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An extremely rare mutation on the Y chromosome may be a genetic marker that is unique to the people who first migrated to the Americas some 30,000 years ago, researchers report.

A group of Stanford University researchers, using a new method of scanning for genetic changes, have identified a mutation that in their sample exists only in Indian populations in North and South America and in Eskimo groups.Peter A. Underhill, lead author of a paper being published Tuesday in the Proceedings of the National Academy of Sciences, said that the Y chromosome mutation occurred in a stretch of DNA that is not related to a gene but is part of "junk" DNA that separates the genes.

Mutations in the Y chromosomes are very rare, and Underhill said the genetic change probably happened in one male and was then passed on to his sons and to later generations.

"In only one chromosome in all the world there was this change, and all Native Americans may be able to trace their heritage back to that one mutational event," Under-hill said in an interview.

Michael Hammer, a research scientist at the University of Arizona in the field of prehistoric genetics, said the marker found by Underhill and his colleagues "is very interesting" and may be used by other researchers to study ancient migration patterns.

Hammer said Underhill's method of looking for mutations "is potentially important," but he cautioned, "It needs to be replicated in other laboratories."

The change involved a switch from what is called the C allele to the T allele in a specific segment of the Y chromosome.

Studies of more than 500 DNA samples from populations around the world, Underhill said, showed that the T allele was present only in specimens from Indian and Eskimo males. The T allele was found, for instance, in Navajo, Mayan and Colombian Indians and among North American Eskimos, but not among samples from Asia, Africa, Europe or Oceania.

The Y chromosome is present only in males and is passed from generation to generation only through fathers to sons. Most of the DNA in this chromosome is very stable, said Underhill, and finding a mutation, or polymorphism, is rare.

Underhill and another Stanford researcher, Peter J. Oefner, developed a laboratory technique that allows a rapid search for rare mutations. Using this technique may make it possible to use the Y chromosome to trace the ancient migration patterns of humans, said Underhill.