DNA Testing: Techniques and Results in the Los Angeles Study

Sidebar to the article Solving Sexual Assaults: Finding Answers Through Research by Nancy Ritter

DNA testing can be a powerful tool in identifying or excluding suspects in sexual assaults. A suspect's DNA profile can be obtained from semen and cells left on the victim. Dried semen, saliva or other body secretions on bedding, clothing or towels can also yield a DNA profile, as can cells left on the exterior or interior of a discarded condom.

The NIJ-funded study of untested sexual assault evidence in L.A. found that:

  • Y-chromosome testing (to determine the presence of male DNA) and conventional serology screening techniques (including microscopic examination to determine the presence of sperm cells) had comparable success rates in leading to positive short tandem repeat results. However, the Y-chromosome technique was more successful in detecting foreign and male DNA in samples taken from the vaginal and external genitalia areas and dried secretions.
  • In developing full and partial profiles, the Y-chromosome screening technique was superior with samples from external genitalia, and conventional serology techniques were more successful with samples from the rectal area. Success was mixed in samples taken from the oral and vaginal areas and from dried secretions.

It should be noted that screening evidence for presence of the Y-chromosome does not yield a male DNA profile; that is, it does not identify the suspect. Also, Y-chromosome screening does not distinguish the tissue type, so the Y chromosome could have come from epithelial cells in saliva, or from semen, blood or skin cells; this type of information could affect the way a crime is eventually charged.

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Date Created: June 15, 2012