In human mitochondrial genetics, Haplogroup X is a human mitochondrial DNA (mtDNA) haplogroup which can be used to define genetic populations. The genetic sequences of haplogroup X diverged originally from haplogroup N, and subsequently further diverged about 20,000 to 30,000 years ago to give two sub-groups, X1 and X2. Overall haplogroup X accounts for about 2% of the population of Europe, the Near East and North Africa. Sub-group X1 is much the less numerous, and restricted to North and East Africa, and also the Near East. Sub-group X2 appears to have undergone extensive population expansion and dispersal around or soon after the last glacial maximum, about 21,000 years ago. It is more strongly present in the Near East, the Caucasus, and Mediterranean Europe; and somewhat less strongly present in the rest of Europe. Particular concentrations appear in Georgia (8%), the Orkney Islands (7%) and amongst the Israeli Druze (26%); the latter are presumably due to a founder effect.
Haplogroup X is also one of the five haplogroups found in the indigenous peoples of the Americas, occurring at a frequency of about 3%. It is found with particular prevalence in the Ojibwa (25%) from the Great Lakes, the Sioux (15%), the Nuu-Chah-Nulth (11%–13%), the Navajo (7%), and the Yakima (5%).
Unlike the four main Native American haplogroups (haplogroup A, haplogroup B, haplogroup C, and haplogroup D), X is not at all associated with East Asia. This has led to speculation that the haplogroup X occurrences might indicate a minority European ancestry for some Native Americans. A particularly concrete suggestion was that the genetic inheritance might reflect transatlantic links perhaps made in about 20,000 BC by the Solutreans, a stone-age culture excavated in south-west France and Spain, who it was suggested might have carried on an Atlantic-spanning maritime life around the margins of the retreating ice-age Atlantic ice fields, similar to the existence of the present-day Inuit.
The speculation abated somewhat when haplogroup X individuals were discovered in Altaia in South Siberia. However, more detailed examination has shown that the Altaian sequences are all almost identical, suggesting that they arrived in the area probably from the South Caucasus more recently than 5000 BC. On the other hand, the North American haplogroup X DNA (now called subgroup X2a) is as different from any of the old world X2 lineages as they are from each other. This suggests that the ancestors of the X2a population presumably separated very early from all of the other X2 lineages, but gives little clue as yet to the true path of their migration from the Near East to North America.
In his popular book The Seven Daughters of Eve, Bryan Sykes named the originator of this mtDNA haplogroup Xenia.