Why can't two fingerprints be exactly the same?

Genetic fingerprint

The principle

The starting point is the genetic material, the DNA (deoxyribonucleic acid; or "deoxyribonucleic acid", or DNA for short). It is a long, spirally twisted molecule on which our genetic information, the genes, is located. The genes are quite far apart on this strand.

Only about five percent of the total human genetic makeup is different in every person and makes us an individual. The rest of the DNA is the same in all people.

In the case of a genetic fingerprint, the DNA is first broken down. This is done with enzymes, a kind of chemical scissors. This results in fragments of different lengths. They can be separated according to their size and made visible with the help of radioactive probes.

The result is a kind of striped pattern like the barcode on a juice bag, in which each strip corresponds to a DNA fragment of a certain length. These patterns are as unique as a fingerprint. No one agrees with another in this pattern.

The more strips or bars of the genetic fingerprints of two people match, the closer they are to each other. Reliable evidence for a paternity test too. If all the strokes of two genetic fingerprints match, there is a very high probability that the traces - around 1 in 30 billion - come from one and the same person.

Copy line for genetic information

Even very small amounts of DNA are enough to create a genetic fingerprint. The samples must not be too small either. You need at least a few millionths of a gram. If there are only minor traces - such as tiny amounts of blood, saliva or semen - the DNA must first be replicated before a genetic test can be carried out.

This duplication takes place by the polymerase chain reaction method (English: "Polymerase Chain Reaction", short: PCR), which is used today in most forensic laboratories. Its inventor, the American Kary Banks Mullis, received the 1993 Nobel Prize in Chemistry for this.

The PCR process is a kind of copy line for genetic information. It provides numerous identical copies of the searched sections, which are framed by specific starting sequences. These start sequences are marked with a so-called primer.

With a well-dosed biochemical cocktail and targeted heating and cooling, the biochemists achieve that the double strand of the DNA separates. The added enzyme polymerase then finally causes each of these individual strands to be supplemented into a whole - the genetic material has doubled.

This process is repeated several times until there is a sufficient amount of material to be able to compare the copied strands with one another. They are then aligned lengthwise in an electric field. The sections you are looking for can be recognized because the primers carry a special dye.

After these processes, the evaluation takes place on the computer, then an evaluation by hand. In the end, the genetic fingerprint is nothing more than a table.

There are special problems with old DNA. The thread-like molecule can break or change chemically. The genetic information can also become unusable if the traces are stored incorrectly, for example in a humid and warm environment. Then bacteria and fungi can develop that destroy the DNA. Then even the most modern laboratory is no longer able to read anything out of the genetic scrap.

Doping sinners and secret fathers

The possible uses of DNA typing are not limited to criminal cases and paternity tests. This method is also used to solve crimes in the areas of sport, biology, ecology and medicine.

For example, during the Olympic Games in Atlanta, Cologne researchers refuted statements made by athletes who could be shown to have doping agents in their urine that these samples did not come from them.

Cellular material can be found in the urine of all people. The small amount of genetic material it contains is sufficient to compare it with the DNA from the blood of athletes. Denial then no longer helps.

There are now other, more refined methods that make the use of the genetic fingerprint interesting. For example, it is now possible to determine the descent of a person either via the maternal line or only the paternal line.

This means that even if there is no comparison material from previous generations, it is possible to determine the relationship between a trace that is well over a hundred years old (such as blood on fabric or hair) with a living descendant of the same line.

For example, it has been shown that the third President of the United States, Thomas Jefferson (1743-1826), had at least one child with a female slave.