There is the old saying, "Blood is thicker than water." The saying is meant to sum up the closeness and devotion of kin. People would do for kin or suffer for kin in ways they might not do or suffer for others.
And in Buddhism, there is talk of "lineage" -- connections that, according to a much-hammered cliche, "reach all the way back to Shakyamuni Buddha." In Zen, there is "mind to mind" transmission from teacher to student and on and on until this very moment. Zen Buddhists might not do for others what kin might, but the closeness that "lineage" supposes is pretty deep or deeply felt or deeply revered or ... well, you get the gist.
And I would not fault "blood" or "lineage" ... but that doesn't mean I can't be curious. After all the wondrous connections are cited, all of the respect is paid, all of the sacrifices are made ... gawd! it can seem closer than close! -- what precisely constitutes "blood" or "lineage?"
Lord knows there is some truth in them somewhere, but it seems to me that the minute anyone speaks of it, "blood" and "lineage" fade like a rose in the fall. In the speaking, the elevating, the praising, the dissecting, or the advertising, what is true about "blood" and "lineage" scampers like a unicorn into the recesses of a forest where all dear things find refuge.
I emailed a Zen monk chum once and asked him what lineage was. He didn't respond and I don't blame him for a second. I know other Buddhist folks who can twist your knickers from now until Christmas on the topic. I guess it's all right as a mild advertising gambit. But I wonder sometimes if they are not vaguely ashamed...speaking of the truth as if it were true.