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The following is a detailed description of the science behind our modern paternity tests
Scientific Background

Every human being has 46 chromosomes. 23 from his/her mother and 23 from his/her father. Two chromosomes each carry the same genetic information with slight variations. Two of the 46 chromosomes are either X- or Y-chromosomes. The combination of XX creates a female, XY creates a male.

The variations within the chromosomes create a unique DNA structure for every human being. Only identical twins (from one egg) have the same DNA. However half of the chromosomes in a person are nearly identical with one half of the paternal or maternal DNA.

This fact allows a comparison of DNA between father, mother and child (or father and child only) to include or exclude paternity. Our analysis compares specific characteristic DNA regions. The regions contain repetitive patterns. The amount of repeats is different from person to person.

We copy and amplify the regions we wish to test, to allow for a big enough sample. We then test for the length and number of repeats within the fragments. Finally the length of the fragments for all persons tested are compared and calculated via computer. This allows us to calculate the probability of paternity.

DNA Amplification

First we have to copy and amplify the regions we wish to test. Simultaneously we mark the DNA fragments with fluorescent dyes. This allows detection in so-called capillary sequencers.

To copy the fragments we use a so called polymerase chain reaction. This chain reaction is created by an enzyme that copies the DNA in several passes.

After 30 passes we have theoretically created up to a billion identical copies. This process is similar to splitting of cells, whereas each cell contains the same DNA information.

Analysis of DNA Fragments

As we copy and amplify the DNA fragments we also mark them with fluorescent dye. Once the samples are exposed laser light these colors will show up in different frequencies.

The dye-labelled fragments are then separated via electrophoresis. The fragments are exposed to an electric field and pushed through a very fine tube (a capillary). This tube is filled with a viscose polymer. Since DNA is negatively charged it will move to the positive pole within an electrical field. Longer fragments will need more time to arrive at the pole than shorter fragments.

At the end of the capillary a laser sends one-colored light. This laser light stimulates the fluorescent dyes to emit light of a certain wave length. The color of the fragments allows us to identify each and every one of them. Since we can measure the time it takes a fragment to arrive at the positive pole we can then deduce the length of the fragment.

Statistical Evaluation

Afterwards the information for all the different lengths we have measured is compiled in the computer. The computer than calculates the congruities between all the fragments of the tested persons. These calculations in turn result in the probability and the chances of actual paternity.

To achieve this, the program uses statistical distribution data of all possible variants of a marker in a certain population group, so-called allele-frequency tables. The frequency for every variant is different (e.g. there are variants that are only found in a few people, and there are variants that are found in many people). Therefore chance of an accidental congruity between two people would be different for every case.

Of course, the markers used for our tests are highly significant. The combination of at least 16 markers allow us to guarantee for a probability of at least 99,99%. This means that in case of total correlation between alleged father and child, the possibility of accidental congruity would be below 0.01%.
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