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Formulas for paternity situations Posers in forensic mathematics Go to home page of Charles H. Brenner |
The formulas below were derived with the help of the Symbolic Kinship Program
The triple proportion A:B:C is listed, where A, B, and C are the likelihoods of the genetic types assuming full-shipship, half-sibship, and unrelated respectively.
From these proportions all likelihood ratios can quickly be written. For example, the likelihood ratio comparing full to half-sibship would be A/B.
Children share two alleles (both are pq) | (1+p+q+2pq) : (p+q+4pq) : 8pq |
Share two alleles doubly (both are pp) | (1+p)^{2} : 2p(1+p) : (2p)^{2} |
Share one allele doubly (they are pp and pq) ^{note} | (1+p) : (1+2p) : 4p |
Share one allele (both heterozygous, pq and pr) ^{note} | (1+2p) : (1+4p) : 8p |
Share no alleles (pq or pp vs. rs or rr) | 1 : 2 : 4 |
Note: Sharing one allele is evidence that the children share at least one parent, but very little evidence that they share one parent rather than two.