# How accurate is DNA evidence? **URL:** \ **Category:** Factual Questions\ **Created:** [January 20, 2010, 3:40am UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657 "2010-01-20T03:40:44Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![doubled](https://avatars.discourse-cdn.com/v4/letter/d/51bf81/32.png) [@doubled](https://boards.straightdope.com/u/doubled)\ **Post date:** [January 20, 2010, 3:40am UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/1 "2010-01-20T03:40:44Z")
Inspired by an endless thread about the murder of JonBenet Ramsey. > **[What happened to JonBenet Ramsey?](https://boards.straightdope.com/sdmb/showthread.php?t=547435)** > > I know we’ve had threads on this topic before, but we were going down that route in the “Which ONE Crime Would You Like the True Answer to” thread, and I figured I’d start a new one up rather than hijack. Here are a couple of posts from the thread. ... Supposedly, ten years after the murder was committed, police discovered trace amounts of unknown DNA on her underwear. How likely is it that this DNA came from the killer? Or put another way – is the DNA testing so sensitive that it could have picked up trace amounts from someone who merely handled the laundry days before, or used a toilet seat before she sat down on it, or something like that? ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [January 20, 2010, 4:00am UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/2 "2010-01-20T04:00:15Z")
> [@doubled](#): > > Inspired by an endless thread about the murder of JonBenet Ramsey. > [What happened to JonBenet Ramsey? - Great Debates - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=547435) > > Supposedly, ten years after the murder was committed, police discovered trace amounts of unknown DNA on her underwear. How likely is it that this DNA came from the killer? Or put another way – is the DNA testing so sensitive that it could have picked up trace amounts from someone who merely handled the laundry days before, or used a toilet seat before she sat down on it, or something like that? You have two questions: how accurate is it, and how sensitive is it? The basic idea is that only a handful of skin cells–fewer than a dozen, perhaps–which are shed all the time, are required to get a DNA result. The DNA from those cells can be amplified and a result given that is reasonably accurate in giving a DNA profile of the donor. So in that sense it’s sensitive and potentially accurate. The question for a given circumstance is the context, though. Yes, it is sensitive enough that cells from a factory worker or anyone else handling the article of her clothing can be the donor of the DNA. It’s not out of the question that cells from me shaking your hand can wind up on my clothes, or clothes I handle. So it’s not “accurate” in determining who the perp is unless you have a candidate perp. Then you can place them at the scene (or at least, place them in context of having handled the clothing, etc). In the Jon Benet case, I’ve never seen an expert analysis of how all the DNA was obtained and handled. Certainly DNA in cells on the underwear could easily cross-contaminate other articles of clothing, and it’s my opinion based on what I’ve read that the touch DNA analysis in her case is nothing close to exculpatory for suspects in the family circle. Mary Lacey, the DA who pronounced her family cleared based on the DNA, did not have sound reasoning in my opinion, and in fact had political and perhaps personal reasons for jumping to that pronouncement. You could have a crime technician walk around your home and find DNA from dozens of sources. We shed skin cells all the time. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [January 20, 2010, 3:08pm UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/3 "2010-01-20T15:08:53Z")
I suppose too it depends what sort of sample - the information we’re missing. If it’s a tiny chunk of skin cells, then who did the laundry in the househld, and could it have transferred from other items during washing, etc.? If it’s a dried stain of something else, it’s a bit harder to suggest it came from accidental contamination. Obviously, DNA, like every other piece of evidence, just lends context. It does not prove anything. The circumstances surrounding where it was and how it could have gotten there are always the critical issues. IIRC, there is an “amplification” process which can create billions of copies of a tiny sample for analysis. In another scenario, a perp sample with a tiny flake of cross-contamination from someone else might yield a debatable result. Then, as pointedout in another thread recently, a small sample may not produce enough information to positively identify anyone. In the ideal case, with a nice clean large sample, the odds of misidentification are pretty much zero - one in billions, or something. There was an episode of CSI:NY (not the best source for accurate information) where they thought they had a serial killer, only to find that a person at the q-tip factory was not wearing her gloves while handling the long yarn of cotton being made into q-tips; she was leaving a residue of skin cells every so often. Labs all across the north-east cameup with a DNA match for the same prson in several different homicides. Sounds a little far fetched, but not impossible. ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [January 20, 2010, 4:41pm UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/4 "2010-01-20T16:41:27Z")
I just happened to watch an episode of Cold Case Files that discussed this… The victim was a young woman who was raped and murdered in Ann Arbor, Mi, in 1969. Many years later, the investigators were able to generate DNA profiles from several stains on her stockings and one single blood stain that was found on her. They convicted one man based on the sweat stains; presumably the size of the stains precluded any accidental transfer. Here’s the surprising bit: the single drop of blood found on the victim was traced to a prison inmate who was a small child at the time of the murder. They questioned him, and though he could recall some dark events in his past, they couldn’t figure how his blood came to rest on the girl’s body. The defense did research and found that a sample of the inmate’s DNA was being processed for his own case in the same laboratory facility as the cold case. The laboratory denied cross contamination and defended their lab practices. They stated quite firmly that the samples had never even been on the same side of the laboratory, and the samples had been carefully contained, and so on. The defense tried to use this to discredit the credibility of the other evidence (the sweat stains), but were unsuccessful. Regardless, it sure makes one wonder how many times cross contamination _does_ happen. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [January 20, 2010, 5:05pm UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/5 "2010-01-20T17:05:44Z")
> [@md2000](#): > > There was an episode of CSI:NY (not the best source for accurate information) where they thought they had a serial killer, only to find that a person at the q-tip factory was not wearing her gloves while handling the long yarn of cotton being made into q-tips; she was leaving a residue of skin cells every so often. Labs all across the north-east cameup with a DNA match for the same prson in several different homicides. Sounds a little far fetched, but not impossible. CSI is pretty freakin’ bogus most of the time, but this was actually based on a real case in Germany. Here’s an [article](http://www.spiegel.de/international/germany/0,1518,615608,00.html) on the incident. ---
**Author:** ![Pray\_for\_peace](https://avatars.discourse-cdn.com/v4/letter/p/5f9b8f/32.png) [@Pray\_for\_peace](https://boards.straightdope.com/u/Pray_for_peace)\ **Post date:** [January 20, 2010, 5:29pm UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/6 "2010-01-20T17:29:31Z")
I’ve never worked as a forensic scientist, but I did work with DNA in a lab doing molecular biology experiments for close to 10 years. I don’t watch CSI, so none of my information is coming from there. 😛 As **Chief Pedant** mentioned, DNA testing is very accurate with respect to identifying the individual who left behind the DNA sample. (Of course, individual cases are fact-specific, e.g., the relevance or weight of the DNA evidence depends, for example, on whether the DNA was located under the victim’s fingernails or simply present somewhere at the scene.) Although other methods are sometimes used, the most common forensic DNA analysis method in the US is STR (short tandem repeat) analysis. All DNA profiles stored in CODIS (see below) are generated using STR. > [@](#): > > Short tandem repeat (STR) technology is used to evaluate specific regions (loci) within nuclear DNA. Variability in STR regions can be used to distinguish one DNA profile from another. The Federal Bureau of Investigation (FBI) uses a standard set of 13 specific STR regions for CODIS. CODIS is a software program that operates local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and missing persons. The odds that two individuals will have the same 13-loci DNA profile is about one in a billion. [Human Genome Project](http://www.ornl.gov/sci/techresources/Human_Genome/elsi/forensics.shtml) > [@](#): > > As its name implies, an STR contains repeating units of a short (typically three- to four-nucleotide) DNA sequence. The number of repeats within an STR is referred to as an allele. For instance, the STR known as D7S820, found on chromosome 7, contains between 5 and 16 repeats of GATA. Therefore, there are 12 different alleles possible for the D7S820 STR. An individual with D7S820 alleles 10 and 15, for example, would have inherited a copy of D7S820 with 10 GATA repeats from one parent, and a copy of D7S820 with 15 GATA repeats from his or her other parent. Because there 12 different alleles for this STR, there are therefore 78 different possible genotypes, or pairs of alleles. Specifically, there are 12 homozygotes, in which the same allele is received from each parent, as well as 66 heterozygotes, in which the two alleles are different. > … > In order to utilize STR information as a means of human identification, the FBI established the frequency with which each allele of each of the 13 core STRs naturally occurs in people of different ethnic backgrounds. To this end, the FBI analyzed DNA samples from hundreds of unrelated Caucasian, African American, Hispanic, and Asian individuals. \*\*Assuming that all 13 STRs follow the law of independent assortment (and they should, as they are scattered widely across the genome) and that the population randomly mates, a statistical calculation based upon the FBI-determined STR allele frequencies reveals that the probability of two unrelated Caucasians having identical STR profiles is approximately 1 in 575 trillion \*\*(Reilly, 2001). [Nature, 2008](http://www.nature.com/scitable/topicpage/Forensics-DNA-Fingerprinting-and-CODIS-736) Bolding mine. However, there can be technical problems with DNA analysis. For example, contamination with other evidence (secondary transfer) is a key source of incorrect DNA profiles and raising doubts as to whether a sample has been adulterated is a favorite defense technique. In addition, the ability of STR to discriminate between two people is lower if the actual source is a person related to the suspect, especially a sibling. While almost all individuals have a single and distinct set of genes, rare individuals, known as “chimeras”, have at least two different sets of genes. There have been several cases of DNA profiling that falsely “proved” that a mother was unrelated to her children. > [@](#): > > Sometimes, the DNA from crime scene evidence is in a very small quantity, poorly preserved, or highly degraded, so only a partial DNA profile can be obtained. When fewer than 13 STR loci are examined, the overall genotype frequency is higher, therefore making the probability of a random match higher as well. For instance, in the fictional case in Table 1, if data were only obtained for the first four STRs listed in Table 1, the likelihood of encountering this genotype would be roughly 1 in 331. In this instance, prosecutors would need additional types of evidence against Suspect B to convince a jury that he or she was the source of the evidence sample. In addition, if an individual happens to have STR alleles that are very common in his or her ethnic group, the genotype frequency can also be quite high, even when all of the core 13 STR loci are examined. It is also important to note that crime scene samples sometimes contain DNA from several different sources. This can make teasing out the sources of the DNA extremely difficult. [Nature, 2008](http://www.nature.com/scitable/topicpage/Forensics-DNA-Fingerprinting-and-CODIS-736) ---
**Author:** ![Pray\_for\_peace](https://avatars.discourse-cdn.com/v4/letter/p/5f9b8f/32.png) [@Pray\_for\_peace](https://boards.straightdope.com/u/Pray_for_peace)\ **Post date:** [January 21, 2010, 4:06am UTC](https://boards.straightdope.com/t/how-accurate-is-dna-evidence/525657/7 "2010-01-21T04:06:36Z")
> [@doubled](#): > > Inspired by an endless thread about the murder of JonBenet Ramsey. > [What happened to JonBenet Ramsey? - Great Debates - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=547435) > > Supposedly, ten years after the murder was committed, police discovered trace amounts of unknown DNA on her underwear. How likely is it that this DNA came from the killer? Or put another way – is the DNA testing so sensitive that it could have picked up trace amounts from someone who merely handled the laundry days before, or used a toilet seat before she sat down on it, or something like that? The location of the DNA evidence does not suggest it was due to incidental contact of the kind you suggest. There was DNA in a drop of blood on JonBenet’s underpants. A separate lab also found matching DNA in two locations on her longjohns (different than her underpants). This DNA was found by scraping the waistband and the sides of the longjohns - the locations where someone would have touched when pulling them off. The amount of DNA identified on the longjohns was enough for it to be processed in the routine way DNA is analyzed. (In other cases, so-called “low copy number DNA” has to be processed in a different way, using more cycles of PCR amplification of the DNA). [Cite.](http://www.cbsnews.com/stories/2008/07/10/earlyshow/main4247767.shtml) DNA from the two sites on the long johns matched genetic material from an unknown male that had previously been recovered from the blood in JonBenet’s underpants. Additional tests were conducted to ensure that the genetic material did not come from law enforcement or medical examiner’s personnel. [Cite.](http://www.cnn.com/2008/CRIME/07/09/jonbenet.dna/)