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.

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 test that includes 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:

1. DNA samples are divided for testing by two independent teams.

2. DNA is purified from the swabs using special chemical agents.

3. 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.

   
   
4. 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.

   
   
5. The raw data from the two teams are compared to verify if they match.
6. 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.

For more information about paternity testing technology, you may wish to refer to the book, "Paternity Testing: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References."

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