Allelic Drop-Out
Definition
Failure of one allele of a heterozygous locus to amplify detectably, a stochastic effect of low-template amplification where sampling of limited template molecules is random.
- Underlying cause
- Random sampling of few template molecules
- Locus state affected
- Heterozygous loci
- Main risk factor
- Low-template or degraded DNA
- Common mitigation
- Increased PCR cycles or LCN protocols
Common questions
How is drop-out different from allelic drop-in?+
Drop-out is the loss of a real allele that should have amplified but did not, while drop-in is the appearance of a spurious low-level allele from contamination or stutter. The two are opposite failure modes and both are modeled separately in low-template interpretation guidelines.
Could a true homozygote be mistaken for a drop-out-affected sample?+
Yes, a single peak at a locus can mean either a genuine homozygote or a heterozygote where one allele dropped out, and peak height alone cannot always distinguish them. Analysts use replicate amplifications and probabilistic genotyping models to assign a likelihood to each explanation.
Related terms
- ESSplex SE QS
- Qiagen kit covering the European Standard Set of 17 loci plus DYS391 and Amelogenin; optimised for EU Prüm database upload.
- GlobalFiler
- Applied Biosystems 25-plex STR kit covering 24 STR loci plus Amelogenin, compatible with both CODIS 20 and ESS17 requirements.
- Haematin
- Degradation product of haemoglobin that inhibits Taq polymerase by chelating Mg2+ ions and binding to the polymerase active site; most common inhibitor...
- Humic Acid
- Organic polymer from soil degradation that inhibits PCR by intercalating into double-stranded DNA and disrupting primer annealing.
- Identifiler Plus
- Applied Biosystems 16-plex STR kit covering the original 13 CODIS loci plus D2S1338, D19S433, and Amelogenin. Widely used through the 2010s; insufficient...
- Investigator 24Plex QS
- Qiagen 26-plex STR kit with Quality Sensor (IPC) and covering 24 STR loci plus DYS391 and Amelogenin. GO! variant uses increased cycles...
- IPC (Internal PCR Control)
- A non-forensic DNA sequence spiked into the PCR reaction at fixed concentration; a shift in IPC Ct value relative to the expected...
- Low-Copy-Number (LCN) PCR
- PCR protocol using increased cycle numbers (typically 34 cycles) developed at the UK FSS to generate profiles from template inputs below ~100...
- PowerPlex Fusion 6C
- Promega 23-plex STR kit covering all 20 CODIS core loci, all 17 ESS loci, DYS391, and Amelogenin across 6 dye channels.
- Probabilistic Genotyping
- Software-based interpretation of complex DNA electropherograms using continuous statistical models (e.g. STRmix, TrueAllele, ArmedXpert). Generates a likelihood ratio for complex mixtures previously...