In a rape case, vaginal swabs are collected. The DNA analyst reports a 'mixed profile' with at least three contributors. The major contributor matches the husband; a minor contributor has a partial profile. Which forensic interpretation challenge is MOST significant?
- A Touch DNA contamination
- B Stochastic effects: allele drop-in and drop-out ✓
- C Mitochondrial heteroplasmy
- D Y-chromosome persistence in sperm-depleted fractions
Explanation
When template DNA is at low levels (as in minor contributors of a mixed profile), stochastic effects become significant: allele drop-out (an allele is not amplified despite being present) and allele drop-in (a spurious allele amplifies) can distort the profile. These phenomena lead to unreliable genotype calls and complicate mixture interpretation, particularly when trying to include or exclude a suspect as a contributor. Mitochondrial heteroplasmy (C) is a separate issue with mtDNA; Y-chromosome persistence (D) is relevant to persistence studies; touch DNA contamination (A) is a pre-analytical issue, not a primary interpretation challenge in collected swabs.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.