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Forensic DNA in Sexual Assault Cases

Forensic DNA testing may be able to establish that sexual contact occurred, however it cannot determine consent.

Semen is composed of seminal fluid and typically spermatozoa. Semen can be identified via chemical tests that detect components of seminal fluid or by microscopic identification of sperm cells. Vasectomized males and those with azoospermia will not have spermatozoa in their semen. Spermatozoa are a rich source of DNA and are responsible for a vast majority of the DNA content of semen.

Vaginal fluid has a high DNA content due to the presence of large amounts of epithelial cells. Unfortunately, there currently is no way to conclusively identify vaginal fluid.

Saliva is another rich source of DNA. It contains high levels of the digestive enzyme amylase. Laboratory detection of amylase is not confirmatory for saliva since not all amylase in the body is of salivary origin. The high DNA content of saliva has made collection of buccal (mouth) swabs the preferred method of DNA reference sample collection, eliminating the need to draw blood from an individual.

The current limitations of body fluid identification sometimes complicate the determination of the significance of DNA results. For example, if a relatively low level of female DNA is found on a penile swab, it is impossible to determine the bodily origin of that female DNA.

Suspected digital penetration cases present an interesting challenge for DNA analysis. Since the amount of male DNA left behind in a vagina by digital penetration is relatively low compared to the amount of female DNA present in a vagina, standard autosomal DNA tests may fail to detect the male DNA.

Y-STR testing is sensitive and specific for male DNA and is essentially able to detect a ‘needle in the haystack’ of tiny amounts of male DNA mixed with large amounts of female DNA. Examples of scenarios that call for Y-STR testing are: digital penetration cases, detecting semen from a vasectomized male on vaginal swabs, and detecting low amounts of male DNA on female fingernail scrapings/clippings.

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