Drop-in
Definition
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 peak height and locus distribution.
- Field
- Capillary electrophoresis / DNA profiling
- Nature
- Sporadic low-level peak
- Cause
- Trace contamination
- Key trait
- Not reproducible across re-amplifications
Common questions
How does an analyst tell a drop-in peak apart from a real minor contributor?+
A drop-in peak is typically much lower than the stochastic threshold, appears at scattered loci without the consistent locus balance expected of a genuine contributor, and fails to reproduce when the sample is re-amplified, whereas a true minor contributor's alleles tend to recur across replicate runs at related loci.
Why does drop-in matter for probabilistic genotyping software?+
Modern probabilistic genotyping models explicitly incorporate a drop-in probability parameter, because ignoring the possibility of contamination-driven peaks can inflate the apparent number of contributors or wrongly implicate an innocent profile that happens to match a stray low-level peak.
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-Out
- Failure of a genuine allele to amplify above the analytical threshold, typically due to low or degraded template. Creates apparent homozygosity at...
- 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...