DNA is a double-helix, so I guess you could say that DNA was
bent into shape.
Hey all you Gary Sinise groupies,
Trace here for DNews.
DNA evidence is often seen as the silver bullet of crime scene investigations…
start with a tiny dab of blood,
add a 10 second lab montage, and the perp is behind bars.
Right? Not much.
As powerful as DNA analysis is,
it’s not the perfect evidence that the general public,
and the court system, make it out to be.
First, a little background.
DNA fingerprinting was developed back in 1984,
and it exploits some basic truths about genetics.
All DNA is comprised of four chemical bases:
腺嘌呤 胞嘧啶 鸟嘌呤 胸腺嘧啶
Adenine, Cytosine, Guanine, and Thymine.
These link together to form base pairs,
and long chains of base pairs link up to form strands of DNA.
Different combinations of those base pairs define different genes,
and thus different traits.
In total, the human genome contains about 3 billion base pairs…
but only about point one percent of those vary from person to person,
and that tiny slice of the pie accounts for everyone’s unique traits.
And it’s that 0.1% that DNA fingerprinting utilizes.
When samples of blood, saliva, or other “leavings” are found at a crime scene,
experts analyze the DNA fragments within those samples
and compare them to a suspect’s DNA…
or to a database of people with criminal records.
The more distinct sets of base pairs you can find,
the more confidently you can match, or rule out, samples.
Since 1984 this procedure has gotten quicker, cheaper,
and more accurate, and has exploded in use.
But here’s the problem:
as useful as DNA testing is,
it’s not necessarily the forensic turn-key solution
that shows like CSI make it out to be.
And that’s largely because the process of collecting and analyzing DNA
can be surprisingly ambiguous.
First off, DNA is everywhere.
一小部分皮肤 唾液 血液
Tiny traces of skin, saliva, blood
or other DNA-containing material are often found
all over a crime scene,
收集样本技术含量低 耗时 易错
and collecting samples is low-tech, time-consuming, and fairly error-prone.
Once samples are in, investigators must figure out
which are relevant to the crime…
it’s just no small task.
Worse, DNA from the perp is often mixed with that of the victim,
requiring lab techs to tease out which is which.
All in all, DNA evidence is frequently messy,
requiring experts to try and extrapolate
whether an incomplete or mixed DNA profile truly matches a reference sample.
It’s sort of like trying to figure out
whether a few scattered puzzle pieces even belong
to the picture on the box, let alone fit together.
This ambiguity can lead to labs fudging results
帮不到研究员 或基因分析错误 或完全错误
to help out investigators, or genuine errors in analysis, or outright fraud.
And all that opens the door for false positives in DNA matches.
Sadly, there are many examples of misused DNA evidence ruining lives.
In 1998 Josiah Sutton was arrested
and charged with raping a woman in Houston, Texas.
A crime lab employee testified at the trial that
Sutton was an “exact match” with the DNA recovered from the scene.
He was convicted, and spent almost five years in prison
before a second round of testing
found he was in no way an “exact match.”
That second test didn’t make use of any new evidence…
it simply interpreted the DNA more accurately.
And then there’s DNA mixups wasting valuable time and money.
For more than 15 years, investigators in Europe searched for the Phantom of Heilbronn,
a criminal mastermind who left identical DNA traces
at more than 40 crime scenes across the continent.
After an exhaustive manhunt,
the DNA in question was finally matched…
to a factory worker who helped make DNA test kits.
所以 看 我们不是争辩
So, look.We’re NOT arguing that,
“DNA evidence doesn’t work because some people don’t use it right.”
DNA fingerprinting has revolutionized forensics,
and to date has exonerated 347 wrongfully-convicted
people in the US alone.
But there is a perception that DNA testing
is both simple and infallible.
Jurors tend to overcommit to physical evidence
when it’s presented as fact by experts.
Researchers have even proposed a “CSI Effect,”
which is the tendency of juries expect
fast and conclusive DNA evidence in every trial.
These are dangerous trends because, as sound as the science is,
it’s no good to anyone unless it’s used within its limits.
Like any science.
If you’d like to learn more about DNA,
check out this video we have on CRISPR
which could actually edit your DNA.
What other questions do you have about forensic science?
Let us know down in the comments,
make sure you subscribe for DNews and thanks for watching.