Haplogroup O1 (Y-DNA)
In human genetics, Haplogroup O1 (MSY2.2) is a Y-chromosome DNA haplogroup. Haplogroup O1 is a descendent branch of the greater Haplogroup O.The great majority of Y-chromosomes within Haplogroup O1 belong to its subgroup O1a (M119).
Haplogroup O1 is generally found wherever its brother haplogroup, Haplogroup O3, is found, although at a frequency much lower than that of Haplogroup O3. A conspicuous exception to this general pattern is presented by the Taiwanese aboriginal populations, among whom Haplogroup O1 generally dominates Haplogroup O3 in frequency. The frequency of Haplogroup O1 has been found to be negatively correlated with latitude, so that, on average, it occurs at relatively higher frequency towards the more southerly parts of its range, but it never attains majority haplogroup status outside of Taiwan. Heightened frequencies of Haplogroup O1 have also been observed in samples of populations from the Philippines and the eastern-southeastern coast of China. This haplogroup is, however, conspicuously absent from populations of the Japanese Archipelago, which seems to preclude a close genetic relationship between the peoples of Japan and the Austronesians, despite many enthusiastic attempts to prove a common origin of the Japanese and Austronesian languages. Haplogroup O1a-M119 Y-chromosomes have also been found to occur at low frequency among various populations of Siberia, such as the Nivkhs (one of 17 sampled Y-chromosomes), Ulchi/Nanai (2/53), Yenisey Evenks (1/31), and especially the Buryats living in the Sayan-Baikal uplands of Irkutsk Oblast (6/13).
Both Haplogroup O1 and Haplogroup O3 are prototypical Chinese patrilines, and they are believed to have first evolved sometime during the late Pleistocene (Upper Paleolithic) in what is now southern or central China. Both O1 and O3 are commonly found among most modern populations of China and Southeast Asia. These two lineages are also presumed to be markers of the Austronesian expansion, probably originating from prehistoric Taiwan or the neighboring southeastern coast of China, when they are found among modern populations of Maritime Southeast Asia or Oceania.
By far the strongest positive correlation between Haplogroup O1 and ethnolinguistic affiliation is that which is observed between this haplogroup and the Austronesians. It is interesting that the peak frequency of Haplogroup O1 is found among the aborigines of Taiwan, precisely the region from which linguists have hypothesized that the Austronesian language family originated. A slightly weaker correlation is observed between Haplogroup O1 and the Han Chinese populations of southern China, as well as between this haplogroup and the Tai-Kadai-speaking populations of southern China and Southeast Asia. The distribution of Daic languages in Thailand and other parts of Southeast Asia outside of China has long been believed, for reasons of traditional linguistic geography, to reflect a recent invasion of Southeast Asia by Daic-speaking populations originating from southeastern China, and the somewhat elevated frequency of Haplogroup O1 among the Daic populations, coupled with a high frequency of Haplogroup O2a, which is a genetic characteristic of the Austro-Asiatic peoples of Southeast Asia, suggests that the genetic signature of the Daic peoples' affinity with populations of southeastern China has been weakened due to extensive assimilation of the earlier Austro-Asiatic residents of the lands which the Daic peoples invaded. Also, it has been noted that Haplogroup O1 lineages among populations of continental Southeast Asia outside of China display a reduced level of diversity when compared with populations of South China and insular Southeast Asia, which may be evidence of a bottleneck associated with the westward migration and settlement of ancestral Daic-speaking populations in Indochina.
The frequencies of Haplogroup O1 among various East Asian and Austronesian populations suggest a complex genetic history of the modern Han populations of southern China. Although Haplogroup O1 occurs only at an average frequency of approximately 4% among Han populations of northern China and peoples of southwestern China and Southeast Asia who speak Tibeto-Burman languages, the frequency of this haplogroup among the Han populations of southern China nearly quadruples to about 15%. It is particularly interesting that the frequency of Haplogroup O1 among the Southern Han has been found to be slightly greater than the arithmetic mean of the frequencies of Haplogroup O1 among the Northern Han and a pooled sample of Austronesian populations. This suggests that modern Southern Han populations possess a non-trivial number of male ancestors who were originally affiliated with some Austronesian-related culture, or who at least shared a genetic affinity with many of the ancestors of modern Austronesian peoples.

# Subgroups
The subclades of Haplogroup O1 with their defining mutation, according to the 2006 ISOGG tree:
- O1 (MSY2.2)
O1*
O1a (M119) Typical of Austronesians, southern Han Chinese, and Tai-Kadai peoples
O1a*
O1a1 (M101)
O1a2 (M50, M103, M110) Occurs among Austronesian peoples of Taiwan, the Philippines, Indonesia, Melanesia, Micronesia, and Madagascar as well as among some populations of continental Southeast Asia
- O1*
- O1a (M119) Typical of Austronesians, southern Han Chinese, and Tai-Kadai peoples
O1a*
O1a1 (M101)
O1a2 (M50, M103, M110) Occurs among Austronesian peoples of Taiwan, the Philippines, Indonesia, Melanesia, Micronesia, and Madagascar as well as among some populations of continental Southeast Asia
- O1a*
- O1a1 (M101)
- O1a2 (M50, M103, M110) Occurs among Austronesian peoples of Taiwan, the Philippines, Indonesia, Melanesia, Micronesia, and Madagascar as well as among some populations of continental Southeast Asia