Maternal typing and test sufficiency in parentage analyses

The contribution of maternal typing to paternity analysis was evaluated to determine how many additional loci to study in one-parent cases.

Four groups underwent paternity analyses with an eight-locus test battery. Files of 25 case trios were retrieved, in which alleged fathers had achieved paternity indices of greater than 100 ("included trios"). Maternal types were omitted and the cases were reanalyzed ("included duos"). Mother-child pairs of the cases were then coupled with unrelated men ("excluded trios"), and the cases were analyzed. Maternal types were omitted from the excluded trios and cases were reanalyzed ("excluded duos").

Paternity indices of men in included duos were markedly reduced when compared to included trios; odds were sufficiently low in 9 of 25 men that paternity remained in doubt. After omission of maternal phenotypes, excluded duos exposed 33 percent fewer genetic inconsistencies than excluded trios; 5 of 25 men in excluded duos demonstrated less than two genetic inconsistencies and 1 man had none. The specific probabilities of paternity exclusion in motherless cases averaged 61 percent per locus of those in case trios. One random man in 52 duos was not excluded by the eight tests versus 1 in 417 trios.

Omission of maternal typing from eight common microsatellite paternity tests reduced conclusive evidence for or against paternity by 30 to 40 percent. False inclusion of random men is an important failing of tests in motherless cases. Cases involving one parent and child (e.g., in immigration) would require examination of an additional five similar loci to compensate for absent maternal data. A change in standards is suggested.