In interpreting STR profiles, why can allele dropout lead to misinterpretation of the number of contributors?

In interpreting STR profiles, why can allele dropout lead to misinterpretation of the number of contributors?

• A. Dropout can mimic homozygosity and reduce apparent allelic diversity
• B. Dropout increases the number of observed alleles
• C. Dropout reveals non-human DNA
• D. Dropout only occurs with degraded samples

Answer: (A)

Allele dropout happens when one allele at a DNA locus fails to amplify during PCR, often because there isn’t enough template DNA or due to amplification bias. The result is that a heterozygous individual can appear homozygous at that locus, showing only one peak instead of two. In mixtures, this loss of an allele can mask the presence of a second contributor, making the profile look like it has fewer alleles and potentially fewer contributors than there actually are. So the observed allelic diversity is reduced, which can lead to misinterpreting how many people contributed to the sample.

Dropout does not increase the number of observed alleles, it does not reveal non-human DNA, and it is not exclusive to degraded samples—it's a stochastic amplification issue most common when DNA is scarce or inhibitors are present.