STR 3/3/26

Peak Heights and Inter Locus Balance

  • Definitions and Interpretations
    • Different interpretations exist for peak heights in loci: peak height itself vs. actual peak height comparison.
    • Important to understand relative heights when assessing inter locus balance.
      • Example: Peak heights of different loci compared can vary significantly, e.g., 800 vs. 4,000.
    • Expectation from students: be able to articulate observations without needing calculations.

Internal Lane Standards (ILS) for Sizing Quality

  • Importance of ILS in electrophoresis
    • Recollection from previous units about ILS usage.
    • Separation observed during capillary electrophoresis (CE) as an indicator of quality.
    • ILS sizing quality value can range from 0 to 1.
      • Value of 1 indicates perfect separation; 0 indicates poor separation.
      • Typical acceptable range is 0.89 or higher, subject to variation based on instruments and PCR amplification kits.
      • Emphasizes the need to examine the context and specific SOP guidelines for interpretation.

Resolution and Profile Quality in Capillary Electrophoresis

  • Definition and Importance of High Resolution
    • Essential for distinguishing between closely sized PCR products.
    • Comparison with gel electrophoresis: CE provides better resolution, enabling detection of differences as small as one base.
      • Example: Differentiating between alleles 9.3 and 10 when differing by a single base.
    • Key aspects include:
      • Standard deviation of allele size is used to evaluate resolution but not for direct student calculations.

Artifacts in Electropherograms

  • Definition of an Artifact
    • Artifacts are peaks not corresponding to true alleles; common artifacts include pull-up spikes.
    • Importance of recognizing artifacts during analysis and distinguishing from true alleles.
    • Discussion of types of artifacts:
      • Technical: Related to instrumentation.
      • Biological: Due to amplification process and individual DNA makeup.
    • Emphasis on recognition and explanation skills for analysts in real-world scenarios.

Analytical Thresholds

  • Definition and Application
    • Analytical threshold: The minimum RFU value for detection above background noise.
    • Typical settings range from 50 to 150 RFUs; class examples derived from a threshold of 75 RFUs.
    • Importance of understanding implications of threshold settings on peak interpretation, including stating visible peaks and background noise.

Components of High Quality STR Profiles

  • Protocol Validation
    • Importance of validated protocols and parameters for achieving high profiles during profiling.
    • Key consumables to monitor:
      • Capillary arrays: Recommended brands must follow manufacturer guidelines for optimal performance.
      • Performance Optimized Polymer (POP): Introduced as crucial for profiling, with importance on viscosity and storage requirements.
      • Formamide: Used to maintain PCR products in single-strand state, highly sensitive to cycle of freezing/thawing.
      • Buffers and sterile water: High quality required without contaminants; must adhere to proper storage protocols.

Understanding Peaks and Alleles in Profiles

  • Evaluation of Electropherogram Peaks
    • Analyze peaks based on height, base size, and threshold values.
    • Importance of checking each peak in relation to expected alleles:
      • Typical sample results yield one or two peaks per genetic marker.
      • Higher than expected peaks may indicate a mixture or a pull-up artifact.

Examples of Pull Up and Other Artifacts

  • Definition and Mechanism of Pull-Up
    • Occurs when a strong peak from one dye channel interferes with the detection of another peak from a different channel.
    • Typical indicators include peaks at similar base positions in adjacent channels with a higher relative RFU compared to baseline levels.
    • Recommended ratios for assessment: Pull up peaks should generally be less than 15% of the true allele’s height.
      • Example for Pull Up Evaluation:
      • Ongoing discussion where students practice identifying and calculating potential pull-ups from electropherograms.
    • Bridge Pull Up phenomenon explained: peaks fused when they are too close together in size and detection, creating a merged peak visualization.

Spikes in Electropherograms

  • Definition and Characteristics
    • Spikes indicate technical errors during electrophoresis, often sharp in morphology.
    • Solutions include re-running the samples to confirm results as spikes are not reproducible artifacts.
    • Importance of recognizing spikes across dye channels and ensuring they fall within designated base size regions.

Summary

  • Class re-emphasizes the significance of precision in genetic analysis.
  • Analysts must not only differentiate between true alleles and artifacts but comprehend the nature of peak values, resolution, and analytical thresholds for interpretation.