Obligatory Paternal Allele

Serological systems such as blood group antigens or HLA typing have fewer alleles per locus and lower discrimination power than STR markers, so an obligatory allele identified serologically excludes fewer alternative fathers than the same logic applied to modern STR profiling, which is why serological testing has largely been superseded by DNA testing.

Serological typing is more prone to technical ambiguity, such as weak antigen expression or cross-reactivity, than DNA-based typing, so an apparent absence of the expected obligatory allele in older case records may reflect a testing artefact rather than a true genetic exclusion, and should be reviewed cautiously if re-examined today.

Cases decided decades ago on the basis of blood group or serological exclusion are sometimes revisited using preserved samples or archived records, and understanding how the obligatory allele was derived from the original serological data is necessary to assess whether the original exclusion or inclusion still holds up.