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Misico dux Vandalorum
"12% percent of modern Ashkenazi Jews are of Khazar descent"
This is "popular genetics" or something like this - in other words, rubbish pseudo-science.
There is no any scientific way to check how many Jews are descended from Khazars.
You should also remember that 10 generations ago each person had even up to 1024 ancestors - who lived more or less at the same time (as one generation). Researching haplogroups gives us answer only about 2 of those 1024 ancestors who lived 10 generations ago - i.e. direct female lineage ancestor and direct male lineage ancestor. But nothing about 1022 ancestors of all lateral lineages - and we also inherit genes from them, not only from 2 direct lineages.
That being said, despite these shortages of haplogroup research, it is still useful to understand how populations used to spread around the globe.