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Accuracy and Precision

Data that is close to a measured value versus data that is grouped closely together.

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A Hairy Question

A Hairy Question


Credit: Thorwald
Source: https://commons.wikimedia.org/wiki/File:A-B-Z-DNA_Side_View.png
License: CC BY-NC 3.0

A strand of hair is found at the murder scene. Since it is dark brown, it obviously does not belong to the female victim (a blonde). This sample is carefully bagged and logged (chain of custody is important) and then taken to the crime lab for analysis. After a long commercial break, the DNA analysis is completed. Comparison with the hair DNA of the primary suspect (the murder victim’s boyfriend) yields a perfect match. The culprit confesses to the crime and is taken away as the closing credits roll.

Amazing But True

  • DNA analysis has been widely used as a means of linking criminal to crime. Finding a blood sample, an intact hair, or saliva on a cigarette butt or chewing gum can provide enough material to determine where the material came from. The details of DNA structure can be elucidated and compared with an authentic DNA sample from the alleged perpetrator.
  • For reliable data, hair follicle material is needed. The hair strand primarily contains protein and very little DNA, making the obtaining of an adequate sample very difficult. The follicle DNA is extracted, chemically treated to make copies, and then fractionated using electrophoresis. The DNA fractions obtained by this process can then be treated with fluorescent compounds to visualize the separate DNA fractions. For forensic purposes, the pattern obtained from the sample at the crime scene is compared to the pattern that is seen in a sample from the suspect. A match provides very good evidence that the two samples were from the same individual. It should be noted that this process takes days to complete, not the few minutes we see on television.
  • Credit: Micah Baldwin
    Source: http://www.flickr.com/photos/micahb37/3080247531/
    License: CC BY-NC 3.0

    The results of DNA electrophoresis should be unique for each individual. This is essentially your DNA signature [Figure2]

  • The reliability of the DNA measurements depends upon a number of factors. Each individual has a unique DNA structure when the entire strand is studied. Depending on how extensive the analysis is, there is a one in 48 billion chance that the DNA sample belongs to someone other than the person being investigated. In other words, it is extremely unlikely that the suspect is innocent and very likely that this individual will be sent to prison.
  • Watch the video at the link below to learn more about forensic DNA analysis: https://www.youtube.com/watch?v=F7D1awC9J2E

Show What You Know

Use the links below to learn more about hair analysis. Then answer the following questions.

  1. What part of the hair contains DNA that can be used for analysis?
  2. What is the process called that makes more chains of the DNA?
  3. What is the likelihood of identification using standard techniques?
  4. What is an exclusion?
  5. Why are several different portions of the DNA strand studied?

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Image Attributions

  1. [1]^ Credit: Thorwald; Source: https://commons.wikimedia.org/wiki/File:A-B-Z-DNA_Side_View.png; License: CC BY-NC 3.0
  2. [2]^ Credit: Micah Baldwin; Source: http://www.flickr.com/photos/micahb37/3080247531/; License: CC BY-NC 3.0

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