If you are really interested in the topic, I suggest that you read works by genetic researchers. A very entertaining, highly readable book is "The Seven Daughters of Eve," likely to be available at your local library.
http://www.amazon.com/exec/obidos/tg/detail/-/0393020185/002-2109841-8098459?vi=glance DNA isn't random. You know that, because you know that paternity tests prove or disprove that a man is the father of a child. DNA tests can also prove that someone is related to someone else, so historians and geneticists have used DNA to prove that one of Sally Hemmings' children was fathered by someone related to Thomas Jefferson, although not necessarily Tom himself. Similarly, they've used DNA to prove that several of the bodies in a mass grave were Tsar Nicholas Romanov, Tsarina Alexandra, and three of their children, but no evidence was found for one of the daughters or the son, Prince Alexei. Among the people who contributed DNA for those tests was the Duke of Edinbrough (Prince Phillip) who is related to the late Tsarina through a maternal line.
I do not disagree with your points, but that still does not prove that there could not be a second line of human DNA.