Observation of null alleles apparently due to deletions.

After examining 2 paternity cases in 17 classical, 4 RFLP and 5 PCR-VNTR systems, isolated pseudo-exclusions were observed in the polymorphism D2S44 (YNH24). In both cases the "exclusions" were due to apparent opposite homozygosity. The application of different restriction enzymes, PCR amplification and varying electrophoretic conditions each led to an equivalent result of a 1-band-pattern with a mismatch between both father/child pairs. From these results the authors conclude that a complete or almost complete loss of the alleles is the most probable explanation.