Not necessarily true. Outbreeding tends to encourage outbreeding to the point that the Jewish self-identity is lost. Hypothetically a bunch of "diluted Jews" could claim 100% Jewishness, but I think the opposite would be true, that within a few generations they would claim no religion or a gentile religion. It is hard to imagine someone who would claim a Jewish identity without an understanding of Jewish religion and practice. Obviously if such a person were intending to keep Judaism prominent, he/she would know that they were not 100% pure and would rectify it through conversion, or if it were a male he simply needs to marry a matrilinially descended Jewish woman and practice Judaism, thus raising children who were both ethnically and spiritually Jewish.
It's not strictly about genes and a direct line -- although I have no doubt that there is a core direct line... but rather it is a large group that is bred around this direct line. Some breed all the way out of the line, others breed themselves back towards the line, while some stay completely in the line.
"Not necessarily true. Outbreeding tends to encourage outbreeding to the point that the Jewish self-identity is lost. Hypothetically a bunch of "diluted Jews" could claim 100% Jewishness, but I think the opposite would be true, that within a few generations they would claim no religion or a gentile religion."
A great deal of outbreeding isn't necessary. The Jews have been in diaspora for 2,000 years. If only %1 of the Jews in each generation engage in outbreeding and raise their kids as Jews, over a period of 2,000 years the Jewish population could end up with a high proprotion of Gentile genes. Over a long period of time a very small amount of outbreeding can produce the effect I describe. This can account for why Jews tend to resemble the people amongst which they live while at the same time still maintain some genetic distinctiveness.