Q&A: Can DNA Be Used To Identify Multiple Assailants In a Four Decade Old Rape?

01 Aug

Q: Is it possible to determine if a woman found dead in sub-zero temperatures was raped by more than one assailant 40 years after the event? If so, how could this be accomplished? Could a pathologist conclude that the woman was raped, as opposed to consensual intercourse, if there is no physical evidence such as bruising? Could evidence obtained in 1969 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 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 can easily yield enough DNA for testing.

As you can see, very small samples may be enough. Perhaps only a single cell will work.

DNA is a fairly hardy molecule and survives time, drying, mixing with other materials, and many other adverse circumstances. It does not survive heating and exposure to acids, however. Both heat and acids denature, or destroy, the DNA strands. 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 protect the DNA from decay and would thus help the coroner by preserving better samples for him to collect. If he then preserved them properly they could still be useful decades later. Typically, the DNA sample is air dried and stored in a non-reactive container.

DNA has been found in Egyptian mummies, exhumed bodies, and samples stored from very old crimes. It was two-decade-old DNA evidence that ultimately linked Gary Leon Ridgway, the Green River Killer, to a string of prostitute murders near Seattle, Washington. This was possible because the DNA was handled and stored properly.

Yes, it would be possible to determine that there had been two or more assailants, since each would leave behind his own distinctive DNA pattern. The finding of two or more 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 tiny blue beads with 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. The same with DNA.

The problem of determining whether a rape occurred or not is a question for the jury. Rape is not a medical term, but rather a legal one. The coroner could possibly 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.

For more about DNA testing check out:

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Posted by on August 1, 2010 in DNA, Q&A


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