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DNA Fingerprinting in a Criminal Investigation

DNA Fingerprinting

Test Overview

DNA fingerprinting is a test to identify and evaluate the genetic information—called DNA (deoxyribonucleic acid)—in a person's cells. It is called a "fingerprint" because it is very unlikely that any 2 people would have exactly the same DNA information, in the same way that it is very unlikely that any 2 people would have exactly the same physical fingerprint. The test is used to determine whether a family relationship exists between two people, to identify organisms causing a disease, and to solve crimes.

Only a small sample of cells is needed for DNA fingerprinting. A drop of blood or the root of a hair contains enough DNA for testing. Semen, hair, or skin scrapings are often used in criminal investigations. See a picture of DNA testing in a criminal investigation.

A person who has DNA fingerprinting done voluntarily usually provides a sample of blood taken from a vein. DNA testing also can be done on cells obtained by a simple mouthwash or a swab of the cheeks inside the mouth, but these methods are not recommended.

Why It Is Done

DNA fingerprinting is done to:

Determine who a person's parents or siblings are. This test also may be used to identify the parents of babies who were switched at birth.
Solve crimes (forensic science). Blood, semen, skin, or other tissue left at the scene of a crime can be analyzed to help prove whether the suspect was or was not present at the crime scene.
Identify a body. This is useful if the body is badly decomposed or if only body parts are available, such as following a natural disaster or battle.

How To Prepare

Tell your doctor if you have had a blood transfusion within the past 3 months. No other special preparation is required before having DNA fingerprinting.

Talk to your doctor about any concerns you have about the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).

How It Is Done

Blood sample from a vein

DNA that is used to establish paternity is collected from a blood sample. The health professional drawing blood will:

Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
Clean the needle site with alcohol.
Put the needle into the vein. More than one needle stick may be needed.
Attach a tube to the needle to fill it with blood.
Remove the band from your arm when enough blood is collected.
Apply a gauze pad or cotton ball over the needle site as the needle is removed.
Apply pressure to the site and then a bandage.

Blood sample from a heel stick

If a DNA blood test is done on a baby, a heel stick will be done instead of a blood draw from a vein.

For a heel stick blood sample, several drops of blood are collected from the heel of the baby. The skin of the heel is cleaned with alcohol and then pricked with a small, sterile lancet. Several drops of blood are collected inside circles on a specially prepared piece of paper. When enough blood has been collected, a gauze pad or cotton ball is placed over the puncture site. Pressure is applied to the puncture site briefly and then a small bandage is usually placed over it.

DNA can be collected from dried blood, skin, saliva, hair, urine, and semen. Bone and teeth samples are used when a body is badly decomposed.

How It Feels

Blood sample from a vein

You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. But many people do not feel any pain, or have only minor discomfort, once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing the blood, the condition of your veins, and your sensitivity to pain.

Blood sample from a heel stick

The baby may feel a brief sting or a pinch when the lancet pricks the skin. While the blood is being collected, there is very little or no discomfort.

The collection of DNA from saliva, urine, or semen does not cause discomfort.

Risks

Blood sample from a vein

There is very little risk of complications from having blood drawn from a vein.

You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.
In rare cases, the vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated with a warm compress applied several times daily.
Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your health professional before your blood is drawn.

Blood sample from a heel stick

There is very little risk of complications from having blood drawn from a heel stick. A small bruise may develop at the puncture site.

There are no risks linked with collecting DNA from saliva, urine, or semen.

Results

DNA fingerprinting is a test to identify and evaluate the genetic information—called DNA (deoxyribonucleic acid)—in a person's cells.

DNA samples can:

Determine who a person's parents are (establish paternity). Tissue samples from two people can also be compared to determine how likely they are to be blood relatives.
Determine whether a suspect was present at a crime scene, by comparing DNA from the scene to the DNA of the suspect.
Positively identify a body. Bone and hair samples can be used to identify a badly decomposed body.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

Sample size. The possibility of having inaccurate test results increases if a very small sample of DNA is available for testing. The chance that the DNA will be mixed up with some other DNA is also higher with a smaller sample.
Having had a blood transfusion within the past 3 months.
Decay of the tissue sample.

What To Think About

Because DNA fingerprinting has been used to identify bodies, such as military personnel killed in action or crime victims, DNA databases (much like fingerprint databases) have been developed. DNA fingerprinting is more accurate than traditional fingerprints, dental records, blood type, or ID tags for this purpose.
Although home paternity test kits are available, they are not as accurate as DNA fingerprinting, and the results cannot be used in a court of law. If you are thinking about using of one of these kits, talk with your doctor.
Other tests that can help determine the likelihood of two people being related or that can detect inherited diseases include karyotyping and genetic testing. For more information, see the topics Karyotype Test and Genetic Test.

References

Other Works Consulted

Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Credits

By	Healthwise Staff
Primary Medical Reviewer	Sarah Marshall, MD - Family Medicine
Specialist Medical Reviewer	Siobhan M. Dolan, MD, MPH - Reproductive Genetics
Last Revised	March 29, 2011

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Last Revised: March 29, 2011

Author: Healthwise Staff

Medical Review: Sarah Marshall, MD - Family Medicine & Siobhan M. Dolan, MD, MPH - Reproductive Genetics

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