The genetic material that determines a person's inherited features (such as sex, eye color, and blood type) is called DNA(deoxyribonucleic acid). DNA is a long molecule that is made up of building blocks called nucleotides. The order of the nucleotides in the DNA is extremely important. The nucleotides are grouped together to form genes. Each person's DNA contains over 40,000 genes. One or more genes determines each inherited feature. Every person's order of nucleotides in the DNA is unique.

DNA makes up the thin strands (called chromosomes) found in the center, or nucleus, of all body cells except red blood cells and platelets. Therefore, DNA fingerprinting can be done using a sample from any tissue in the body. If a blood sample is used, DNA is usually obtained from white blood cells. Only a small sample is needed for DNA fingerprinting. A drop of blood or a hair bulb contains enough DNA for testing.

To obtain a DNA fingerprint:

• Cells containing DNA are collected and purified.
• The cells are broken open and their DNA is separated. If only a tiny amount of DNA is available, it may be duplicated (by a process called amplification) before proceeding with the analysis.
• The DNA is treated with proteins (called restriction enzymes) that cut it at specific locations, producing DNA fragments of different lengths. Because each person's DNA sequence is unique, the pattern of DNA fragments varies among individuals.
• Using an electrical current, the DNA pieces are separated by size on a thin slab of material similar to gelatin.
• The DNA on the gel is transferred to a nylon membrane, which is more sturdy than the gel.
• The nylon membrane is soaked with a solution containing radioactive chemicals called probes that specifically bind to certain types of DNA fragments. The probes will bind to only a few (up to about 20) of the hundreds of DNA fragments.
• A photo is taken that shows the location of the probes. The pattern of the visible DNA fragments is called the DNA fingerprint, autoradiograph, or autorad. The fingerprint looks similar to bar codes that are read by a store checkout scanner.

DNA fingerprinting can be done on a sample of blood or tissue from any part of the body. 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.

DNA Fingerprinting
Results

DNA fingerprint patterns of tissue samples from a crime scene can be compared to those from a suspect. Results of the comparison usually indicate how likely it is that the two tissue samples belong to the same person. For example, there may only be one chance in a billion that the two DNA fingerprints are from different people. This would mean that the two samples are most likely from the same person.

DNA fingerprint patterns of tissue samples from two people can be compared to determine how likely it is that they are blood relatives. The probability may range from very high to impossible.