Amryn Cross
Author Forensic Question: Planting DNA Evidence
Question:
I’m considering writing a crime novel that involves the antagonist framing others for crimes he has committed. He is a genius level sociopath who studies his victims’ habits by analyzing their trash. His day job is with the local trash company (handy for him.)
Now the question. Can he use semen from a condom (if it’s not too old) to plant on/in a victim? He rapes, kills, and then plants the evidence along with other clues that lead to his intended second victim?
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Forensic Question: Solving Cold Cases
What that might mean for your DNA results is that you get a mixture—say, the victim’s blood and another unknown profile. Now, does that profile belong to the killer or just the detective or crime scene tech that handled the evidence without gloves? Without something to compare back to, you won’t be able to say.
In most cases, a forensic anthropologist would be the person to make that determination. Some states have one on staff while others call in an expert like Dr. Bass (founder of the Body Farm) when they are needed. I would say the time frame for that sort of determination is at least a couple weeks, though I’m sure there are cases where it could be done faster. And I should also mention, many forensic anthropologists like to be present when the team is recovering the bones to make sure none are missed and to make observations based on the position of the bones.
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Forensic Question: DNA Testing Turn Around Time
Amanda asks:
Amryn says:
Each lab will have its own turnaround time for DNA. For most state crime labs or local agency labs (meaning those that are attached to a law enforcement agency) the turnaround time is likely to be several weeks/months. This is because they may receive dozens of cases per day, certainly per week, and cases are usually worked in the order they are received. Some cases may only have one or two samples that need DNA testing, while others may require 40 or 50 (this is not as common, but not unheard of either.)
In some cases, a district attorney or someone in charge of the lab may declare the case a “rush” or an “expedite” (usually documented in the form of a letter), in which case it jumps to the front of the line. A lab may drop everything else to work on this case, depending on the scope of the case. Usually one analyst is assigned a case, but may ask others for help to get the results out faster.
Most places don’t do blood typing anymore because it’s kind of time consuming and obviously not as unique as a DNA profile. The most that could be said in a preliminary report is to verify that the stain on the knife is, in fact, blood. It seems silly, but they do have to verify that it is blood and not cocktail sauce or something on the weapon.
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Forensic Question: Differentiating Twin Murder Suspects
Identical twins are a hot topic in forensic DNA. Up until now, it’s been thought that identical twins have identical DNA and therefore are the perfect twist to a murder mystery. We’re finding out now that’s not necessarily true.
Forensic Question: DNA Analysis
This question came to me via my blog as a comment but for some reason didn’t post so I put Amryn to the task of answering it but am only using the poster’s initials since she didn’t give permission directly to use her full name.
It is related to Amryn’s post on DNA analysis that you can find here.
S.C. Asks:
I understand that it becomes more difficult with half-siblings. But with full-siblings, if say, a brother suspected that someone was his sister, could he, through a blood sample, confirm the lineage if he did not have access to the parent’s DNA?
Amryn Says:
This would definitely be possible. The best way to go about it would be to do mitochondrial DNA testing which traces the mother’s lineage. So a brother and sister would have the same mitochondrial DNA because they share the same mother. To try to connect two people as siblings with the more common type of DNA testing is more difficult and would involve statistical calculations. The chance that full siblings will share one of their numbers (see the example in the post) at one of those locations on a chromosome is about 50%. So, yes, the brother could determine someone was his sister, but his best bet would be to have a mitochondrial DNA test done.
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Amryn Cross: How to Research Your CSI or Detective
1. Where does your story take place?
2. When does your story take place?
This may go without saying, but if your story is set in the 1980’s or 1990’s, having your detective catch the bad guy with DNA is not very believable. Technology and tests are always changing in forensics.
3. What’s your character’s job title?
As cool as it looks on CSI, I can’t think of a place where the same person collects the evidence at a crime scene, determines the cause of death, runs the test on the evidence, interrogates the suspect, goes on stakeouts/undercover ops, and make the arrest. There are different jobs for each of these, and overlap will vary from place to place. In a small town, a detective might collect the evidence, interrogate the suspect, and make the arrest, but he or she won’t be determining the cause of death (that’s the medical examiner or coroner) or running the tests on the evidence (if it’s a small town, the evidence will probably be sent to the nearest crime lab.) At the same time, your CSI may collect evidence and nothing else if they work in a large city. Other places, the same people collect the evidence and test it.
4. What agency does your character work for?
Research that specific agency and know what they specialize in. Most law enforcement agencies will work a variety of crimes, but many people are surprised to know that the FBI doesn’t typically get involved in murder cases or witness protection even though they’re often depicted doing just that. Find out how people within that agency are titled. Are they detectives, agents, inspectors, officers? Who carries a gun within that agency? You might be surprised. In some states, even the lab personnel are considered commissioned officers and carry a weapon.
5. Don’t be afraid to ask questions.
There are lots of experts out there who are willing to answer questions about their experiences (myself included.) Most law enforcement agencies have a public relations person or department that can answer some questions for your specific region. One of my favorite online sources is the Crime Scene Writer’s group. This is a pool of people from all sections of law enforcement who are willing to answer writer’s questions. Make sure you search the messages first because there’s a lot to be gleaned from past questions.
Forensic Author Question: Fetal Bones
Forensic Topic: DNA Analysis
I’m so pleased to have forensic expert Amryn Cross back with a question from a reader.
Welcome back, Amryn!
The Body Farm
Forensic specialist Amryn Cross is here for her monthly post on that elusive Body Farm.
Welcome back, Amryn!