NIJ Training Course for Forensic DNA Officers of the Court: Evaluating Major and Minor Contributors in a Laboratory Report

Sometimes an evidence sample might contain a  large amount of material from one DNA source  and only a little from another. Consider in this  display two known samples: one is a male shown  in the top electropherogram the fact that he is  male is apparent by the two green peaks visible  at the left representing an X and a Y allele; the  other known sample is a female shown in the bottom  electropherogram. Her electropherogram displays  only one tall green peak at the left representing  her two X alleles. The center electropherogram is  that of the evidence sample. Note that every peak  in the evidence samples electropherogram matches  to a peak from the electropherogram of one of  the two known samples, indicating that those  individuals could not be excluded as being the  two contributing sources of the DNA found in the  evidence sample. Notice also that the peaks in the  evidence samples electropherogram that correspond  to those from the females electropherogram are not  as tall as those that match the male. This means  that more of the male's DNA than of the females  was present in the evidence sample. The male  therefore would be referred to as the major  contributor to the evidence sample and the female  would be referred to as the minor contributor.