Stutter is a by-product of the amplification of STR loci whereby a minor product, typically one repeat smaller than the primary allele, is generated. Sequence analysis of stutter products of STR loci has shown that the product is missing (or occasionally gained) one core repeat unit relative to the main allele. Stutter is thought to occur due to slipped strand mispairing during the amplification process. This happens during generation of the newly synthesized strand where one repeat unit temporarily dissociates, or "slips" away, from the template strand. When the polymerase works to re-associate the two strands, it miss pairs the two strands by one repeat unit thereby making the product strand one repeat unit shorter than the template. While this phenomenon can complicate the interpretation of forensic DNA profiles, stutter fortunately occurs in a reproducible and predictable fashion. The proportion of the stutter product relative to the main allele (percent stutter) is measured by dividing the height (or area) of the stutter peak by the height (or area) of the main allele peak.⁰¹

Typically, stutter is affected by:

• The repeat unit length (2 base pair repeats have higher stutter than 3 basepair, etc).
• The degree of homogeneity of repeats (the more homogenous, the higher the stutter).
• The length of the allele within a locus (the larger the alleles have higher stutter).

Read more about interpretation in this course.

he scientific community, as well as individual forensic laboratories, has conducted validation studies to determine the expected range of stutter percentages. In general, stutter percentages do not vary significantly.

There are two cases in which variability in the stutter percentages can be seen:

• Low-level samples (low RFUs)
• Samples exceeding the detection level of the instrument (excess DNA)

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