Cases of disputed paternity arise when there is uncertainty about the biological father of a child. Resolving such disputes is crucial for legal reasons (e.g., child support, inheritance, custody), social reasons (e.g., establishing family ties, identity), and medical reasons (e.g., genetic disease risk assessment). Historically, paternity was often inferred through social recognition or less reliable methods like blood group analysis. However, recent advancements in molecular biology, particularly DNA profiling, have revolutionized paternity testing, providing highly accurate and conclusive results.

Recent Techniques for Paternity Testing:

The primary and most reliable technique used today is DNA Profiling (also known as DNA Fingerprinting or Genetic Fingerprinting), specifically using Short Tandem Repeat (STR) analysis.

1. Principle of DNA Profiling (STR Analysis):

   • Every individual inherits half of their DNA from their biological mother and half from their biological father.
   • STRs are short sequences of DNA (typically 2-6 base pairs long) that are repeated multiple times in a row at specific locations (loci) on chromosomes. The number of repeats varies significantly among individuals, making them highly polymorphic.
   • An individual inherits one copy (allele) of each STR locus from their mother and one from their father.
   • By analyzing multiple STR loci, a unique genetic profile can be generated for each individual.

2. Procedure:

   • Sample Collection: DNA samples are collected from the child, the mother, and the alleged father. Common sources include buccal (cheek) swabs, blood, or other biological materials.
   • DNA Extraction: DNA is extracted from the collected samples.
   • PCR Amplification: Specific STR loci are amplified using the Polymerase Chain Reaction (PCR) technique. This creates millions of copies of the target STR regions, making them detectable.
   • Fragment Separation and Detection: The amplified STR fragments, which differ in length based on the number of repeats, are separated by size using capillary electrophoresis. A laser detects fluorescent tags attached to the fragments, generating a unique electropherogram (a graph showing the size and quantity of each STR allele).
   • Data Analysis and Comparison: The STR profiles of the child, mother, and alleged father are compared. For each STR locus, the child must have inherited one allele from the mother and the other from the biological father. If the alleged father's alleles match the child's paternal alleles across all tested loci, paternity is highly probable. If there are multiple mismatches, paternity is excluded.

3. Interpretation and Accuracy:

   • Modern DNA paternity tests typically analyze 16-24 or more STR loci.
   • If the alleged father is the biological father, the probability of paternity is usually 99.9% or higher.
   • If the alleged father is not the biological father, paternity is excluded with 100% certainty (barring rare mutations or sample mix-ups).
   • The high number of loci analyzed ensures statistical power and virtually eliminates the possibility of a random match between unrelated individuals.

Other (Less Common or Older) Techniques:

• Mitochondrial DNA (mtDNA) Analysis: Used to trace maternal lineage, as mtDNA is inherited exclusively from the mother. While not directly used for paternal identification, it can be useful in complex cases involving maternal relatives.
• Y-Chromosome Analysis: The Y-chromosome is passed directly from father to son. This can be used to establish paternal lineage among males (e.g., if the alleged father is deceased, his male relatives can be tested).
• Blood Group Analysis (Historical): ABO, Rh, and other blood group systems were used in the past. However, these methods could only exclude paternity (e.g., if the child has blood type A and the alleged father has blood type B, and the mother has blood type O, paternity is excluded). They could not definitively confirm paternity and have been largely superseded by DNA testing.

In conclusion, DNA profiling using STR analysis is the gold standard for resolving disputed paternity cases due to its unparalleled accuracy, reliability, and ability to provide definitive answers, thereby ensuring legal and social justice.