Paternity Testing - A Scientific Summary
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Reprinted courtesy of DNA Diagnostics, Inc. (DDC)
(dnacenter.com).
Ó 1995-2012
DDC All Rights Reserved
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DNA in the Human Body
DNA is inherited from both biological
parents.
DNA, also known as deoxyribonucleic
acid, is found in all cells of the body. It is composed of four different
molecules—adenine (A), thymine (T), cytosine (C), and guanine (G). Like the
letters of an alphabet, these four molecules are arranged in strings of
specific sequences that spell out instructions for our body’s everyday functions.
DNA contains information that dictates our physical characteristics, such as
facial features, height, and even health.
During conception, the father’s sperm
cell and the mother’s egg cell, each containing half the amount of DNA found in
other body cells, meet and fuse to form a fertilized egg, called a zygote. The
zygote contains a complete set of DNA molecules, a unique combination of DNA
from both parents. This zygote divides and multiplies into an embryo and later,
a full human being.
At each stage of development, all the
cells forming the body contain the same DNA—half from the father and half from
the mother. It is this fact that enables scientists to use a variety of
sampling methods for DNA testing. We can take samples at virtually any stage of
development and from any part of the body—and still obtain the same results,
because these samples contain the same DNA.
Each person has a unique genetic profile
that reflects this inheritance.
(See
image on next page)
How Genetic Markers Are Used
Special locations (called loci) in human
DNA display predictable inheritance patterns that could be used to determine
biological relationships. These locations contain specific DNA sequences,
called markers, that forensic and DNA scientists use as identifying marks for
individuals. In a routine DNA paternity test, the markers used are Short Tandem
Repeats (STRs), short pieces of DNA that occur in
different repeat patterns among individuals.
Each person’s DNA contains two copies of
these markers—one copy inherited from the father and one from the mother.
Within a population, the markers at each person’s DNA location could differ in
length and sometimes sequence, depending on the markers inherited from the
parents.
The combination of marker sizes found in
each person represents his/her genetic profile. DDC examines a minimum of 16
STR markers to create a genetic profile for each tested person in a paternity,
identity, or family relationship test.
How
DDC takes DNA samples from each person
to create individual genetic profiles
DDC obtains DNA samples from buccal (cheek) swabs for testing and analysis. Buccal swabs are like cotton swabs, but we use a special
Dacron® polymer instead of cotton to provide a consistent surface for DNA
collection. When the samples reach our laboratory, our staff checks each sample
package as it arrives, checking for signs of tampering. If the package is
intact, it then undergoes our proprietary Dual Process™ DNA testing procedure.
Two independent teams do the following:
Review accompanying documents to make
sure all forms are properly filled out.
Assign the case number and create a
record for the sample.
Take half of the buccal
swab samples and purify DNA from the swabs. DNA purification and isolation is
performed using our Biomek® robotics systems.
Perform the polymerase chain reaction
(PCR) to make billions of copies of the DNA markers.
Analyze the PCR products using the ABI
Prism® 3100 genetic analyzer. These detect the marker sizes in each sample,
generating the raw data.
The staff carefully tracks the samples
and verifies completion of each step throughout the Dual Process™. The raw data
from the two teams are reviewed by the laboratory director. If the data shows
no anomalies, statistical calculations are performed on the genetic profiles of
each tested party.
How Paternity Is Determined
In a paternity test, the genetic
profiles are compared to see if the child’s profile has pieces matching the
tested father and mother
A paternity test report lists the
genetic profiles of each tested party, noting the allele sizes of the different
markers tested. It also lists the Paternity Index (PI) for each marker—a
statistical measure of how strongly a match at a particular locus signifies
paternity. The table below shows partial results of a paternity test. The sizes
of three of each person’s markers are shown. The numbers in red italics
signify markers the child inherited from the mother, and those in blue bold face signify markers inherited from the father.
Locus PI Mother Child Alleged Father
1 D8S1179 10.30 13,
14 14, 16 13,
16
2 THO1 2.32 7, 9.3 8, 9.3 7, 8
3 CSF1PO 17.75 10, 11 7, 10 7
*In the above table, only one value is given
for the alleged father’s CSF1PO—this means that he inherited the same size of
markers from his parents.
The paternity index (PI) is zero if none
of the child’s markers at a specific locus matches the alleged father’s
markers. The PI is 1.0 or greater if there is a match, and the actual value
depends on the frequency of the marker in the population. For example, CSF1PO
in line 3 of the table above shows that there is a 1 in 17.75 chance that
another random, untested person (instead of the tested alleged father) could
have passed on the same marker to the tested child. This PI is stronger than
the other two, probably because those two markers are more commonly found in
the population.
The PI’s for
each marker are multiplied with each other to produce the Combined Paternity
Index (CPI), which represents the overall odds that another random, untested
male would have the same results if his genetic profile were compared with the
child’s. The CPI is then converted into a Probability of Paternity value, which
specifies the probability that the tested man is the father.
Our laboratory often achieves a
Probability of Paternity of at least 99.999%—indicating that there is only a
minute, 0.001% probability that another random individual in the population
could have the same paternity test results and be the child’s biological
father.
Related Uses Of
Genetic Marker Testing
In other family relationship tests,
genetic profiles of participants are compared to see if the expected degree of
shared DNA exists
The DNA test report in other family
relationship tests, such as grandparentage and siblingship tests, is similar to a paternity test report.
Instead of the Combined Paternity Index, a different value, such as a Siblingship Index, is reported.
The report shows the genetic profiles of each tested person. If there are markers shared among the tested individuals, the probability of biological relationship is calculated to determine how likely the tested individuals share the same markers due to a blood relationship.
Last Updated January 6, 2012
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