In human genetics, Haplogroup H is a human mitochondrial DNA (mtDNA) haplogroup. Haplogroup H is the most recent genetic variation to evolve, but is also the most common - 40% of people have it.
Mitochondrial haplogroup H is a predominantly European haplogroup that participated in a population expansion beginning approximately 20,000 years ago. Today, about 30% of all mitochondrial lineages in Europe are classified as haplogroup H. It is rather uniformly distributed throughout Europe suggesting a major role in the peopling of Europe, and descendant lineages of the original haplogroup H appear in the Near East as a result of migration. Future work will better resolve the distribution and historical characteristics of this haplogroup.
The Cambridge Reference Sequence (CRS), the human mitochondrial sequence to which all other sequences are compared, belongs to haplogroup H.
About one half of Europeans are of mt-DNA haplogroup H. The haplogroup is also common in North Africa and the Middle East. According to FamilyTreeDNA, currently the largest genetic genealogy testing firm, approximately 32% of the their database is haplogroup H. Of those H results, approximately 21% of them has a 519C mutation, which is volatile and generally not useful in characterizing subclades.
Haplogroup H is a descendant of haplogroup HV.
People in haplogroup H
Marie Antoinette, and her matrilineal ancestors back to Bertha von Putelendorf (died 1190), were of haplogroup H. (): the extended sequence is: HVR1 T16519C; HVR2 T152C, C194T, A263G and N315.1C.) If all pedigrees are assumed correct, the following other matrilineal relatives of Marie Antoinette were of haplogroup H: Maria Theresa of Austria, Pedro II of Brazil, Philip III, Duke of Burgundy, Leopold II of Belgium, Victor Emmanuel II of Italy, August III the Saxon, and Stanisław August Poniatowski. 
Among other famous members of group H are Empress Alexandra Fyodorovna, wife of the last Russian tsar Nicholas II, and all of her children. Their membership in haplogroup H was established when genetic testing was done on their remains to authenticate their identity.
As a consequence, all of relatives on the matrilineal line also have this haplogroup. These include Alexandra Fyodorovna's relatives along the matrilineal line all the way back to her ancestor Blanca Nçez de Lara (approx. 1317 - 1347), which include her grandmother Queen Victoria and her cousins the Duke of Edinburgh, Lord Mountbatten, and Wilhelm II of Germany. Other matrilineal descendants of Blanca Núñez de Lara include Marie de' Medici, Charles II, James II, and William III of England, Leopold I of Belgium, Louis XV of France, and Queen Sophia of Spain.
Advantage in infections
mtDNA haplogroup H is very common in Caucasoids, reaching frequencies of ~50%. This certainly suggested that it may confer some advantage. It would be interesting to do a study on its frequency in several archeo-DNA samples that are now available compared to that in modern populations.
Mitochondrial DNA and survival after sepsis: a prospective study
The Lancet 2005; 366:2118-2121 Simon V Baudouin et al.
Human genome evolution has been shaped by infectious disease. Although most genetic studies have focused on the immune system, recovery after sepsis is directly related to physiological reserve that is critically dependent on mitochondrial function. We investigated whether haplogroup H, the most common type of mitochondrial DNA (mtDNA) in Europe, contributes to the subtle genetic variation in survival after sepsis. Although haplogroup H is the most recent addition to the group of European mtDNA, paradoxically it is also the most common. Increased survival after sepsis provides one explanation for this observation. MtDNA haplotyping offers a new means of risk stratification of patients with severe infections, which suggests new avenues for therapeutic intervention.