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DNA is
extracted and purified (from the buccal swabs, blood, etc.) using
an enzyme which releases it into solution. To determine sizes of
DNA in a particular region the DNA region is either:
- amplified
using DNA primers, targeted to the regions, and a DNA polymerase
enzyme (PCR method), or
- cut with
restriction enzymes (RFLP method).
The
DNA samples are then ran through a system of capillaries using a
chromotography method (called capillary electrophoresis) to resolve sizes.
An example of an older procedure called gel electrophoesis is shown in Figure
1 to assist in illustrating the separation effect of
chromotography. A computer-generated electropherogram (Figure 2)
demonstrates how the DNA sizes can be obtained using a fluorescent
detection system. It is similar to reading a barcode. This procedure has
replaced the older gel electrophoesis method
illustrated in Figure 1.
To
visualize the DNA, either:
- the primers
are labeled before the PCR reaction (PCR method), or
- a labeled
probe is used to hybridize to the specific region of DNA
being examined (RFLP method).
Once the sizes
of DNA in the specific regions are determined, they are
represented by a universal number (e.g., 9, 10, 11), and
referred to as alleles.
Each person
should have 2 alleles at each system examined for
paternity testing (with exception given to either, a) a person having the
same allele presented twice, or b) the possibility of a 'silent' allele).
One allele is passed from the biological father and the other allele is
passed from the biological mother.
Each known
allele has a determined frequency of occurrence in the general
population and among different ethnic groups (Table 1). These
frequencies are used to calculate the probabilities of paternity,
siblingship, grand-parentage, etc.
If
you have any questions regarding this process please let us know and we
will be happy to assist you with answering them.
Figure
2: An electropherogram (or chromatograph) showing the peaks that represent
the 2 alleles established at each chromosomal location tested for one
individual (i.e., D13S317, D7S820 and D16S539). In this case, the data
collected is that from Lane 2 (Child)
in Figure 1
.
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Figure
1: An autoradiograph representing genomic DNA sample detected using silver
staining. Lanes labeled L contain allelic ladders for the respective loci.
Numbers to the right of the image indicate the alleles established for the
samples ran in lanes 1 (Alleged Father) and 2 (Child).
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