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DNA Testing Technology
 The
discovery of DNA and its properties has opened up a
wide field of applications. One of these is in the field
of human identity testing—creating genetic fingerprints
that can help not only to identify a person but also
to trace his family relationships. DNA testing has had
a large impact in the paternity testing field over the
past decade.
Why DNA?
Old methods of paternity testing used blood. Blood has
many different antigens that were traditionally used
to determine tissue compatibility between organ donations
and recipients. However, paternity testing using blood
typing methods proved difficult because of the following
reasons:
- Blood typing using the common A, B, O antigens did
not have a very high power of exclusion (40%) because
many people share the same blood type.
- Blood typing using other blood antigens (serological
and HLA testing) called for more complex analytical
procedures that required large samples and therefore
could not be used with very small children. The power
of exclusion was higher (up to 80%), but it could
not differentiate among blood relatives.
- Blood test results were affected by blood transfusions
and bone marrow transplants. DNA, on the other hand,
offered certain advantages that made it useful for
DNA testing:
- DNA contains certain locations that are highly variable
across the human population, but are inherited in
a predictable pattern. Thus, unrelated individuals
would have differences on these DNA locations, but
related individuals would have similar patterns.
- DNA is the same in all cells of the body, regardless
of age. When a sperm and egg cell meet, the DNA that
they contain is copied over and over as the fertilized
egg divides and develops into a full human being.
Even after death, the DNA in preserved tissues is
the same DNA that was there at conception. Thus we
can use all types of samples, such as the easily collected
cheek swab, from all stages of growth and beyond,
in a DNA test.
Genetic Markers
Half of a child's genetic material (alleles) come from
the mother, while the other half is contributed by the
father. A series of genetic systems (loci) are analyzed
in an attempt to ascertain the biological father of a
child. Each genetic system in a person has two allele,
these alleles are numerically labeled. In paternity testing,
the alleles from the child are compared to those of the
"parents" to determine if it is possible for
either or both parents to have contributed the particular
alleles present in the child. For instance, assume that
a child has a 10 and 11 allele for a particular genetic
system and the child's mother is known to possess a 10
and a 12 allele for this system. The mother must have
contributed the 10 allele and the 11 allele must be paternal.
In this example, any man who does not possess an 11 allele
could not be the child's father (barring the possibility
of mutation that converts one allele to another - something
that is unlikely but can be taken into consideration if
needed). In the event that a man is not excluded, the
likelihood that a randomly chosen man might also be able
to provide the allele in question to the child can be
determined by examining the allelic frequencies from a
relevant population database.
DNA in Paternity Testing
In paternity testing, 16 specific locations on the DNA
are tested to generate a profile for each individual.
At each location, there are two copies of the DNA. These
are called alleles. One allele is inherited from the mother,
and the other is inherited from the father. Thus, in a
paternity test, the child’s DNA profile shows which
alleles match the mother and if the other alleles match
the tested male (alleged father).
Each allele is assigned a value called the Paternity
Index (PI). It is a measure of how strongly the allele
contributes to the paternity evidence if there is a
match, based on how rare the allele is found in the
population. The paternity indexes from all 16 alleles
are combined to form the Combined Paternity Index (CPI),
which is used to calculate the probability of paternity.
A probability of 0 is given for all exclusions because
if there are a sufficient number of non-matching alleles,
there is statistically no chance that the tested man
could be the biological father. Inclusions are given
as probabilities of 99.99% and higher for a standard
paternity tests that include the child, and alleged
father, and mother.
DNA Testing in the Lab
In a paternity test, buccal swab samples are collected
from the child, alleged father, and mother and sent
to our laboratory for testing and analysis. In the lab,
the following steps are performed:
- DNA samples are divided for testing by two independent
teams.
- DNA is purified from the swabs using special chemical
agents.
- The 16 genetic locations are amplified using the
Polymerase Chain Reaction (PCR).
- The DNA in the swabs contains all the tested
party’s genetic information. PCR takes the 16
small pieces of genetic information and makes
billions of copies of each to facilitate analysis
by our scientists.
- Our laboratory can test up to 24 genetic locations
in unusual genetic situations, such as when testing
closely related alleged fathers or when mutations
are observed.
- The products from PCR are analyzed to determine
each allele’s size.
- The child’s allele sizes are inherited from
the mother and father.
- The paternity index calculation for each allele
depends on the allele sizes present.
- The raw data from the two teams are compared to
verify if they match.
- Statistical analysis is performed to calculate the
combined paternity index.
- One of our PhDs examines the allele sizes from
step 3 and calculates the paternity indexes using
special software.
- Our laboratory uses the largest database in
the industry, giving the most accurate and conclusive
paternity testing results.
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