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.