COARSEGOLD, Calif. — The six-page, single-spaced letter that Nancy Dondero and about 50 of her relatives received last month was generously salted with legal citations and footnotes. But its meaning was brutally simple. “It is the decision by a majority of the Tribal Council,” the letter said, “that you are hereby disenrolled.”

And with that, Ms. Dondero’s official membership in the Picayune Rancheria of the Chukchansi Indians, the cultural identity card she had carried all her life, summarily ended.

“That’s it,” Ms. Dondero, 58, said. “We’re tribeless.”

Ms. Dondero and her clan have joined thousands of Indians in California who have been kicked out of their tribes in recent years for the crime of not being of the proper bloodline.

For centuries, American Indian tribes have banished people as punishment for serious offenses. But only in recent years, experts say, have they begun routinely disenrolling Indians deemed inauthentic members of a group. And California, with dozens of tiny tribes that were decimated, scattered and then reconstituted, often out of ethnically mixed Indians, is the national hotbed of the trend.