According to wikipedia, "the theoretical risk of a coincidental match is 1 in 100 billion (100,000,000,000) although the practical risk is actually 1 in 1,000 because monozygotic twins are 0.2% of the human population", so it looks like some twins will cause an issue for DNA-based convictions, but I think a non-twin sibling is probably diverse enough? I have different eye colour, ears, food intolerances to my brothers, so I feel like our DNA is pretty different ("pretty" on a scale of human to human comparisons, since 99.9% of DNA is identical between humans or something like that).
I'd argue that while a DNA match could be a very good starting point for a conviction, other evidence should also be presented, for the reasons you suggest. For example, there is the case of the Phantom of Heillbron (https://en.wikipedia.org/wiki/Phantom_of_Heilbronn), where European police found DNA evidence of a woman being at many diverse crime scenes, but it turned out the cotton swabs used for testing were contaminated by at worker at the factory (The swabs weren't certified for police work)
That is a coincidental match, though! It's the risk of a false positive when you compare exactly two samples.
The problem is that we're proposing a DNA database, so you're now comparing 1 sample found at a crime scene with 300ish million people. That's a 300M in 100B chance, or 1 in 333, of some person having a false positive match.
Now combine that with thousands of samples being tested every day (because let's be honest, why wouldn't you use such a database for literally every single crime?) and you're looking at dozens of false-positives per day. It is only a matter of time before it isn't someone with an airtight alibi at the other side of the country, but someone reasonably-close who was just sitting alone on their couch at home all weekend. How are they going to prove they weren't involved in the crime?
This is why large-scale DNA matching is a horrible starting point for a conviction. It gives you a false sense of security, and it makes the investigators lazy. In most cases it is far better if DNA is used as the final nail in the coffin when you've already got a solid case against a suspect.
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According to wikipedia, "the theoretical risk of a coincidental match is 1 in 100 billion (100,000,000,000) although the practical risk is actually 1 in 1,000 because monozygotic twins are 0.2% of the human population", so it looks like some twins will cause an issue for DNA-based convictions, but I think a non-twin sibling is probably diverse enough? I have different eye colour, ears, food intolerances to my brothers, so I feel like our DNA is pretty different ("pretty" on a scale of human to human comparisons, since 99.9% of DNA is identical between humans or something like that).
I'd argue that while a DNA match could be a very good starting point for a conviction, other evidence should also be presented, for the reasons you suggest. For example, there is the case of the Phantom of Heillbron (https://en.wikipedia.org/wiki/Phantom_of_Heilbronn), where European police found DNA evidence of a woman being at many diverse crime scenes, but it turned out the cotton swabs used for testing were contaminated by at worker at the factory (The swabs weren't certified for police work)
That is a coincidental match, though! It's the risk of a false positive when you compare exactly two samples.
The problem is that we're proposing a DNA database, so you're now comparing 1 sample found at a crime scene with 300ish million people. That's a 300M in 100B chance, or 1 in 333, of some person having a false positive match.
Now combine that with thousands of samples being tested every day (because let's be honest, why wouldn't you use such a database for literally every single crime?) and you're looking at dozens of false-positives per day. It is only a matter of time before it isn't someone with an airtight alibi at the other side of the country, but someone reasonably-close who was just sitting alone on their couch at home all weekend. How are they going to prove they weren't involved in the crime?
This is why large-scale DNA matching is a horrible starting point for a conviction. It gives you a false sense of security, and it makes the investigators lazy. In most cases it is far better if DNA is used as the final nail in the coffin when you've already got a solid case against a suspect.
Sounds like perfect for rich people, they'll have the resources to argue in court.