Stutter is a by-product of the amplification of STR loci whereby a minor product one repeat smaller than the primary allele is generated. Sequence analysis of stutter products of STR loci has shown that the product is missing one core repeat unit relative to the main allele.02 Although the mechanism is not entirely understood, stutter 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 effected by:
- The repeat unit length (2 basepair repeats have higher stutter than 3 basepair, etc).
- The degree of homogeneity of repeats (the more homogenous, the higher the sutter).
- The length of the allele within a locus (the larger the alleles have higher stutter).
In known single source samples, stutter is identifiable by its size and position. However, with mixed samples, stutter and alleles can overlap, complicating interpretation.
Click here to read more about interpretation in Subject 06, Module 03.
The 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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