DNA is inherited from both biological
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
Each person has a unique genetic profile that reflects this inheritance.
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.
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
- 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®
- 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.
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 signify markers the child inherited
from the mother, and those in blue signify markers inherited from
|*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
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.
There are a variety of DNA markers that can be used
to determine family relationships, identify individuals, even deduce
their ancestry. For more information, please read our article, Solving
Crimes and Mysteries.