In numerous recent studies, the mitochondrial DNA of Greeks was examined and was found to be predominantly Caucasoid with only infrequent presence of "erratic" sequences from non-Caucasoid sources. Mitochondrial DNA ("mtDNA") is inherited from one's mother and is thus a good way to establish the maternal ancestry of a population.
The most comprehensive European-wide study of mtDNA is  in which 125 Greeks were sampled among thousands of Europeans. The Greeks and the Albanians appear in the "Mediterranean-East" category of the study. Greeks tested belonged overwhelmingly to the Caucasoid-specific haplogroups ("Seven Daughters of Eve" popularized by Bryan Sykes' book).
The "erratic" sequences include a Sub-Saharan African (L1a) sequence, which was derived from the Albanian part of the sample . The other two sequences non-attributed to a European founder are members of haplogroups prevalent in Asia, M and D. Thus, the total percentage of erratics in the Greek sample was 1.6%. The Greeks, like most Europeans are fairly pure in terms of their maternal ancestry.
It is sometimes argued that the Greeks absorbed large numbers of Negro slaves or immigrants. There is no evidence of such an event in Greek mtDNA. If it ever took place, it was so limited in scope that not a single sequence in a total of 125 could be found.
The number of non-European sequences in the rest of Europe is also small, while in the Near East it is about 5%, only slightly larger. One can easily verify that Sub-Saharan African admixture (L sequences) has been detected in Scandinavia (Sweden, Norway and Iceland - 0.6%), Southeastern Europe (Bulgaria/Romania - 0.5%), Central Mediterranean (Italy and Sardinia - 1.7%; mostly in Sardinia), the Mediterranean West (Spain and Portugal - 3.7%), North Central Europe (Poles, Czechs, Germans, Danes - 0.9%), North Western Europe (Britain, Ireland and France - 0.4%). In another recent study  on Norwegians, an L2 Sub-Saharan African sequence was found in the sample of 74 Norwegians (1.4% Sub-Saharan admixture). Finally  showed 0.5% to 1.2% introgression of Sub-Saharan African genes into the European American gene pool.
The main conclusion to be drawn from these studies, is that Caucasoids of European descent have negligible traces of non-Caucasoid maternal admixture. Sub-Saharan African traces of such ancestry are found at levels of about 1% in many populations.
A new study  has collected mtDNA haplogroup frequency data from several populations, including Greece, and the islands of Crete, Lemnos, and Rhodes. The frequency data are shown in the table below. The Sub-Saharan influence amounts to 2 L sequences in Crete, for a total of 1% of the island, and 0.4% of the total studied Greeks.The European average of L sequences (N=10,859) is 0.7%. M sequences cannot be attributed to a racial origin, since haplogroup M is divided into different clades which occur at appreciable frequencies in the Caucasus, Middle East, North Africa, South as well as East Asia, and could thus have diverse origins; Greek M sequences have been attributed to the West Eurasian M1 clade . M sequences occur in Greece at a total rate of 1.7%. The average of them in Europe (N=10,859) is 0.8%.
- Richards et al., Tracing European founder lineages in the Near Eastern mtDNA pool. American Journal of Human Genetics, 67, 1251-1276. Online paper and supplementary data in Vincent Macaulay's home page.
- Michele Belledi et al., Maternal and paternal lineages in Albania and the genetic structure of Indo-European populations, European Journal of Human Genetics, 8, 480 - 486 (01 Jul 2000)
- Giuseppe Passarino et al., Different genetic components in the Norwegian population revealed by the analysis of mtDNA and Y chromosome polymorphisms, European Journal of Human Genetics10, 521 - 529 (23 Aug 2002)
- Esteban J. Parra et al., Estimating African American Admixture Proportions by Use of Population-Specific Alleles, Am. J. Hum. Genet., 63:1839-1851, 1998
- A. Achilli et al., Mitochondrial DNA Variation of Modern Tuscans Supports the Near Eastern Origin of Etruscans, American Journal of Human Genetics
(in press) (2007)
- A. Olivieri et al., The mtDNA Legacy of the Levantine Early Upper Palaeolithic in Africa, Science, Vol. 314. no. 5806, pp. 1767 - 1770 (2006)