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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2:18 minutes

Problem E.6b

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. Is there a failure to exclude any of the suspects? Explain.

Verified step by step guidance

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.

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.

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.

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.

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.

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Key Concepts

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

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.

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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.

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Textbook Question

The results shown are from a DNA test for four genes used in a paternity identification case. DNA for the mother (M) and her child (C) are shown along with DNA from two possible fathers, F1 and F2. What can you conclude based on the DNA results available?

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Textbook Question

The results shown are from a DNA test for four genes used in a paternity identification case. DNA for the mother (M) and her child (C) are shown along with DNA from two possible fathers, F1 and F2. Based on the exclusion principle, is either man excluded as the possible father? Explain.

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Textbook Question

The results shown are from a DNA test for four genes used in a paternity identification case. DNA for the mother (M) and her child (C) are shown along with DNA from two possible fathers, F1 and F2. In the 'C' column, label the DNA bands contributed by the mother with 'M' and the DNA bands contributed by the father with 'F.'

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