Deoxyribonucleic acid, generally abbreviated as DNA, is a complex molecule found in virtually every cell of the human body and in all living organisms.

DNA carries the genetic instructions in the form of a code, used for the development and function of both cells and the organism as a whole and is the mechanism whereby this genetic information is passed from one generation to the next.

The vast majority of human DNA is exactly the same between individuals but small variations in the code are responsible for different physical characteristics such as height, eye colour, skin tone and hair colour etc. Half our DNA is inherited from our mother and half from our father.

Non-identical siblings will inherit different combinations of DNA from the same parents and are therefore similar but different. Except for identical siblings, each person's DNA is unique.

Most cells in your body contain DNA. This is critical for the forensic uses of DNA.

If saliva, blood, semen, skin or anything else containing cells is left behind by a person at a crime scene, then there is a good chance that this material may be recovered and a DNA profile obtained. The DNA profile can then be used to identify a potential link between a person and a crime scene.

DNA samples are currently taken from anybody arrested for a recordable offence. Technically, recordable offences are those set out in the National Police Records (Recordable Offences) Regulations 2000 (S.I. 2000 No 1139).

In practice they include any offence punishable with imprisonment and some additional offences specified in the schedule to the regulations.

People may be asked to provide DNA samples for elimination or volunteer purposes to assist an inquiry. These profiles will not be added to the NDNAD unless explicitly requested by the person providing the sample.

Perhaps the best way to get a feel for the severity of offences that are recordable is as follows - if you get stopped for speeding, it isn't a recordable offence and a DNA sample wouldn't be taken from you; if you get stopped for drink driving and are found to be over the limit, a DNA sample would be taken from you.

DNA samples can be taken from cells lining the inside of a person's mouth. These samples are taken using swabs.

The swabs are a sort of thick, serrated blotting paper on a stick that is rubbed on the inside of the cheek to collect mouth (Buccal) cells.  It doesn't hurt, it is similar to rubbing the inside of your cheek with a toothbrush.  The blotting paper sample is then pushed off the stick into a plastic tube.

Two samples are taken, but only one is normally required to obtain a DNA profile. After a DNA profile has been obtained from the sample, all remaining sample material is stored in freezers by the laboratory that profiled the sample.

DNA profiling was discovered by Sir Alec Jeffreys during the 1980s.  He discovered a way of distinguishing an individual's DNA: a biological identification system.

DNA profiling targets areas of the DNA that are known to differ widely between individuals. Apart from a gender test, these areas do not code for any physical characteristic or allow definitive determination of any medical condition.

The current system of DNA profiling used in the UK is known as SGM Plus. It examines ten sequence areas of DNA plus a gender test and produces a numeric DNA 'profile' which can be loaded electronically onto the NDNAD. This contains two numerical representations of the DNA at each area examined: one inherited from the mother and the other from the father.

When you look at a DNA profile held on the NDNAD, you will see it as a list of numbers, along with two letters (XX or XY) which show the result of the gender test. The number provides information about a feature of your DNA at each area we examine. For example, '11' informs us that at the particular sequence area of DNA examined, a very short sequence is repeated 11 times.  When you see a '6', it means there are 6 repeats, and so on.

An example of an SGM Plus profile may look like this:

15,18; 6,9; 11,13; 22,22; 31,32.2; 14,17; 17,20; 11,12; 13,16.3; 15,16; XY

Although each person's DNA is unique (apart from identical siblings) DNA profiling does not examine all variations between individuals and is therefore a DNA profile is not unique to an individual. It does however examine those areas of the DNA that discriminate widely between individuals and the chance of two unrelated individuals having matching full SGM Plus profiles is less than 1 in a billion (that is, a thousand million).

A DNA profile, as held on the NDNAD, is comprised of numbers relating to short repeating sequences present at the areas examined in a person's DNA, along with the result of the male/female gender test.

If a person has been arrested for a recordable offence, then that person will have had their DNA sample taken. The DNA profile obtained from this sample will be loaded and held on the NDNAD.

Two DNA profiles are said to match if the corresponding numbers and the result of the male/female gender test are the same.

If a crime has been committed, biological material such as blood may have been left at the crime scene. It may be possible to obtain a DNA profile from any blood recovered there, which can be submitted for inclusion on the NDNAD.

If the person who left the blood at that crime scene already has their DNA profile held on the NDNAD, then the numbers representing their DNA profile will match the corresponding numbers of the DNA profile from the blood, together with the result of the male/female gender test.

When the DNA profile from a person matches the DNA profile from a crime scene, a DNA match report will be sent from the NDNAD to the police force where the crime occurred, and to the police force which had sampled the person.

Identical twins and identical triplets will share the same DNA profile, there are currently (October 2009), seven sets of triplets and 3,844 confirmed pairs of identical twins on the NDNAD.

Two brothers or two sisters (blood relatives) are much more likely to share more similar profiles than two unrelated people.

There is a very small, but finite chance that two unrelated people could share the same DNA profile. For court purposes, when two full DNA profiles are said to match, a conservative match probability of one in a billion (that is, a thousand million) is used to describe the power of the match.

To date (October 2009), no two unrelated people have been found with the same SGM Plus DNA profile on the NDNAD.

Although the chances of any two unrelated people having matching DNA profiles are very small, it is a crucial point of principle. For this reason you cannot convict a person on DNA evidence alone and there must be additional corroborating evidence available.

The only DNA information from a person that is held on the NDNAD are the 20 numbers that make up your DNA profile along with the result of the male/female gender test.

Alongside the DNA information, additional information is held: name; date of birth; ethnic appearance and the gender of the person, as recorded when the sample was taken.

Further information is held about the police force who took the sample, unique reference numbers which link a sample to police records and laboratory processing details.

DNA left at a crime scene may decompose over time due to bacteria, UV light, environmental conditions among other things. 

As a result, if some of the DNA cannot be examined successfully, information from that person's DNA profile may be missing. In such cases the profile may be incomplete.

If blood left at a crime scene initially had a profile such as this...

15,18; 6,9; 11,13; 22,22; 29,32.2; 14,17; 17,19; 11,12; 13,16.2; 16,18; XY

...but was collected some time later, all that may be obtained of the profile might be:

15,   ; 6,9;  11,13;    ,      ; 29,32.2; 14,17;     ,19;    ,12; 13,16.2; 16,18; XY

This is an example of an incomplete DNA profile. It can still be used to establish a potential link between an individual and a crime scene but the match probability drops as a DNA profile gets more incomplete. However, even very incomplete profiles can still be used to conclusively eliminate a person from an investigation.

The rate at which DNA decomposes is very difficult to predict; sometimes relatively fresh samples degrade quickly and at other times complete DNA profiles can be obtained from relatively old samples. 

If two people leave their DNA together at a crime scene, and this DNA is recovered, a mixed DNA profile may be obtained.

There are strict rules for placing a person's DNA profile on the NDNAD, when it has been obtained from a mixture of DNA.

A mixed profile might look like this:

11 15 16 18;6 7 9 9.3; 10 11 13 18; 17 18 22 22; 29 32 32.2 35; 12 14 17 18; 17 18 19; 11 12 15 16; 13 16.2 17 19; 15 16 18 22; X Y.

If you know the DNA profile of one of the people who has contributed to a mixed DNA profile, it may be possible for this DNA profile to be taken away from the mixed one, leaving the DNA profile of the other person.  This may be possible in cases where both a victim's DNA and that of an assailant are mixed together. An elimination DNA profile from a victim can be sufficient to identify the DNA profile of an assailant. If neither person is known it may be very difficult to separate the DNA profiles of each person who has contributed to the mixed profile.

It is hard to say how many detections have resulted from the use of DNA as every case is different and other forms of evidence will also contribute to detections.

However, we are able to provide figures for the number of detected crimes in which a DNA match was available from profiles loaded to the NDNAD. In 2008/09, 17,607 crimes were detected in which a DNA match was available, including 70 'homicides' (this includes murder and manslaughter) and 168 rapes.

We also measure 'additional' detections which arise from the original case involving the DNA match. These happen where, for example, a suspect is presented with DNA evidence linking him to one crime and confesses to further offences. In 2008/09 there were 14,602 'additional' detections.

This brings the total number of crimes detected in 2008/09 in which a DNA match was available or played a part to 32,209.

We also measure the total number of DNA matches on the NDNAD, including those that do not result in a detection. A 'match' means that DNA found at a crime scene matches DNA from a person and includes cases that do not result in a detection, for example where someone has a legitimate reason for being at a crime scene. Even if there is no direct link to the offender, this can still be useful to the police, as it produces further information and speeds up the investigation.

There were over 410,000 crimes with DNA crime scene to person matches during the period 1998/99 to 2008/09.

The number of DNA detections (excluding 'additional' detections) has fallen by 11% over the last 5 years, from 20,489 in 2003/04 to 17,607 in 2008/09. 

This needs to be seen against the background of falling crime. In the same period total recorded crime in England and Wales fell by 22% (from 6,013,759 in 2003/04 to 4,703,814 in 2008/09).

However, there has been an even more significant fall in those volume crime offences where crime scenes are examined for DNA by police scenes of crime officers. 

Last year 2008/09

• 51.4% of all crime scenes examined by scenes of crime officers for DNA were burglary offences.  In the last five years burglary offences have dropped by 29% (from 820,013 to 581,397).
  
• 20.4% of all crime scenes examined by scenes of crime officers for DNA were theft of and from vehicles offences.  In the last five years offences against vehicles have dropped by 40% from 985,006 to 592,117.

More relevant to the performance of the NDNAD in this discussion is the improvement in match rates. In 2002/03, a new crime scene DNA profile being loaded to the DNA database had a 45% chance of matching a persons DNA profile. In 2008/09 this had risen to just under 60%. To put this in perspective, should your house have been burgled and DNA found there was a 45% chance of the NDNAD identifying the donor of the DNA in 2002/03. Last year there was just under a 60% chance of the donor being identified.

In 2008/09, there were 36,727 crimes with suspect-to-crime-scene DNA matches, including 252 homicides, 580 rapes and 175 other sex offences. A 'match' means that DNA found at a crime scene matches DNA from a person when loaded to the NDNAD. This includes cases where someone has a legitimate reason for being at a crime scene.

A match provides police with a lead as to the possible identity of the offender. Even if there is no direct link to the offender, matches can still be useful to the police, as they produce further information and speed up the investigation.

The costs of the NDNAD are made up of the costs of operating and running the database and the costs of monitoring and accrediting forensic suppliers who submit profiles to the NDNAD.

Since 2006/07 there has been an increase in the number of forensic suppliers and this has meant that monitoring and accreditation costs have risen. The NDNAD Delivery Unit charges suppliers for these services, and the resulting income is not taken into account in the total costs figure provided.

The 2008/09 costs have also been inflated by the costs of the project to transfer the physical operations and maintenance of the NDNAD from the Forensic Science Service (FSS) to NPIA.

The UK was the first country in the world to set up a National DNA Database and has been adding profiles to the database for 14 years. The UK takes DNA from all individuals who have been arrested for a recordable offence. Many other countries have now set up their own DNA databases, including the US, Canada, Australia and most European countries, and the size of these databases will increase over time.

Access to the NDNAD is strictly controlled and is limited to a small number of employees of the NDNAD Delivery Unit and the Forensic Science Service (approximately 30 staff in total), all of whom have received the necessary security clearances. Police forces do not have access to the NDNAD but are informed of matches by the NDNAD Delivery Unit.

Occasionally requests are made to release information on the NDNAD for research purposes. There are strict controls over the circumstances in which such information can be released. Requests are considered by the NDNAD Strategy Board and the NDNAD Ethics Group and are approved only if they have a clear operational benefit for the police. Further details about research requests can be found in the NDNAD Annual Report 2007/09.

All DNA profiles taken from children aged under ten by English and Welsh forces had been deleted from the NDNAD by 5th March 2009, following an announcement from the then Home Secretary that such profiles would no longer be retained. DNA from a child under 10 can only be taken with the consent of a parent or guardian. Police forces have been instructed that they should no longer submit DNA profiles of under tens to the NDNAD, and the NDNAD is continually monitored to ensure that this remains the case.

Responsibility for policing in Scotland is devolved and is not the responsibility of the Home Secretary. As the age of criminal responsibility in Scotland is eight years old, police forces in Scotland may still submit profiles of eight and nine year olds to the NDNAD.