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STR Data Analysis and Interpretation for Forensic Analysts

Step 1: Internal Size Standards and Allelic Ladders

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Commercial kits for short tandem repeats (STR) typing include allelic ladders and internal size standards (ISS) that are added to each sample.

Read more about Capillary Electrophoresis in course: Amplified DNA Product Separation.

Each sample from a run or gel should be assessed to determine if the ISS peaks have been called correctly. In general, the peaks or bands from an ISS are uniform in size or intensity. Lack of uniformity or miscalled peaks can indicate problems with the sample, injection, and/or run.

Image of the 250 peak
National Institute of Justice (NIJ) (see reuse policy).

The ISS is particularly useful in determining the precision of a capillary electrophoresis run. For example, using Applied Biosystems software, the 250 peak is not assigned in the ISS and the size of this peak (in each sample) can be used to determine the in-run precision. Temperature fluctuations can cause the in-run precision to exceed 1 base pair and evaluation of the ISS can assist analysts in identifying this issue.

  • Allelic ladders should be assessed to ensure that all peaks have been called correctly. Ladder peaks that have not been called or have been miscalled can indicate a problem with the ladder sample, injection, and/or run.
Profiler Plus® allelic ladder (loci D3S1358, vWA, FGA)
Profiler Plus® allelic ladder (loci D3S1358, vWA, FGA) (View larger image.)
National Institute of Justice (NIJ) (see reuse policy).

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