Stochastic Effects
Definition
Random sampling errors arising when few template molecules are present in a PCR reaction. Manifest as allele drop-out, allele drop-in, elevated stutter and peak-height imbalance.
- Cause
- Few template molecules present in a PCR reaction
- Manifestations
- Allele drop-out, allele drop-in, elevated stutter, peak-height imbalance
- Context
- Touch DNA, hair, bone, teeth and challenging substrates
Common questions
Why does elevated stutter become a bigger interpretive problem in low-template samples?+
Stutter peaks are normally a small, predictable fraction of the parent allele's height, but at low template levels their relative height can rise enough to be mistaken for a true minor contributor allele, complicating mixture interpretation.
How do forensic laboratories mitigate stochastic effects when working with degraded or trace samples?+
Common approaches include increasing PCR cycle number cautiously, running replicate amplifications and applying a consensus or composite profile, and using probabilistic genotyping software that explicitly models dropout and drop-in probabilities rather than a simple presence or absence call.
Related terms
- Allele Drop-in
- Appearance of a low-level allele in a PCR product from stochastic amplification of a contaminant molecule. More common in low-template reactions where...
- Allele Drop-Out
- Failure of one allele at a heterozygous locus to amplify in a low-template PCR reaction, producing an apparent homozygote. Increases dramatically below...
- EDTA Decalcification
- Demineralisation of bone powder using 0.5 M EDTA at pH 8.0, which chelates calcium from hydroxyapatite and releases DNA from the mineral...
- Petrous Bone
- The petrous portion of the temporal bone, the densest bone in the human skull. Its dense hydroxyapatite matrix protects enclosed DNA from...
- Secondary Transfer
- The movement of a glass fragment from a primary surface (which contacted the broken glass) to a secondary surface (which contacted the...
- Touch DNA
- Minute quantities of epithelial cells transferred by skin contact with a surface, without leaving a visible stain. Collected by swabbing contact points...