Pelasgians (pre-Minoan Greeks, or Helladic Greeks) belonged to an admixture of I, E-V13

Ancient Greeks : Pelasgians (pre-Minoan Greeks, or Helladic Greeks) belonged to an admixture of I, E-V13, T and G2a. E-V13 and T probably arrived in Greece from the Levant (and ultimately from Egypt, hence the small percentage of T) in the early Neolithic, 8,500 years ago. G2a came from the Caucasus approximately 6,000 years ago as herders of sheep and goats (and early miners ?).
Minoan Greeks migrated from Mesopotamia via Anatolia. They were mostly J2 people, but probably had some E too.
Mycenaean Greeks arrived around 3,600 years ago from the Ukrainian steppes. They were an Indo-European people belonging to R1a. So were the Macedonians and the Thracians (hence the higher density of R1a in northern Greece).Greece was invaded by the Dorians around 1200 BCE. Nobody knows who they were or where they came from, but the high percentage of R1b in the regions where they settled (Peloponese, Crete) strongly suggest that they were R1b people. The events are linked to the Sea Peoples (see below), who were probably R1b people from the north-east of the Black Sea, or early Celts from central Europe.
Greek historians sometimes mention that the Dorians were the descendants of the Trojans who came back to avenge their ancestors. The Trojans were an Indo-European people related to the R1b Hittites (see below). This would also explain why there is about the same percentage of R1b and R1a in modern Greece. Each correspond to a different wave of Indo-European invader. They only make up 12% of the population (each) because the Neolithic farmers (especially E and J2) were already well-established and numerous by that time.

By definition, the Pelasgians are the autochthonous, native inhabitants of Mesolithic Greece. They did not speak Greek, but their language may have contributed to a few loan words in ancient Greek. Modern Greece is a melting pot. The Pelasgians were first there probably haplogroup I2, then came early farmers from the Near East (E1b1b and J2), herders from the Caucasus and Anatolia (G2), then the Mycenaeans (I would think R1a), the Dorians (possibly R1b) and others. Ancient Greek language and religion is surely an admixture of all this, although the Indo-European component is stronger than the rest. I suppose that this is because the Mycenaeans and Dorians were the last invaders, whose language and culture eventually stuck, after some adaptation to local idiosyncrasies.

http://vieilleeurope.wordpress.com/2011/02/06/eupedia-maciamo-y-dna-haplogroups-of-ancient-civilizations/