Blood quantum is what is used by the tribes and the government to determine tribal membership. Tell them they are wrong.
1) Who is "them"?
2) It is only used by indians because the US Government has forced it upon the Indians for it's own convenience, NOT for the convenience of the Indians.
3) To the best of my knowledge, the Indians have never claimed that it's a scientifically valid methodology, as you vehemently argue that it is (it's NOT). It's simply a loose method of establishing an extremely simplistic (and inaccurate) classification of heritage. It categorically does NOT establish "genetic" quanta. It only indicates a probability, and even then, only superficially. If ANYONE claims otherwise, they ARE wrong.
4) The practice of the blood quantum criterion, is not sustainable due to the inherent limitations of the testing process as well as the need for genetic diversity within the tribes.
5) The whole blood quantum institution, is ultimately meaningless except for the purposes of legalities.
For those who choose to live in a world of historical and scientific reality: