Eli5 How do DNA tests trace not only your relatives?

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I assume for tracing your heritage they look at DNA segments that are common in certain areas but how do they trace back your lineage if they only recently started DNA testing. Did they dig up all the corpses and swab them?

In: Biology
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They start with a large sample size of people who already had their heritage known and documented, looked for similarities and differences within those groups, and use that information.

Certain communities share “genetic markers” that are unique to them, therefore if you have said marker, your DNA or History includes that region. It’s like a DNA map of where your eyes been and who you’ve been with

There are genetic mutations that arise in DNA over time. While we often associate mutations with disease, the vast majority of those mutations are harmless. Within a geographic region where people have been intermarrying and having offspring for many generations, mutations can get passed along and become quite common within a population. As an example, you might sequence the DNA of living people on Crete and find that they have two or three mutations that occur at a high frequency and are also unique to their population. If someone in Australia or the United States is found to have the same mutation, it may be because they have an ancestor from Crete. I say “may” because it is not a certainty. The type of mutation and its frequency in the population allow scientists to assign a probability to the likelihood of someone having ties to a geographic area.