Originally posted to DNA for Everyone.
So, 60 Minutes has broadcast a report on the tracing of DNA. Leslie made a statement, "The are just two bits of DNA which remain pure. The Y chromosome which passes directly from father to son. And something called mitochondrial DNA which passes unchanged from mother to child." Logically speaking, if the mDNA really passes unchanged with every generation, then everyone has the exact same mDNA. However, that is not the case. A limited number of aberrations have occurred in the mDNA over time. Those changes are called markers and passed from mother to child. Identification of these markers and estimating when and which groups they occurred is the process behind identifying to whom an individual is related.
Leslie did make a really good point. As you trace back through the Y chromosome and mDNA, the further back one goes, the smaller the ratio of these markers can provide. So going back one generation, you can see info on both individuals. Going back to the second generation, you can only see 2 of 4 individuals, three see 2 of 8, eight see 2 of 256, etc.
Another fuzziness the report failed to explain is the testing really only matches individuals to currently living individuals who share similar markers. So, you don't really see who your ancestors are. An African-American woman showcased, got back several matches to individuals belonging to tribes in Sierra Leone, Ivory Coast, and Senegal. I have first and second cousins all over the place. Is it right to assume only individuals taken from Africa and brought to the Americas are the ones who have left the tribe? It seems hard to believe family members left in Africa occupy the same huts as when we left.
A good book to read on Y chromosome and mDNA tracing to determine the origins of all the world's population to common ancestors in Africa is The Journey of Man by Spencer Wells. That is a little further back.