into the investigation via unexpected matches known as ‘cold hits’. A match does not necessarily prove the individual was the perpetrator of the crime as there may be an innocent explanation for their DNA being at the scene, or the match could have occurred in error.


How much does a DNA database cost?

Costs will vary in different countries and depend on policies about how many DNA profiles are analysed a year and other factors. In the UK, the cost of analysing a single DNA sample has been estimated at 40 British pounds (USD 60) and the cost of storing a DNA sample at 1 British pound a year (USD 1.5). This does not include police costs (including training) or the cost of analysing crime scenes. Costs for operating and hosting the database total more than 2 million British pounds (USD 3 million) a year, but can be much more when major upgrades are needed. If a new database is set up, or one is significantly expanded, there will also be the costs of setting up new laboratories and training police, crime scene examiners and the legal profession in how to collect, analyse and use DNA evidence.


How can DNA exonerate an innocent person?

Sometimes a crime scene DNA profile has been obtained which is highly likely to be from the perpetrator of the crime. This might be DNA in semen from a rape or DNA on the handle of a knife used in a stabbing, for example. If an innocent person has been accused or convicted of a crime without checking their DNA profile against the crime scene DNA profile, showing that their DNA profile does not match the crime scene DNA profile can be sufficient to exonerate them. DNA databases of crime scene DNA profiles can help exonerations, but DNA databases of individuals’ DNA profiles are only used to implicate individuals as suspects for a crime. This is because an innocent individual who is a suspect can always have their DNA tested without being on a database.


What kind of errors can occur with the use of DNA?

DNA evidence is not foolproof. The biggest sources of error are mistakes in the laboratory or when evidence is collected from the crime scene. DNA on evidence such as clothing or weapons can get accidentally transferred from one item to another and DNA samples can get mixed up or contaminated with DNA from another person. If crime scene DNA becomes contaminated with an innocent person’s DNA that person can be wrongly convicted of the crime. False matches between DNA profiles can also occur by chance and this becomes more likely the larger a DNA database is and the more searches that are done. Often, DNA profiles from crime scenes are not complete and this increases the chances of a false match with the wrong person’s DNA. False matches are also more likely to occur with close relatives (for example, the brother of the person who committed the crime), or if the DNA profile of more than one person is mixed together (this is often the case with DNA from rapes) or found only in tiny samples (which are more difficult to analyse). The presence of DNA at a crime scene can also be misinterpreted, for example DNA on a cigarette butt could have dropped before the crime took place, or planted there afterwards.


What are the human rights concerns about DNA databases?

Because DNA is left wherever a person goes, DNA databases can be used to track individuals and their relatives, even if they haven’t committed any crime. For example, DNA collected from coffee cups at a political meeting can be used to track the people who were there and identify their relatives, if their DNA profiles are stored on a computer database.  A DNA database can also be vulnerable to abuse by anyone who infiltrates the system, and can even be used to track down victims, including children. DNA evidence is not foolproof and DNA evidence can be planted to try to implicate a person in a crime. Errors and mistakes can also occur, including mix-ups in laboratories. Often, DNA databases can be discriminatory by keeping profiles mainly from people from ethnic minority groups. If records of arrests are kept, linked to a person’s DNA, these can be used to refuse people jobs or visas, even if they were never convicted of a crime. Safeguards are therefore needed to protect human rights, whilst allowing the use of DNA in criminal investigations.

© 2014 Forensic Genetics Policy Initiative