Mitochondrial DNA is a type of DNA that is passed from mother to child. During the event of fertilisation, the egg is fertilised by the male gamete. The human spermatozoid contributes with the Y chromosome and with 22 autosomal chromosomes. Everything else necessary for the development of the embryo comes from the egg. This includes, of course, the mitochondrial DNA. A mitochondria is a cellular organite responsible for the production of the energy that allows eukaryotic cells to work properly. It is of great use to the study of human populational migrations, since it is passed unchanged from mother to son and from mother to daughter.

Scientists believe that there is about one change in the mtDNA (through mutation) every 10.000 years from the first modern human, referred to as mitochondrial Eve, who lived approximately 150.000 years ago in Africa. This allowed them to group mtDNA into families called haplogroups. An mtDNA haplogroup is therefore determined by polymorphisms [polymorphisms are the different possible genetic sequences for a given loci] that occurred thousands of years ago. Haplotypes are sub-clusters of haplogroups, and the polymorphisms that determine them are less prevalent, and a lot more recent. Most of the polymorphisms that determine haplogroups are continent specific. An haplogroup is coded with a capital letter (e.g. U) while its sub-clusters are coded with a number (e.g. U6).

The earliest work with mtDNA started in the early 80s. Scientists either digested a small number of DNA samples using a large number of restriction enzymes [restriction enzymes are proteins that are able to cut DNA sequences in precise known locations], or they digested a large number of samples with a single enzyme. In either situation, if the DNA molecules are any different from each other, the enzymes will cut the DNA chains in different places (given the specificity of the enzymes), thus producing chopped sequences of different sizes.

The analysis of mtDNA became an important tool for the study of human populational structure and history (Stoneking et al, 1993). Most phylogenetic analyses of mtDNA have been based on sequence variation in the first hypervariable segment of the control region (HVS-1) which is the most variable part of mtDNA. RFLP studies [an RFLP is a Restriction Fragment Length Polymorphism, and they are the chopped sequences obtained through the digestion of a DNA chain by restriction enzymes] of the mtDNA coding region have been used to classify mtDNA haplogroups.

Mitochondrial DNA haplogroups

As it was previously mentioned, the first modern human lived approximately 150.000 years ago in Africa. Soon afterwards, an early expansion of modern humans populated Africa and left its genetic fingerprints in mtDNA (the L1 haplogroup) found particularly in the KhoiSan (Bushmen) people today. This early expansion appears not to have left Africa, probably due to the harsh ice age climate and to the presence of the Man of Neanderthal in Eurasia. Tens of thousands of years later, an east African expansion (60-80.000 years ago) repopulated Africa (L2 and L3 types) and ultimately led to a migration out of Africa of at least one mtDNA subtype (L3a).

L1 represents 29% of all African mtDNA's, while L2 encompasses some 34%. The remaining 37% belong to haplogroup L3, an heterogeneous group of 4 lineages. One of these lineages, defined by the loss of the Ddel site at the base pair 10394, represents only a few percent of the African mtDNAs, but appears to be the progenitor of roughly half of all European, Asian and native American mtDNAs.

European mtDNAs can be subdivided into two groups according to the presence (25%) or absence (75%) of the DdeI site at bp10394. About 99% of the European mtDNAs can subsumed within 9 mtDNA haplogroups, designated H, I, J, K, T, U, V, W and X (Torroni et al. 1996), and these haplogroups can be further subsumed within four mtDNA haplogroup clusters, HV, UK, TJ and WIX (Richards et al. 1998). In Asia, 77% of all mtDNA are encompassed within a super-haplogroup M defined by a DdeI site gain at bp 10394 and an AluI site gain at bp 10397 (Ballinger et al. 1992a, Torroni et al. 1993, Chen et al. 1995b, Wallace 1995). Haplogroup M is subdivided into smaller sub-haplogroups designated C, D, G and E. Most of the remaining Asian mtDNA are encompassed by haplogroups A, B and F (Torroni et al. 1994). Essentially all Native American mtDNAs fall into haplogroups A, B, C and D (Torroni & Wallace 1994).

African mtDNA haplogroups

L1 (L1a, L1b, L1c) - Populations carrying L1 are the descendants of the KhoiSan speaking peoples

L2 - This haplogroup is east African and it is related with the Bantu expansion

L2a, the most common clade (62% of the total L2), is the only one widespread all over Africa. Not surprisingly, it is also the one more associated with the Bantu and their expansion

L2b and L2c are more concentrated in western Africa (particularly Senegal) and are virtually absent in eastern Africans, in Biakaand Mbuti Pygmies, and is rare in southern Africans. It is common in some Senegalese populations.

L2d is the rarest, but it also appears mainly restricted to western Africa (particularly Western Sahara and Mauritania/Senegal).

L3 (L3b, L3d, L3e)- This haplogroup is east African and it is related with the Bantu expansion

L3* (also known as N/M/L3) - It is particularly important to make it quite clear (particularly to Neo-Nazis like "Refuting Racial Myths") that this group is specific to sub-Saharan Africa, since it is present in a great deal of the European populations including the Finns [Passarino et al. (2002) even found L2 lineages in a Norwegian sample, so the presence of African markers introduced in Nordic populations during the Neolithic shouldn't be a surprise]. This African haplogroup, mostly introduced in Europe during the Neolithic (as explained in Gonzalez et al. 2003) is distinguished from the Eurasian haplogroups M and N at the nucleotide positions 10400 and 10873, respectively (Quintana-Murci et al. 1999).

Asian and Amerindian mtDNA haplogroups

M (comprises C, D, E, G) - these 4 haplogroups represent about 77% of the Asian mtDNA

A, B, F - these haplogroups represent about 23% of the Asian mtDNA

A, B, C, D - most Amerindian mtDNA fall within these four haplogroups

M1 - This haplogroup is not Negroid, despite what Anon says. Originally it was proposed an African origin, due to the presence of the subclade M1 in Eastern Africa. However, a posterior return from Asia to Africa of these lineages is a more plausible explanation because the genetic diversity of M is much greater in India than in Ethiopia. In fact, M1 could be a branch of the Indian cluster M as ancestral motifs of the African M1 are found in M*, M3 and M4 Indian subclusters. This supposed Indian expansion to the west also reached northern areas, since evolved representatives of M4 have also been detected in Central Asia. We may consider the upper bound for this return to Africa 25.000-47.000 yr BP, the age calculated for M1 in Eastern Africa.

European mtDNA haplogroups

About 99% of the European mtDNA haplogroups fall within haplogroups H, I, J, K, T, U, V, W and X (Torroni et al. 1996). Haplogroup U6, present in the Portuguese population, is of North African (Caucasoid) origin.

taked from '' Refuting Anon's
Refuting Racial Myths ''