Originally Posted by Jonathan Morningstar
Let's say an African American has a DNA match in Nigeria. Does that actually mean the ancestry is Nigerian? What if the ancestors where in Ghana or Sierra Leone 300yrs ago?
Isn't the African database too small?
Ususally this type of test does either the Paternal line (Y-DNA) or the Maternal line (mtDNA). It just shows the likely origin of each haplogroup.
So for instance if you go back, as you say, 300 years you will have approximately 10 generations (30 years each). That would give you approximately 1000 individual ancestors. The above tests only show you one ancestor for each line (Paternal and Maternal) - where your Y-chromosome came from and where your mtDNA came from. This is based on the statistical measure of those markers being most prevelant for those areas - say for instance Nigeria.
So as an example someone could look totally 'white' yet have a Y-Chromosome that is most likely from Nigeria because somewhere (most likely far back) he had an ancestor that came from this region - a 'black' man.
All other genetic info needs to be tested for (Autosomal DNA) to account for all the other individuals in your ancestry (the remaining 998).
As far as the database being to small - I am not sure. It would not be hard to find out.
Here is some info Genealogical DNA test - Wikipedia, the free encyclopedia
Hope that helps!