Drop-Out
Definition
Failure of a genuine allele to amplify above the analytical threshold, typically due to low or degraded template. Creates apparent homozygosity at a heterozygous locus.
- Field
- Capillary electrophoresis / DNA profiling
- Nature
- Failure of a genuine allele to amplify
- Cause
- Low or degraded template DNA
- Visible effect
- Apparent homozygosity at a heterozygous locus
Common questions
Why is drop-out a particular concern in low-template or touch DNA casework?+
With very little starting template, amplification of each allele becomes a near-random stochastic event, so one allele of a true heterozygous pair can simply fail to reach detectable levels while the other amplifies normally, producing a profile that looks homozygous and can understate the number of contributors if not accounted for.
How do analysts guard against misinterpreting a drop-out affected profile?+
Laboratories set validated stochastic thresholds below which drop-out risk is flagged, often run replicate amplifications, and use probabilistic genotyping software that models drop-out probability directly, rather than reading a low-template profile the same way as a high-template single-source sample.
Related terms
- Allelic Ladder
- A multi-allele reference standard containing the full range of known alleles at each STR locus in the panel, co-injected with samples to...
- Capillary Electrophoresis (CE)
- A separation technique that resolves DNA fragments by size through a polymer-filled glass capillary under an applied electric field. Single-base resolution is...
- Drop-in
- Sporadic appearance of a low-level peak not reproducibly present in re-amplifications, typically caused by trace contamination. Distinguished from genuine minor-contributor alleles by...
- GeneMapper ID-X
- Applied Biosystems analysis software for STR genotyping. Current-generation tool used in the majority of US, UK, and Australian forensic DNA laboratories.
- OSIRIS
- Open-source STR analysis software developed by NIST (US National Institute of Standards and Technology), used as an independent or supplementary analysis tool...
- Pull-Up (Bleed-Through)
- A spectral artefact where a tall peak in one dye channel generates a spurious smaller peak in an adjacent dye channel at...
- Spectral Matrix
- A mathematical correction factor that decouples the overlapping emission spectra of the fluorescent dyes used in a multiplex STR kit. Must be...
- Stochastic Threshold (ST)
- A validated minimum peak height below which allele calls are considered uncertain due to stochastic amplification effects. Profiles with peaks between the...
- Stutter Peak
- A PCR artefact appearing one repeat unit below (and occasionally above) a genuine allele peak, caused by polymerase slippage. Typically 5-15% of...