An allelic ladder helps forensic staff compare DNA between evidentiary and reference samples. Like a sort of ruler, these ladders measure the fragment lengths. The overall purpose is to rule out or confirm a match from a sample of evidence.
Researchers look at data from several hundred of people before constructing allelic ladders, which really show the most common alleles at each point. Alleles within specific places vary significantly from one person to the next. During testing, microvariants show up frequently, representing an incomplete repetition for an allele.
When forensic scientists analyze samples, a 14-allele designation indicates that there are 14 matches on the fragment. A designation of 12.2 means there are 12 repeats and two additional bases. An allele represents the traits of one copy of a certain gene, or one copy of a set location on a chromosome.
In cases in which alleles do not provide a match, they may be new to characterization. If this happens, rerunning the sample before running a new characterization is preferable because of the prevalence of false positives. The number of samples run to establish the existing set of ladders is high enough to justify confidence in the findings at least until the technician can run another set.