Two-man and two-sibling paternity cases.

Traditional genetic marker systems rarely fail to resolve paternity disputes when two or more men are accused, except when men are brothers. A sibling of the biologic father may not be excluded by these laboratory tests and sometimes yields calculated odds of paternity that are equal to or higher than the true male parent. Resolved two-brother cases were compared with resolved cases involving two unrelated men. In each case, the residual odds of paternity were determined for each man and the greater was divided by the lesser to produce a paternity fraction. The paternity fraction is a useful indicator of biologic parentage when it exceeds a value of 10 (log10 of-the-odds score greater than or equal to 1). Tests for alleles at highly heterozygous loci are indicated in initial laboratory evaluations of cases involving brothers. Human leukocyte antigen and variable number of tandem repeat polymorphisms appear suitable.