From my limited understanding of the religion being Jewish requires being born of Jewish parents or following some moderately complex procedures. From that I would assume that every Jew alive today is either a decedent of Abraham or is a first generation convert. A quick search on line indicates this might not be the case. But I am unsure of the validity of them.
With genetic testing, this figure can only get more and more accurate over time. This article argues that science proves that 75% of Jews trace back to the middle east:
Im not sure it would be possible to trace much more accurately than this.