The theoretical Khazar contribution to the bloodline of modern Ashkenazi Jews proposed by some historians is politically sensitive. Current genetic studies show that Jewish Y-DNA tends to come from Middle Eastern populations whereas studies that take into account mtDNA show no relation to middle eastern ethnic groups. [1] So although Khazars might have been absorbed into the Jewish population it is unlikely that they formed a large percentage of the ancestors of modern Ashkenazim. [2]

http://en.wikipedia.org/wiki/Khazars