Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

18. Molecular Genetic Tools

Methods for Analyzing DNA

Genetics

18. Molecular Genetic Tools

Methods for Analyzing DNA

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2:05 minutes

Problem E.6a

Textbook Question

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. Do the STR results exclude any of the three suspects? Explain.

Verified step by step guidance

Step 1: Understand the context and the role of CODIS STR markers. CODIS (Combined DNA Index System) uses specific Short Tandem Repeat (STR) loci to compare DNA samples. Each STR marker consists of repeating units of DNA, and individuals differ in the number of repeats at each locus, making STR profiles highly individual-specific.

Step 2: Compare the STR profiles from the crime scene DNA (under the victim's fingernails) with the STR profiles of each suspect (A, B, and C). For each of the six STR markers, check if the alleles (repeat numbers) in the crime scene sample match those of each suspect.

Step 3: For each suspect, identify any STR markers where the alleles do not match the crime scene DNA. A mismatch at even one STR locus can exclude a suspect because it indicates that the suspect's DNA profile is not consistent with the DNA found at the crime scene.

Step 4: Summarize the comparison results by listing which suspects have matching STR profiles at all six loci and which have mismatches. Those with complete matches cannot be excluded, while those with mismatches can be excluded as the source of the DNA under the victim's fingernails.

Step 5: Conclude by explaining that the exclusion or inclusion of suspects is based on the presence or absence of matching STR alleles across all tested loci, emphasizing the power of STR analysis in forensic identification.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Short Tandem Repeats (STRs)

STRs are short sequences of DNA, typically 2-6 base pairs long, repeated multiple times in a row at specific loci. The number of repeats varies among individuals, making STRs highly useful for genetic fingerprinting and forensic identification. Comparing STR patterns helps determine if DNA samples come from the same person.

DNA Profiling and Matching

DNA profiling involves analyzing multiple STR markers to create a genetic profile unique to an individual. Matching STR profiles between crime scene samples and suspects can include or exclude individuals as contributors. A suspect is excluded if their STR alleles do not match the crime scene DNA at one or more loci.

Forensic Interpretation of STR Results

Forensic analysis compares STR profiles from evidence and suspects to assess identity. If all STR markers match, the suspect cannot be excluded; if any marker differs, the suspect is excluded. Understanding allele matching and the significance of multiple loci is essential for interpreting forensic DNA evidence accurately.

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