Paternity testing analyzes an individual’s genetic code to establish biological relationships. This process uses DNA to compare genetic markers between individuals. Polymerase Chain Reaction (PCR) plays a foundational role, enabling scientists to amplify minute quantities of DNA found in samples. This amplification generates enough genetic material for detailed comparisons.
Understanding Polymerase Chain Reaction
Polymerase Chain Reaction (PCR) is a laboratory technique that creates millions of copies of a specific DNA segment from a small initial sample. It operates through a cyclical process involving rapid temperature changes. First, the DNA sample is heated, causing the double-stranded DNA to separate into two single strands.
Next, the temperature is lowered, allowing short DNA sequences called primers to attach to target regions on each single strand. An enzyme called DNA polymerase then extends these primers, building new complementary DNA strands. This three-step cycle of denaturation, annealing, and extension is repeated multiple times, exponentially increasing the amount of target DNA. PCR’s ability to amplify tiny amounts of DNA makes it a tool for genetic analysis, especially with limited samples.
The DNA Blueprint for Paternity
Paternity testing focuses on Short Tandem Repeats (STRs). These are short DNA sequences, typically two to six base pairs long, repeated multiple times. The number of repeats varies significantly among individuals, making them highly informative genetic markers.
Individuals inherit one copy of each STR marker from their mother and one from their father. For example, if a child has 10 repeats at an STR location, they inherited a 10-repeat allele from one parent and another allele with a different number of repeats from the other. This inheritance pattern makes STR alleles ideal for determining biological parentage.
Applying PCR in Paternity Testing
PCR is central to paternity testing, enabling detailed analysis of an individual’s genetic profile. The process begins with obtaining a DNA sample, commonly collected through a buccal (cheek) swab. This extracted DNA serves as the template for amplification.
PCR then amplifies the target STR regions within the DNA sample. Primers bind to unique sequences flanking each STR region, directing DNA polymerase to copy these areas. This creates millions of copies of each STR marker, ensuring sufficient material for subsequent analysis. Following amplification, the DNA fragments are separated by size using capillary electrophoresis. This method distinguishes STR alleles based on their varying lengths, as longer fragments take more time to pass through the capillary.
Fluorescent labels attached to the amplified DNA fragments allow for their detection and precise sizing. The output is a “DNA profile” for each individual, listing the STR alleles they possess at each tested genetic location. A typical paternity test analyzes a panel of 15 to over 20 different STR markers to ensure accuracy and reliability.
Interpreting Paternity Test Results
Once DNA profiles are generated for the child, mother, and alleged father, these profiles are compared. The child must have inherited one allele at each STR locus from their mother and the other from their biological father. Analysts examine each STR marker to see if the alleged father’s profile contains the allele the child did not inherit from the mother.
If the alleged father’s DNA profile contains all necessary paternal alleles across every analyzed STR locus, he cannot be excluded as the biological father. Conversely, if there are multiple mismatches where the alleged father does not possess the paternal allele required by the child’s profile, he is excluded as the biological father. The outcome of paternity testing is typically expressed as a statistical probability. For confirmed paternity, the probability is usually reported as 99.9% or higher.