DNA is half truth half fiction. It is a useful tool to determine if you are related to a person or not or the obtain a general view from where you come from.
When you talk about ancient Greeks, you must specified which ones you mean. Greeks living in Islands The so called "Pelasgoi" could not possibly have exactly the same DNA as the Athenians or Spartans, or those from Thebe or Macedonians or The Thraciens. Because of the continuous wars and mixed marriages the DNA was mixed.
I just do not really understand where did you learn your Greek history. What do you think happens when the Romans took over Greece? What about the Byzantine Empire. How many emperors where really Greek??
Anyway, I wish you the best of luck in your endeavours..
PS: I do have a PhD in science from the Vienna University.