DNA paternity testing is a highly accurate scientific method used to establish a biological link between a child and an alleged father. This process relies on comparing specific segments of genetic material, known as DNA markers, inherited by the child from both the mother and the father. The test determines if the alleged father’s unique genetic profile contains the necessary markers the child must have inherited from their biological father.
The Foundation: Short Tandem Repeats
The primary type of DNA marker used in modern paternity testing is known as a Short Tandem Repeat (STR). An STR is a short, specific sequence of DNA bases, typically ranging from two to six base pairs in length, that is repeated multiple times at a particular location on a chromosome. The actual marker being analyzed is the number of times this core sequence is repeated, which determines the length of that specific DNA segment.
These repeated segments are located at specific sites, or loci, across the human genome. Since every person inherits one set of chromosomes from each parent, they have two copies of the STR at each locus. Standard paternity tests typically analyze a profile of 20 or more distinct STR markers to ensure high statistical confidence.
This high number of tested loci, combined with the variability found in STRs, makes the test powerful. The length of the repeated sequence varies significantly between unrelated individuals, allowing scientists to establish a unique genetic profile for each person tested.
Why STRs Are Ideal Markers
STRs are considered the gold standard for relationship testing because they possess several characteristics. The most significant trait is their high polymorphism, which refers to the vast number of different repeat counts, or alleles, that exist within the human population at each locus. This means that the chance of two unrelated individuals sharing the exact same profile across many loci is extremely low.
The structure of STRs also makes them highly amenable to laboratory techniques, particularly Polymerase Chain Reaction (PCR) amplification. PCR allows scientists to quickly and efficiently make millions of copies of the specific STR segments, even from minute or degraded DNA samples. This efficiency is important for modern, high-throughput testing.
Furthermore, most STR loci are found in the non-coding regions of the genome, meaning they do not contain instructions for making proteins and therefore do not affect an individual’s physical traits or health. They follow predictable patterns of Mendelian inheritance, which is essential for accurate relationship analysis.
The Comparison Process: Allele Matching
The raw data from STR analysis is used to determine the alleles—the specific number of repeats—that each tested person possesses at every locus. Since a child inherits one allele from the mother and one from the biological father, the testing procedure focuses on identifying which allele the child inherited from the alleged father. The mother’s genetic contribution is first factored out of the child’s profile to isolate the paternal contribution.
The remaining paternal allele in the child’s profile must then match one of the two alleles possessed by the alleged father at that same locus. This process is repeated across all 20 or more tested STR loci. If the alleged father is the biological father, he must share an allele with the child at every comparison point.
A single mismatch at multiple loci is typically enough to exclude the alleged father as the biological parent. This finding is known as an exclusion, resulting in a 0% probability of paternity. Conversely, an inclusion requires a perfect match between the child’s obligate paternal allele and one of the alleged father’s alleles across the entire profile of markers tested.
Interpreting the Statistical Results
While a full match across all tested STR markers establishes an inclusion, the final paternity test result is presented as a statistical probability rather than a simple yes or no answer. This statistical certainty is built upon two successive metrics: the Paternity Index (PI) and the Combined Paternity Index (CPI). The PI is calculated for each individual locus and represents the strength of the match at that single point.
The Combined Paternity Index (CPI) is calculated by multiplying the Paternity Index values from every tested locus together. This multiplication results in a very high number that indicates how many times more likely the alleged father is to be the biological father compared to a random, unrelated man from the general population. A CPI value greater than 1000 is considered strong support for paternity.
The final reported metric is the Probability of Paternity (POP), a percentage derived directly from the CPI. This figure converts the odds ratio into an easily understandable percentage that answers how likely the tested man is the father, given the genetic evidence. When the genetic evidence overwhelmingly supports paternity, the POP is reported at a minimum of 99.9% or higher, often reaching 99.999%.