Genetic research to evaluate plus stutter on D18S51 locus

Abstract

 The PCR amplification of STR loci typically produces a minor byproduct called minus stutter (or plus stutter) that is one repeat unit shorter (or one repeat unit longer) than the original product. These stutters cause some problems that affect the result of STR typing and complicate when comparing results of STR typing. It is necessary to remove the stutter in the forensic STR typing. Minus stutter is filtered out automatically by setting of the analysis software. On the other hand, plus stutter at most loci in STR Kits does not have such setting and it is in particular difficult to distinguish between the original peak and stutter peak.

 In this study, we calculated plus stutter ratio for each long allele (25 and over) at D18S51 and compared it with minus stutter ratio. In addition, we also calculated plus stutter ratio using various STR Kits or changing the annealing temperature conditions.

 The results showed stutter ratio and the number of repeat units were in positive correlation and stutter ratio was not affected by the STR Kit. Plus stutter ratio had more variability than minus stutter ratio regardless of the number of repeat units or STR Kit. It showed a clear difference between minus stutter and plus stutter. The results also suggested minus stutter ratio and annealing temperature were in positive correlation, but plus stutter ratio and annealing temperature were not in positive correlation. It seemed therefore likely that minus stutter may be possible to set a threshold for each allele, but it is difficult to set a threshold in the same way for plus stutter.