DNA Sequencing · Pathology

In Sanger sequencing chromatograms, what does a broad, double-peak pattern beneath a single nucleotide position typically indicate?

  1. A homozygous variant
  2. A failure of the polymerase to extend
  3. A heterozygous insertion/deletion (indel)
  4. Contamination of the reaction with primer-dimer artifacts
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Correct answer: A heterozygous insertion/deletion (indel)

A heterozygous indel causes a frameshift in the sequencing reaction. After the site of the indel, the two alleles are out of sync, resulting in overlapping, double-peak traces (mixed signal) that appear as noise. This often requires cloning or alternative assays to resolve.

Difficulty: Medium Question 10 of 20

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