[ergothus]

My understanding is that DNA is basically unique, but the patterns use for conventional DNA analysis are not. While they may be statistically close enough to unique to trust over the whole population, there have been different issues when you start scoping things to different racial groups or areas with smaller gene pools (not dangerously small, just smaller).

Because actually sequencing and comparing someones entire DNA would be prohibitively expensive if done for every case, they just look for a set of markers and assume that a large enough collection is close enough without accounting for distribution patterns of those markers AMONG THE SAMPLE POPULATION. (e.g. you don't have to look hard in small towns or in populations of the same heritage to notice a lot of physical characteristics that are quite common in that sub-population while being pretty distinct in the human population at large, the same is true for these markers.) . Since crimes often involve suspects from the same location and/or racial profile, a rule that is pretty reliable for the earth is not so reliable for this town/community/group of suspects.

So the science is a problem even before you get to human error. The odds of a false match take hits of multiple orders of magnitude.

Note: I'm not qualified to speak on the topic, but I've followed the topic with interest at a layman level for a while - be glad to hear from someone who IS qualified to speak on it to shed more light on why I'm right/wrong.

[bsder]

> My understanding is that DNA is basically unique

It is, in a lab environment with lots of source material.

If you are taking small samples of DNA from terribly dirty places and then amplifying the hell out of it, you get massive amounts of contamination.

Rape kits are "mostly" reliable under most circumstances, but have some problematic edge cases. Swabs from crime scenes, on the other hand, are generally garbage.

[coldtea]

>My understanding is that DNA is basically unique, but the patterns use for conventional DNA analysis are not.

Even if they were, unless it's something like blood DNA after an attacker gets hurt, liquids from a rape, etc. it's trivial:

1) for the culprit to take another's DNA and place it on a crime scene

2) for an innocent to leave DNA at what would later be a crime scene

3) for police (or anyone with access) to add someone's DNA at a crime scene [1]

And the worst thing is, the DNA match then is considered "irrefutable" evidence...

[1] If you (3) is far-fetched, you haven't been following 100 years of police tampering to frame people, even in the USA, and much more in places like Latin America, etc. Just two recent examples:

https://www.buzzfeednews.com/article/melissasegura/detective...

https://www.huffpost.com/entry/baltimore-cop-drug-video-indi...