A victim of murder is found to have scrapings containing skin cells under several of her fingernails. Genetic analysis confirms that the DNA isolated from these cells came from the same individual and does not match the DNA of the victim. The results shown below are for six CODIS STR markers from the crime scene DNA (from under the victim's fingernails and presumed to be the murderer's), and from three suspects (A, B, and C) who have been detained for questioning about the murder. Is there a failure to exclude any of the suspects? Explain.

Verified step by step guidance

1. Step 1: Understand the concept of CODIS STR markers. CODIS (Combined DNA Index System) uses specific Short Tandem Repeat (STR) loci to compare DNA profiles. Each individual has two alleles per STR marker, one inherited from each parent, and these alleles are represented by the number of repeat units.
2. Step 2: Compare the STR alleles from the crime scene DNA (under the victim's fingernails) with the STR alleles of each suspect (A, B, and C) for all six markers. For each marker, check if the suspect's alleles match one or both alleles found in the crime scene sample.
3. Step 3: Determine if there is a failure to exclude a suspect by verifying if the suspect's alleles are consistent with the crime scene profile at every marker. A failure to exclude means the suspect's DNA profile could have contributed to the sample, i.e., no mismatches at any marker.
4. Step 4: If any suspect has one or more STR markers with alleles that do not match the crime scene DNA, that suspect is excluded as the source of the DNA under the victim's fingernails.
5. Step 5: Summarize your findings by stating which suspects, if any, cannot be excluded based on the STR marker comparison, and explain that a failure to exclude does not prove guilt but indicates that the suspect's DNA profile is consistent with the evidence.

Key Concepts

CODIS STR Markers

CODIS (Combined DNA Index System) uses Short Tandem Repeat (STR) markers, which are specific regions in DNA where short sequences repeat. These markers are highly variable among individuals, making them useful for identity matching in forensic analysis. Comparing STR profiles from crime scene samples and suspects helps determine if there is a genetic match.

Failure to Exclude in Forensic DNA Analysis

Failure to exclude means that a suspect's DNA profile cannot be ruled out as a source of the crime scene DNA. If the STR markers from a suspect match all the markers found in the evidence, the suspect is not excluded and remains a possible contributor. This does not confirm guilt but indicates the suspect’s DNA is consistent with the evidence.

Interpreting DNA Profiles for Identification

DNA profiles consist of alleles at multiple STR loci. To interpret, each locus from the crime scene is compared to suspects’ profiles. A complete match across all loci suggests the suspect’s DNA is present, while mismatches exclude suspects. Understanding how to read and compare these profiles is essential for forensic conclusions.