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Forensic Biology · Tools
Compute the paternity index, combined index and probability of paternity from allele frequency.
PI = 1 ÷ 2p (heterozygous), W = CPI ÷ (CPI + 1)
Standard trio, one locus at a time
From an allele frequency database for the relevant population.
Applies this locus PI across the stated number of loci, for illustration only.
Convention is 0.5, meaning paternity is treated as equally likely either way before the DNA is considered.
Combined paternity index
3.052e+10
PI = 1 / 2p = 1 / (2 x 0.1) = 5.000, CPI = PI^15
W = (prior/(1-prior) x CPI) / (1 + prior/(1-prior) x CPI). With the conventional prior of 0.5 this reduces to CPI / (CPI + 1).
Real reporting computes a different PI at every locus from the actual genotypes and population frequencies, then multiplies those distinct values. Raising one locus PI to a power, as this tool does for illustration, is not a valid report. Mutation, silent alleles, and an untested close relative of the alleged father each change the calculation materially.
The paternity index is a likelihood ratio: how much more probable the child's genotype is if the alleged father is the biological father than if a random man is. For a standard trio where the alleged father is heterozygous and shares one obligate allele, that reduces to 1 divided by twice the allele frequency.
Multiplying across loci gives the combined index, and converting to a probability of paternity requires a prior, conventionally 0.5.