how do dna tests work? such as 123 and me, ancestry dna, etc. they probably don’t have the dna of random people from 100s of years ago right? and if it goes off of last names, how is it reliable?
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They don’t go off last names
The sequence your DNA, literally decoding it into all the base pairs that make up your DNA. They look in the database for people with similar DNA. You get half your DNA from your mom and half from you dad, so if they find someone who has ~50% overlap with you, then it’s your mom or dad or a sibling. ~25% would be a aunt, uncle, niece or nephew, or grandparents. 12.5% would be a first cousin etc
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