Forensic Files by D. P. Lyle, MD
Q&A: Can DNA Be Used To Identify Multiple Assailants In a Three Decade Old Rape?
Q: Was it possible in 1969 (or even today for that matter) to determine if a woman found dead in sub-zero temperatures was raped by more than one assailant. If so, how could this be accomplished? Could a pathologist conclude that the woman was raped, as opposed to consensual intercourse, even if there is an absence of physical evidence such as bruising? What language would the pathologist employ when writing his conclusions? Could evidence from 1969 be preserved (how would it be preserved?) and used today to identified suspects through DNA testing?
A: DNA for testing comes from the genetic material found in the nuclei of the body’s cells. Essentially every cell in the body contains a nucleus. The notable exception is the Red Blood Cells (RBCs), which do not contain nuclei. But, White Blood Cells (WBCs) do. DNA testing of blood tests the DNA found in the nuclei of the WBCs.
Adequate DNA samples for testing have been gleaned from semen stains, bite marks, sweat, sputum, hair, and saliva. Even from the saliva left behind by licking a stamp or sealing an envelope. In the case of saliva from stamps or bites, the DNA tested comes from the cells that line the mouth (called buccal cells), which are constantly shed into the saliva. Hair does not contain cells and thus no DNA, but hair follicles do. A single hair follicle may yield enough DNA for testing.
As you can see, very small samples may be enough.
DNA is a fairly hardy molecule and survives time, freezing, drying, mixing with other materials, and many other adverse circumstances. It does not survive heating, however. Heat denatures, or destroys, the DNA strands. It is important to note that DNA testing does not require intact cells, merely intact DNA. This means that clotted blood, dried semen, and tissue fragments found under victims’ fingernails might yield enough DNA for conclusive testing.
The sub-zero temperatures in your scenario would serve to protect the DNA and would thus help the coroner by preserving better samples for his evaluation.
Yes, he would be able to determine that there had been two assailants, since each would have his own distinctive DNA pattern. The finding of two different DNA patterns in the semen sample obtained from the victim would prove this and when the suspects were apprehended, each could be matched to his own contribution to that sample. Mixing the semen would not alter this finding in any way since each DNA strand would be unchanged. It’s not like mixing blue paint with yellow paint to make green paint but rather like mixing a bunch of tiny blue beads with tiny yellow beads. From a distance, they might appear as though they had melted together to form a green mixture, but on close examination, each tiny bead would be seen to have remained intact and separate. DNA strands don’t “melt” into one another.
DNA can last for years, decades, even centuries. It has been found in Egyptian mummies, exhumed bodies, and samples stored from very old crimes. Recently, DNA evidence linked Gary Leon Ridgway to the famous string of prostitute murders know as the Green River Murders in Washington State. The DNA evidence connected him to murders that occurred in the early 1980s. This was possible because the DNA was handled and stored properly. Typically, the sample is dried and placed in a non-reactive container such as a glass vial.
The problem of determining if a rape occurred is a question for the jury. Rape is not a medical term, but rather a legal term. The coroner could determine if penetration occurred and if semen were present. If he found trauma to the vagina or to other body parts that might suggest the victim was struck or restrained, he might conclude that in his opinion the intercourse was not consensual. Still, it would require a judge or a jury to determine whether a rape occurred or not.
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