3 ms·

That's not how it works, the chances of two people matching as identical (on the SNP tested by 23andme) by random is astronomically low - orders of magnitude hi

by ig1 8y ago

That's not how it works, the chances of two people matching as identical (on the SNP tested by 23andme) by random is astronomically low - orders of magnitude higher than the total number of humans who've ever lived. Generally at a 7cM overlap you have 50:50 chance whether you're related to that person or not. Anything over say 150cM you're virtually certain to be related (ignoring endogamous populations). At 3,400cM (full-match) it's not even worth considering randomness.

Retric 8y ago
You’re assuming a lot with those statistics. Paternity tests involve random ~50% of your dna. Sperm samples are generally contaminated and thus less clear cut. But, people’s relatives have been used in the past for these tests dropping the bar even further. However, being related genetically is not enough. Some people have twin siblings they don’t know about pointing out being related is not nessisarily mean you know anything about the other person allowing for false positives even at 3,400cM. Unlikely sure, but harm comes in many forms. Saying I can’t think of anything is a long way from saying it’s safe.
- ig1 8y ago
  Those aren't false positives in that they represent real genetic relationships (i.e they're identical by descent), as the database size increases you don't increase the false positive rate in any meaningful sense at higher match levels. If you've got a identical twin then you're right that their child will be genetically indistinguishable from your child. But that's down to the nature of genetics rather than anything to do with the reliability of testing or database size.
  - Retric 8y ago
    False positives don't necessarily mean the test was executed incorrectly. It often means you are testing for something closely related to the information you want. If a test failed because _ then it failed independent of why it failed. If your long lost twin causes you to be involved with a paternity suit or homicide investigation then that's both a false positive and a significant problem.

by ig1 8y ago

Retric 8y ago

You’re assuming a lot with those statistics. Paternity tests involve random ~50% of your dna. Sperm samples are generally contaminated and thus less clear cut. But, people’s relatives have been used in the past for these tests dropping the bar even further. However, being related genetically is not enough. Some people have twin siblings they don’t know about pointing out being related is not nessisarily mean you know anything about the other person allowing for false positives even at 3,400cM. Unlikely sure, but harm comes in many forms. Saying I can’t think of anything is a long way from saying it’s safe.

ig1 8y ago
Those aren't false positives in that they represent real genetic relationships (i.e they're identical by descent), as the database size increases you don't increase the false positive rate in any meaningful sense at higher match levels. If you've got a identical twin then you're right that their child will be genetically indistinguishable from your child. But that's down to the nature of genetics rather than anything to do with the reliability of testing or database size.
- Retric 8y ago
  False positives don't necessarily mean the test was executed incorrectly. It often means you are testing for something closely related to the information you want. If a test failed because _ then it failed independent of why it failed. If your long lost twin causes you to be involved with a paternity suit or homicide investigation then that's both a false positive and a significant problem.