Stutter percentages for 13 CODIS STR loci (data adapted from AmpFlSTR manuals). Alleles for each STR locus are shown from smallest to largest. Each locus has a different average stutter percentage but all loci show the trend of increasing stutter with larger alleles (longer number of repeats).
Because stutter products are the same size as actual allele PCR products, it can be challenging to determine whether a small peak is a real allele from a minor contributor or a stutter product of an adjacent allele.
Mixture interpretation requires a good understanding of the behavior of stutter products in single source samples. Often a laboratory will quantify the percentage of stutter product peak heights compared to their corresponding allele peak heights. The percentage of stutter product formation for an allele is determined simply by dividing the stutter peak height by the corresponding allele peak height.
A plot of the alleles from each of the 13 standard STR loci reveals variation in the percentage of stutter for each locus as well as alleles from the same locus (Figure 6.3). Such a plot illustrates several important principles. First, each locus has a different amount of stutter product formation. Second, longer alleles for a STR locus exhibit a greater degree of stutter than smaller alleles for the same locus. Third, stutter percentage with the standard tetranucleotide repeats is generally less than 15% for all 13 CODIS core STR loci under standard amplification conditions.