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
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.
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.
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.
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
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
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%.