Eli5 How does DNA testing show if someone is specifically from a certain heritage like Italian or Jewish?
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They got DNA results from lots and lots of people who self-reported their heritage, and now have a large database of those. So when you do a DNA test, they compare it to the database they’ve collected earlier.
They may, indeed, ask you to self-report _your_ heritage and add _that_ data.
When DNA mutates that change is mostly permanent. A protein in a certain section changes from one type to another. When that person procreates and their children procreate, the mutation spreads in the population. In old times, when travel was less common, it allowed a group to have a higher than normal prevalence for a particular mutation.
Now, companies like Ancestry.com take the DNA from millions of people and match it up with family tree and personal data from all of them to show that people with a certain gene expression linked to freckles have family histories in Ireland, including families that never left Ireland.
So, if you have that particular gene expression, you will get recorded as being partly from that region.
There are an astronomical number of gene variations, and the more of them they can match up, the more confident they are as to where your family came from and what groups they didn’t come from.
My DNA marked me as being from a small county in Poland, and had no markers for Jewish ancestry, meaning I am Eastern European (in part) but not Jewish.
In the case of 23and me, SNPs are used. SNP stands for Single Nucleotide polymorphism. From what I can gather in a couple of searches, most companies like ancestry also use SNPs.
To get into how these SNPs work as a tool for identification, we need some background first. DNA is made out of a bunch of nucleotides, some are very important and when changed can have a large impact on genes and thus the human the DNA belongs to but most don’t. These nucleotides that do not matter can mutate and not cause issues, so can be passed on in generations. If this is a mutation in a single nucleotide causing several options or polymorphism this is a SNP.
When you know an ancestor long ago had certain mutations, all their offspring likely has at least some of these mutations. So when reading the DNA, you look for these SNP patterns. All humans are more or less related and people in a single ethnic group are generally more closely related. So if one Italians has a certain SNP, his distant cousin who is also Italian is likely to also have that SNP.
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