In human genetics, Haplogroup I (M170) is a Y-chromosome haplogroup.
Haplogroup I is native to Europe and is relatively widespread in all European populations, most common in Scandinavia but also very common in Croat populations. Its initial spread is believed to be connected to the migrations that occurred at the last glacial maximum (LGM).
Haplogroup I is a branch of haplogroup F*. According to current theories, haplogroup I first arrived in Europe around 20,000-25,000 years ago from the Middle East. It is believed to be associated with the Gravettian culture. ()
The highest frequency of the I Haplogroup can be found in Scandinavian populations as well as populations in the Adriatic region. This lends support to the hypothesis that the Adriatic region of modern-day Croatia served as a refuge for northern populations during the last glacial maximum. The hypothesis states that after the LGM there was a migration from the north east by the people whose offspring today form a significant portion of the Scandinavian populations. These groups seem to be the ancestors of about 38% of modern day Croats (75% of Bosnian Croats).
There are also indications that this haplogroup is tied to the Celtic culture. The spread of the I group in western Europe is could be consistent with the Celtic expansion that occurred in the mid-first millennium BC.
Some subclades of I, such as I1a, were previously believed to be also of Scandinavian populations. However, recent research done by Scandinavian researchers has shown this assumption to be false. I1a seems to be recently, (within the last 1000 years), from Normandy and to be tracable to the Anglo-Saxon migrations North from Southern Europe. Its true source is still not known. Of additional interest is that another subclade of I, I1b, appears to be more highly concentrated in Greece, and is also not of Scandinavian origin. Thus, the term "I haplogroup" is misleading because its subclades are not necessarily, and most likely not, of the same origins as the straight "I" group. Because of these mismatches in origins, it is possible that more study needs to be done, and as of this writing more haplogroups have been discovered, which previously were not seen, which seems to indicate that the classification of haplogroups is still in its infancy.
Baric et al (2003), Y chromosomal heritage of Croatian population and its island isolates, European Journal of Human Genetics 11 535-542
The Genographic Project, National Geographic, Atlas of the Human Journey