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And along those lines, I've heard that DNA tests aren't reliable either.

by ColanR 8y ago

sjwright 8y ago
DNA tests prove a very specific narrow fact, and it’s up to lawyers to string together a hypothesis in order for it to have relevance. Unfortunately all too often the DNA test result is used as proof of a hypothesis, not of the mere presence of surprisingly similar DNA in a swab sample.
mjevans 8y ago
The problem with the DNA tests we use today, assuming even perfect cases with zero contamination or mistakes (whole other argument there), is that (in my non-expert understanding): * Taking the samples and using several solutions to snip them at given patterns. * Taking another solution and promoting replication of the fragments. * Using some dye and a weight sorting channel to bin the fragments by weight. This results in a kind of 'bar code' that graphs the distribution of weights as a VERY crude hash of samples of DNA. It's pretty useful for determining things like IF it is LIKELY that individuals are related. It's also pretty good at confirming negatives (We're sure someone is NOT related to X). It's not that great at confirming positives. That is, the results are both subjective and ambiguous given the hashing. A "positive" result here is really more of an "OK, it's likely we should run the real and expensive check, evaluate if someone might be a suspect by other merits, etc." I would, offhand, consider a "positive" above to be enough evidence to produce /suspicion/ and /warrants/ to locate other specific evidence to ascertain an actual guilt or innocence based on harder evidence. If an actual "sequence the whole set of samples" option enters the realm of feasible tests then it would also warrant actually doing that. Come to think of it, I'm not sure how the Ancestry/etc novelty DNA tests work. I'd assume they've isolated a few specific markers they're looking for and the processes are optimized towards identifying those and comparing combinations for those specific traits.
- samatman 8y ago
  You’ve described electrophoresis DNA fingerprinting correctly, then jumped to the unjustified conclusion that it’s a VERY crude hash. Not so, it’s a fairly excellent hash, one with few collisions. Better than a fingerprint. If it looks like your DNA, it most likely is. Source: was an electrophoresis tech (not forensic) in a previous life. And yes, the usual genetics sites do SNP tests, which are cheaper than full sequencing but mean that they only find what they’re already looking for. The price of full sequencing is dropping rapidly, I look forward to it being a standard part of medical practice, it will save a lot of lives and improve quality of care.
  - Dylan16807 8y ago
    Better than a fingerprint is still not great!
    - samatman 8y ago
      Well, several decimal orders of magnitude better than a fingerprint, to be accurate.
EliRivers 8y ago
Apropos of this, but about human incompetency rather than DNA tests, the Phantom of Heilbronn. A serial killer so well concealed that her very existence was only given away by DNA traces. https://en.wikipedia.org/wiki/Phantom_of_Heilbronn https://en.wikipedia.org/wiki/Phantom_of_Heilbronn
- hopler 8y ago
  Your Wikipedia link says that the Phantom is a myth and the DNA was contamination from the kit factory.
  - EliRivers 8y ago
    Yes it does, good reading skills. That's the point. Human incompetency, related to DNA forensics. I didn't outright say that, though; the idea was that I would prepare you for a story of a serial killer so well concealed that her existence could only be inferred from trace DNA, but THEN there's the surprise twist - there is no serial killer, it's a trick of human incompetency. BOOM! The twist has a much greater effect because I didn't give it away at the start, but the twist also ties back in to my original statement about human incompetency - but NOT where you may have been expecting, and subverted expectations are a key part of surprise twists - and the story is also about DNA forensics. That's the previous subject again, the reader having been led around in an interesting and surprising circle.

And along those lines, I've heard that DNA tests aren't reliable either.

by ColanR 8y ago

And along those lines, I've heard that DNA tests aren't reliable either.

DNA tests prove a very specific narrow fact, and it’s up to lawyers to string together a hypothesis in order for it to have relevance. Unfortunately all too often the DNA test result is used as proof of a hypothesis, not of the mere presence of surprisingly similar DNA in a swab sample.

mjevans 8y ago

The problem with the DNA tests we use today, assuming even perfect cases with zero contamination or mistakes (whole other argument there), is that (in my non-expert understanding): * Taking the samples and using several solutions to snip them at given patterns. * Taking another solution and promoting replication of the fragments. * Using some dye and a weight sorting channel to bin the fragments by weight. This results in a kind of 'bar code' that graphs the distribution of weights as a VERY crude hash of samples of DNA. It's pretty useful for determining things like IF it is LIKELY that individuals are related. It's also pretty good at confirming negatives (We're sure someone is NOT related to X). It's not that great at confirming positives. That is, the results are both subjective and ambiguous given the hashing. A "positive" result here is really more of an "OK, it's likely we should run the real and expensive check, evaluate if someone might be a suspect by other merits, etc." I would, offhand, consider a "positive" above to be enough evidence to produce /suspicion/ and /warrants/ to locate other specific evidence to ascertain an actual guilt or innocence based on harder evidence. If an actual "sequence the whole set of samples" option enters the realm of feasible tests then it would also warrant actually doing that. Come to think of it, I'm not sure how the Ancestry/etc novelty DNA tests work. I'd assume they've isolated a few specific markers they're looking for and the processes are optimized towards identifying those and comparing combinations for those specific traits.

samatman 8y ago
You’ve described electrophoresis DNA fingerprinting correctly, then jumped to the unjustified conclusion that it’s a VERY crude hash. Not so, it’s a fairly excellent hash, one with few collisions. Better than a fingerprint. If it looks like your DNA, it most likely is. Source: was an electrophoresis tech (not forensic) in a previous life. And yes, the usual genetics sites do SNP tests, which are cheaper than full sequencing but mean that they only find what they’re already looking for. The price of full sequencing is dropping rapidly, I look forward to it being a standard part of medical practice, it will save a lot of lives and improve quality of care.
- Dylan16807 8y ago
  Better than a fingerprint is still not great!
  - samatman 8y ago
    Well, several decimal orders of magnitude better than a fingerprint, to be accurate.

EliRivers 8y ago

Apropos of this, but about human incompetency rather than DNA tests, the Phantom of Heilbronn. A serial killer so well concealed that her very existence was only given away by DNA traces. https://en.wikipedia.org/wiki/Phantom_of_Heilbronn https://en.wikipedia.org/wiki/Phantom_of_Heilbronn

hopler 8y ago
Your Wikipedia link says that the Phantom is a myth and the DNA was contamination from the kit factory.
- EliRivers 8y ago
  Yes it does, good reading skills. That's the point. Human incompetency, related to DNA forensics. I didn't outright say that, though; the idea was that I would prepare you for a story of a serial killer so well concealed that her existence could only be inferred from trace DNA, but THEN there's the surprise twist - there is no serial killer, it's a trick of human incompetency. BOOM! The twist has a much greater effect because I didn't give it away at the start, but the twist also ties back in to my original statement about human incompetency - but NOT where you may have been expecting, and subverted expectations are a key part of surprise twists - and the story is also about DNA forensics. That's the previous subject again, the reader having been led around in an interesting and surprising circle.