Genetic basis
A child normally inherits one autosomal allele at each STR locus from each biological parent. Parentage testing compares the child’s genotype with the alleged parent’s profile across multiple validated markers.
Key principles for parentage testing
Paternity Index
The Paternity Index (PI) compares the probability of the observed evidence if the tested man is the biological father with the probability under a stated alternative hypothesis, commonly an unrelated man from a relevant population.
Combined Paternity Index
The CPI combines evidential weight across informative loci. Probability of paternity may then be calculated using a stated prior probability. It is not the percentage of loci that match.
Exclusion
Multiple confirmed genetic inconsistencies can support exclusion, subject to the laboratory’s validated mutation and interpretation policies. Unexpected findings should be reviewed for sample identity, mutation, technical issues and family circumstances.
Related possible fathers
If another possible father is a close biological relative of the tested man, standard unrelated-man assumptions may overstate the evidence. The laboratory should be informed before analysis so additional testing or appropriate alternative hypotheses can be considered.
Legal versus private testing
A scientifically valid DNA method does not make a privately collected sample automatically court-admissible. Legal testing requires defensible participant identification, collection, traceability and chain of custody.
Forensic/legal use note: This material is educational. Evidentiary admissibility, collection authority, consent and procedural requirements are jurisdiction- and case-specific. Follow applicable law, court directions and the requesting authority’s requirements.
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