On her blog today, Rebecca Skloot encourages the genealogical world to take a deep breath. As her December 2003 PopSci article argued, DNA tests for ethnic heritage are pretty good at proving what you are, but pretty lousy at proving what you're not.
These tests can be fun, and they have some definite use in medical research, but they simply can't tell you anything definitive about your heredity unless you're testing your DNA and comparing it to someone else's to find out if you're related. These tests most certainly can't tell you what you're not -- as in, you're not African-American. [Link]Genealogy has lately been cast as the ugly step-sister of genetealogy—liable at any moment to be undermined by science. But Skloot cautions that one shouldn't lop off a limb of the family tree just because a DNA lab turns up nothing. According to family legend, Skloot's great-great-grandmother Elenor was black, but you wouldn't know it from her test results.
To date, no ancestry test can rule out part of a person's history simply because it doesn't detect it. When I told Noah Rosenberg that my DNA tests found no evidence of Elenor Hickenbottom, he chuckled. "Let's put it this way," he said. "I wouldn't discount your family story just yet." [Link]