DNA Evidence Considerations
For DNA evidence to be meaningfully analyzed it must first be recognized, documented, collected, preserved, and subjected to a chain of custody. Because forensic DNA testing technology has advanced dramatically in terms of sensitivity and discrimination power since its inception in the mid-1980’s, it can be difficult to recognize all possible types of DNA evidence.
Human blood is rich in DNA which is present in the white blood cells. Semen may or may not contain spermatozoa, which are rich sources of DNA. Seminal fluid devoid of spermatozoa contains other cellular material that has far less DNA. Saliva is a rich source of DNA based on the sloughed cells it typically contains. Of course, there is no longer a need to draw blood as a DNA reference sample since buccal (interior) cheek cells provide enough DNA to be tested several times over. There are tests in existence (some presumptive, some confirmatory) to indicate whether blood, semen, or saliva may be present.
However, there are numerous other bodily sources of DNA that cannot yet be specifically identified. These sources include but are not limited to perspiration, skin cells, and vaginal fluid. While the DNA from these types of sources may be detected, it cannot be associated with the specific body fluid or cellular material it originated from.
In general, it is impossible to determine with DNA testing when DNA detected on a sample was first deposited. There is a rule of thumb that sperm may be found in the vagina of a living woman up to 5 days after it was first deposited. The persistence of the sperm depends on a host of factors.
Further complicating matters, DNA can be deposited on an item either directly or indirectly. If a person sneezes or perspires on an object or person, they may transfer their DNA without direct contact. Similarly, if two people shake hands they will likely each transfer DNA to their handshake partner. If that handshake partner then touches another object, they may transfer the other person’s DNA to the object. It is therefore entirely possible that your DNA can end up on an object or a person you never had contact with. One of the best and alarming examples is the case of Lukis Anderson: https://www.themarshallproject.org/2018/04/19/framed-for-murder-by-his-own-dna
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