DNA profiling is a highly accurate method for establishing biological relationships, such as determining paternity or maternity. This scientific process compares a child’s unique genetic characteristics with those of an alleged parent to confirm a biological link. Modern DNA testing is extremely reliable, offering a definitive answer in nearly all cases of questioned parentage. By examining specific segments of an individual’s genetic code, scientists determine the statistical likelihood of a biological connection.
Identifying the Genetic Markers
The foundation of parentage testing relies on analyzing specific, non-coding regions of DNA known as Short Tandem Repeats (STRs). These markers are short sequences of two to six base pairs repeated multiple times within the genome. Since the exact number of repeats varies significantly between individuals, STRs are highly informative for distinguishing one person from another.
Scientists examine a standardized set of STR locations, or loci, across the chromosomes of the tested individuals. Each person inherits two copies of every chromosome, meaning they have two different versions, or alleles, for each STR marker. The child receives one allele for each STR location from their biological mother and one allele from their biological father. A genetic profile is built for comparison by comparing the child’s inherited alleles against those of the alleged parent.
Sample Collection and Preparation
The process begins with obtaining a DNA sample, most often done using a simple buccal swab. This involves gently rubbing a sterile swab against the inside of the cheek to collect epithelial cells. This non-invasive method provides sufficient cellular material to extract a high-quality DNA sample for laboratory analysis.
For a legal or court-admissible test, a strict “chain-of-custody” protocol is enforced. This ensures the samples are collected by an authorized third-party collector and that the identity of the donors is verified, usually with photo identification. Private “peace-of-mind” testing does not require this stringent documentation, though the laboratory analysis remains the same. Once received at the lab, the DNA is extracted from the cells and purified, making it ready for the amplification and profiling process.
Analyzing and Interpreting the Results
After the DNA is extracted, Polymerase Chain Reaction (PCR) is used to create millions of copies of the specific STR markers. This amplification generates enough genetic material to accurately determine the exact number of repeats, or the allele size, at each tested location. The resulting profiles—a set of allele numbers for each marker—are then compared between the child and the alleged parent.
The core of the analysis is a systematic comparison of the child’s two alleles at each locus against the alleged parent’s two alleles. For the alleged parent to be included, one of their two alleles at every location must match one of the child’s two alleles. If the alleged parent is the biological parent, they must have contributed the specific allele the child did not inherit from the other known parent.
An “exclusion” occurs if the child possesses an allele not present in the alleged parent’s profile at two or more different STR locations. While a single non-match may sometimes be explained by a rare genetic mutation, multiple exclusions definitively rule out a biological relationship. When all markers match, scientists proceed to a statistical calculation to determine the strength of the evidence.
This calculation involves determining a Paternity Index (PI) for each matching marker, which compares the likelihood that the tested man contributed the matching allele versus a random man in the population. These individual PIs are then multiplied together to produce the Combined Paternity Index (CPI). The CPI represents how many times more likely the alleged parent is to be the biological parent compared to an unrelated man.
The CPI is then converted into the final percentage known as the Probability of Parentage (POP) or Probability of Maternity (POM). For a positive result, this probability must exceed a very high threshold, typically 99.9% or higher, to confirm the biological relationship. This final percentage represents the statistical certainty that the tested individual is the biological parent. The entire analytical process is identical whether confirming paternity or maternity.