The Science of DNA Testing
DNA science and technology have revolutionised paternity and family relationship testing. Today, DNA testing provides an extremely high power of discrimination among individuals, involves a painless sample collection process, and allows for the fast turnaround of test results.
Following is an overview of the science behind paternity and other family relationship DNA testing.
Children inherit their DNA from their biological parents.
DNA is the vehicle of inheritance from generation to generation. Known as the blueprint of the body, DNA contains the coding that dictates a person’s physical characteristics and provides instructions for the body’s functions.
All children inherit their DNA from their biological parents: half from the mother, and half from the father. A child’s DNA is set at conception, when a zygote is formed from the mother’s egg cell (containing one-half of the mother’s DNA) and the father’s sperm cell (containing one-half of the father’s DNA). This zygote contains a complete set of DNA molecules that is a unique combination of DNA from both parents.
Every individual has a unique genetic profile that reflects this DNA inheritance.
Scientists have found special locations in human DNA that show predictable patterns of inheritance and, therefore, are useful in verifying biological relationships. These locations, or loci, contain specific DNA sequences called DNA markers that genetic analysts can use as identifying marks for individuals. In a standard paternity test, the DNA markers used are called Short Tandem Repeats (STRs), which are short sequences of DNA that are repeated multiple times in succession.
Every person’s DNA contains two copies of these markers, one copy inherited from each parent. In addition, the length of these markers and the number of times they are repeated distinguish a person from others in the population.
The unique combination of a person’s marker sizes represents his or her genetic profile. To create the genetic profiles we use in our paternity and other family relationship DNA tests, we examine a minimum of 16 STRs for each tested party.
DDC UK generates individuals’ genetic profiles from their DNA samples.
The tested parties in a paternity test or other family relationship test have their DNA samples collected using the painless, noninvasive buccal swabbing technique. (Read more about our sample collection process.)
When the samples arrive at our laboratory, they are closely examined for evidence of tampering. If intact, the samples begin our special Dual Process™ procedure, whereby they are carefully divided into two groups for separate testing by two independent teams. Separately, both teams:
- Extract the DNA from the buccal swab samples, and purify it.
![]() Make millions of copies of the targeted DNA markers using the polymerase chain reaction (PCR).![]() Analyse the products of the PCR process and generate raw data about the DNA markers.
At this stage, one of our expert laboratory directors compares the teams’ raw data and looks for discrepancies between them. If no anomalies are found, statistical calculations can be performed on the tested parties’ DNA profiles.
DDC UK compares the tested parties’ genetic profiles to look for patterns of inheritance.
The result report of a paternity test shows the DNA profiles of the tested parties, listing the different sizes of the DNA markers that were examined and used in the statistical calculations. It also shows a Paternity Index (PI) value for each marker tested. The PI is a statistical measure of how strongly a match at a specific marker indicates paternity.
Remember: Every person has two copies of each DNA marker, one inherited from each parent. If neither of the marker sizes at a particular locus matches between a child and an alleged father, the PI for that locus is 0. If there is a match, the PI for that locus is 1.0 or higher, depending on how common the marker size is within the specific race population.
Once all the designated DNA markers have been analysed and assigned PI values, the PIs are multiplied together to generate the Combined Paternity Index (CPI). This value represents the odds that a random male in the population would have the same test results if his genetic profile were compared with the child’s.
Finally, the CPI is converted into a probability of paternity.
- A probability of paternity of 0% means the alleged father is excluded as the biological father of the child—he is absolutely not the father.
![]() A probability of paternity of 99.9% or higher means the alleged father cannot be excluded as the biological father of the child. In other words, there is only a minute possibility that a random male in the population could have the same paternity test results and be the child’s biological father.
With other family relationship DNA tests—such as grandparentage, maternity, and siblingship tests—we perform similar statistical calculations to determine the likelihood of the tested biological relationship.
For more information about any of our DNA testing services, please telephone us at 0800 860 0865 or fill out our online contact form.
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