Haplogroup I (Y-DNA)
In human genetics, Haplogroup I (M170, P19, M258) is a Y-chromosome haplogroup.
Haplogroup I (the letter I, not the number 1) can be found in most present-day European populations, most commonly in Scandinavia, Sardinia, and the Slavic & Bulgarian populations of the Western Balkans in southeastern Europe.
Based on DNA dating analyses, Haplogroup I first arrived in Europe around 20,000-25,000 years ago from the Middle East, just prior to the onset of the last glacial maximum (LGM).  Some speculate this migration was associated with the initial spread of the Gravettian culture. Haplogroup I is closely related to Haplogroup J, most common in Semitic peoples; both Haplogroup I and Haplogroup J are descendants of  Haplogroup IJ (S2, S22). Haplogroup IJ is in turn derived from Haplogroup F that formed a founding middle east population 30-40kya.
There are two principal subgroups of Haplogroup I (Y-DNA):
- I-M253/I-M307/I-P30/I-P40 has highest frequency in Scandinavia, Iceland, and northwest Europe.
- I-S31 includes I-P37.2, common in the Balkans and Sardinia, and I-S23/I-S30/I-S32/I-S33, reaching its highest frequency along the northwest coast of continental Europe.
Rootsi et al. 2004 suggest that each of the ancestral populations now dominated by a particular subclade experienced an independent population expansion, believed to reflect different migrations of people during and immediately after the ice age. Haplogroup I Y-chromosomes have also been found among some populations of the Middle East, the Caucasus, and Central Asia, but they are found at frequencies exceeding 10% only among populations of Europe and Asia Minor, particularly among Germanic, Slavic, Uralic, and Turkic peoples, as well as among the Romance-speaking populations of France, Romania, Moldova, and Sardinia, the Albanian-speaking population of Albania, and the Greek-speaking population of Greece.
Within Europe, several populations are distinguished by having a significantly lower frequency of Haplogroup I than the surrounding populations: these depressions in the frequency of Haplogroup I distinguish the populations of Italy and Switzerland from Germany and Sardinia, Iberia from southern France and Normandy, Greece from Albania and the Slavic peoples, and the Baltic Latvians from the Finnic Estonians. In all these areas, Haplogroup I populations are small relative to the dominant European haplogroups (R1b in Western Europe, R1a1 in Eastern Europe, and N in Northeastern Europe).

# Subgroups
The subclades of Haplogroup I with their defining mutation, according to the 2006 ISOGG tree:
- I (M170, M258, P19)
I*
I1 (P38)
I1*
I1a (M253, M307, P30, P40) Typical of populations of Scandinavia and Northwest Europe, with a moderate distribution throughout Eastern Europe
I1a*
I1a1 (M227) Appears to be limited to a marginally low frequency of approximately 1% among Slavic and Uralic peoples of Eastern Europe; also detected in a single Lebanese man
I1a1a (M72)
I1a2 (M21)
I1b (S31)
I1b*
I1b1 (P37.2) Typical of the South Slavic peoples of the Balkans, especially the populations of Bosnia and Croatia; also found with high haplotype diversity values, but lower overall frequency, among the West Slavic populations of Slovakia and the Czech Republic; a node of elevated frequency in Moldavia correlates with that observed for Haplogroup I1b2 (but not for Haplogroup I1a)
I1b1*
I1b1a (M26)
I1b1a1 (M161)
I1b1b (P41.2 (M359.2)) Typical of the population of the so-called "archaic zone" of Sardinia; also found at low frequencies among populations of Southwest Europe, particularly in Castile, Bearn, and the Basque Country
I1b1b*
I1b2 (S23, S30, S32, S33) Occurs at a moderate frequency among populations of Northwest Europe, with a peak frequency in the region of Lower Saxony in central Germany; minor offshoots appear in Moldavia and Russia (especially around Vladimir, Ryazan, Nizhny Novgorod, and the Republic of Mordovia)
I1b2*
I1b2a (M223, S24)
I1b2a*
I1b2a1 (M284) Generally limited to a low frequency in Great Britain
I1b2a2 (M379)
I1b2a3 (P78)
I1b2a4 (P95)
- I*
- I1 (P38)
I1*
I1a (M253, M307, P30, P40) Typical of populations of Scandinavia and Northwest Europe, with a moderate distribution throughout Eastern Europe
I1a*
I1a1 (M227) Appears to be limited to a marginally low frequency of approximately 1% among Slavic and Uralic peoples of Eastern Europe; also detected in a single Lebanese man
I1a1a (M72)
I1a2 (M21)
I1b (S31)
I1b*
I1b1 (P37.2) Typical of the South Slavic peoples of the Balkans, especially the populations of Bosnia and Croatia; also found with high haplotype diversity values, but lower overall frequency, among the West Slavic populations of Slovakia and the Czech Republic; a node of elevated frequency in Moldavia correlates with that observed for Haplogroup I1b2 (but not for Haplogroup I1a)
I1b1*
I1b1a (M26)
I1b1a1 (M161)
I1b1b (P41.2 (M359.2)) Typical of the population of the so-called "archaic zone" of Sardinia; also found at low frequencies among populations of Southwest Europe, particularly in Castile, Bearn, and the Basque Country
I1b1b*
I1b2 (S23, S30, S32, S33) Occurs at a moderate frequency among populations of Northwest Europe, with a peak frequency in the region of Lower Saxony in central Germany; minor offshoots appear in Moldavia and Russia (especially around Vladimir, Ryazan, Nizhny Novgorod, and the Republic of Mordovia)
I1b2*
I1b2a (M223, S24)
I1b2a*
I1b2a1 (M284) Generally limited to a low frequency in Great Britain
I1b2a2 (M379)
I1b2a3 (P78)
I1b2a4 (P95)
- I1*
- I1a (M253, M307, P30, P40) Typical of populations of Scandinavia and Northwest Europe, with a moderate distribution throughout Eastern Europe
I1a*
I1a1 (M227) Appears to be limited to a marginally low frequency of approximately 1% among Slavic and Uralic peoples of Eastern Europe; also detected in a single Lebanese man
I1a1a (M72)
I1a2 (M21)
- I1a*
- I1a1 (M227) Appears to be limited to a marginally low frequency of approximately 1% among Slavic and Uralic peoples of Eastern Europe; also detected in a single Lebanese man
I1a1a (M72)
- I1a1a (M72)
- I1a2 (M21)
- I1b (S31)
I1b*
I1b1 (P37.2) Typical of the South Slavic peoples of the Balkans, especially the populations of Bosnia and Croatia; also found with high haplotype diversity values, but lower overall frequency, among the West Slavic populations of Slovakia and the Czech Republic; a node of elevated frequency in Moldavia correlates with that observed for Haplogroup I1b2 (but not for Haplogroup I1a)
I1b1*
I1b1a (M26)
I1b1a1 (M161)
I1b1b (P41.2 (M359.2)) Typical of the population of the so-called "archaic zone" of Sardinia; also found at low frequencies among populations of Southwest Europe, particularly in Castile, Bearn, and the Basque Country
I1b1b*
I1b2 (S23, S30, S32, S33) Occurs at a moderate frequency among populations of Northwest Europe, with a peak frequency in the region of Lower Saxony in central Germany; minor offshoots appear in Moldavia and Russia (especially around Vladimir, Ryazan, Nizhny Novgorod, and the Republic of Mordovia)
I1b2*
I1b2a (M223, S24)
I1b2a*
I1b2a1 (M284) Generally limited to a low frequency in Great Britain
I1b2a2 (M379)
I1b2a3 (P78)
I1b2a4 (P95)
- I1b*
- I1b1 (P37.2) Typical of the South Slavic peoples of the Balkans, especially the populations of Bosnia and Croatia; also found with high haplotype diversity values, but lower overall frequency, among the West Slavic populations of Slovakia and the Czech Republic; a node of elevated frequency in Moldavia correlates with that observed for Haplogroup I1b2 (but not for Haplogroup I1a)
I1b1*
I1b1a (M26)
I1b1a1 (M161)
I1b1b (P41.2 (M359.2)) Typical of the population of the so-called "archaic zone" of Sardinia; also found at low frequencies among populations of Southwest Europe, particularly in Castile, Bearn, and the Basque Country
I1b1b*
- I1b1*
- I1b1a (M26)
I1b1a1 (M161)
- I1b1a1 (M161)
- I1b1b (P41.2 (M359.2)) Typical of the population of the so-called "archaic zone" of Sardinia; also found at low frequencies among populations of Southwest Europe, particularly in Castile, Bearn, and the Basque Country
I1b1b*
- I1b1b*
- I1b2 (S23, S30, S32, S33) Occurs at a moderate frequency among populations of Northwest Europe, with a peak frequency in the region of Lower Saxony in central Germany; minor offshoots appear in Moldavia and Russia (especially around Vladimir, Ryazan, Nizhny Novgorod, and the Republic of Mordovia)
I1b2*
I1b2a (M223, S24)
I1b2a*
I1b2a1 (M284) Generally limited to a low frequency in Great Britain
I1b2a2 (M379)
I1b2a3 (P78)
I1b2a4 (P95)
- I1b2*
- I1b2a (M223, S24)
I1b2a*
I1b2a1 (M284) Generally limited to a low frequency in Great Britain
I1b2a2 (M379)
I1b2a3 (P78)
I1b2a4 (P95)
- I1b2a*
- I1b2a1 (M284) Generally limited to a low frequency in Great Britain
- I1b2a2 (M379)
- I1b2a3 (P78)
- I1b2a4 (P95)
Note that the naming of some of the subgroups has changed, as new markers have been identified, and the sequence of mutations has become clearer.  Names from older literature which have now been superseded include: I1b (P37.2) now I1b1  I1b2 (M26) now I1b1b   I2 (M223), later I1c (M223) now I1b2a   I2a (M284), later I1c1 (M284) now I1b2a1. Future changes may occur, according to Ken Nordtvedt , if the P38 SNP is made equivalent to the M258, P19, and M170 SNPs. Which would change I1a to I1, I1b1 to I2a & I1b2a to I2b.

## I*
The composite subclade I- contains individuals directly descended from the earliest members of Haplogroup I, bearing none of the subsequent mutations which identify the remaining named subclades.
Several haplogroup I- individuals, with some of the greatest Y-STR diversity, have significantly been found among the populations of Turkey (8/741), Adygea (2/138), and Iraq (1/176), even though as a whole Haplogroup I occurs at only very low frequencies among modern populations of the Middle East and Caucasus.  This is consistent with the belief that the haplogroup first appeared in that region.  Overall, the highest frequencies of Haplogroup I- appear to be found among the Andalusians (3/103), French (4/179), Slovenians (2/55), and the Saami (1/35).

## I1a
Haplogroup I1a (M253, M307, P30, P40) displays a very clear frequency gradient, with a peak frequency of approximately 35% among the populations of southern Norway, southwestern Sweden, and Denmark, and rapidly decreasing frequencies toward the edges of the historically Germanic-influenced world.
Outside Fennoscandia, distribution of Haplogroup I1a is closely correlated with that of Haplogroup I1b2; but among Scandinavians (including both Germanic and Uralic peoples of the region) nearly all the Haplogroup I Y-chromosomes are I1a.  Another characteristic of the Scandinavian I1a Y-chromosomes is their rather low haplotype diversity (STR diversity): a greater variety of Haplogroup I1a Y-chromosomes has been found among the French and Italians, despite the much lower overall frequency of Haplogroup I1a among the modern French and Italian populations.  Taken together, this suggests that the Haplogroup I element of the ancestry of Scandinavians might be descended from a very small Paleolithic population of Southern European extraction, which became distinct from the ancestral population of Haplogroup I1a individuals outside Scandinavia.
It is conjectured that this shared ancestral population of I1a and I1b2, distinct from I1b1*, may have weathered the last ice age in a refuge located somewhere in the Iberian Peninsula or southern France, or perhaps the Italian Peninsula; after the end of the ice age, some of them headed northward and repopulated Northwest Europe and Scandinavia. This population appears to have carried haplogroups I1a and I1b2 at significant frequencies, with a numerical superiority of Haplogroup I1a. Their descendants are primarily found among the Germanic, Uralic, and Celtic populations of Northern Europe, although almost always overshadowed by the more prevalent carriers of Haplogroup R. If R is considered more specifically, as it is, split between R1b & R1a, then I1a shares almost equally the areas of population where it is at its peak frequencies.

## I1b1*
Haplogroup I1b1- (P37.2) accounts for most of the Haplogroup I component in the Y-chromosome diversity of Eastern European populations, reaching its peak in the Western Balkans, most notably in Dalmatia and Bosnia-Herzegovina (40 - 50%).
The high frequency and diversity of Haplogroup I1b1- among populations of the Western Balkans lends support to the hypothesis that the Adriatic region of modern-day Croatia served as a refuge for populations bearing Haplogroup I1b1- during the last glacial maximum.

## I1b1b
Haplogroup I1b1b (M359), which is found outside of Sardinia only at low frequencies in Southwest Europe, accounts for almost all of the Haplogroup I Y-chromosomes, which comprise more than 40% of all patrilines among the Sardinians.
A distinct Western European Paleolithic population that bore Haplogroup I1b1b (M26) must have existed somewhere west of the Apennines in eastern Iberia, southern France, or western Italy, from which it succeeded in the first substantial colonization of the island of Sardinia approximately 9,000 years ago. Despite the fact that the predominantly Sardinian Haplogroup I1b1b-M359 is derived from the predominantly Balkan Haplogroup I1b1P37.2, the derived Haplogroup I1b1b is practically absent east of France and Italy, while it is found at low but significant frequencies outside of Sardinia in the Balearic Islands, Castile, the Basque Country, the Pyrenees, southern and western France, and parts of the Maghreb in North Africa, Great Britain, and Ireland. Thus, Haplogroup I1b1b appears to be strongly associated with Southwest Europeans of Paleolithic ancestry, and its carriers bear only a distant relictual relationship to the I1b1bearing populations of the Balkans. It is also interesting that, although the distributions of Haplogroup I1b1b and the predominantly Scandinavian Haplogroup I1a overlap in parts of western France and the British Isles, and both haplogroups appear to have a very long history in Southwest Europe (and particularly France), the populations bearing these two haplogroups appear to have differentiated at a very early date and have not extensively mixed since that time. Haplogroup I1b1b appears to be the only subclade of Haplogroup I found among the Basques, although subclades of Haplogroup R1b comprise the vast majority of that people's Y-chromosome diversity. It is notable that Haplogroup I1b1b appears to be found at somewhat higher frequencies among the general populations of Castile in Spain and Bearn in France than among the population of ethnic Basques.  Lastly, the unique phylogeography of this clade has caused some scientists (Gatto, Michael) to posit that the clade should be associated with the spread of Chalcolithic Megalith culture in Western Europe.  The M26 mutation is found in native males inhabiting every geographic region where megaliths may be found, including such far-flung and culturally disconnected regions like the Canary Islands, the Balearic Isles, Corsica, Ireland, and Sweden.  As of 2007, no specific study has confirmed this hypothesis, although the data in several previous studies (Rootsi, et al., Flores, et al., etc.) support this hypothesis.

## I1b2
The distribution of Haplogroup I1b2 (S23, S30, S32, S33) is closely correlated to that of Haplogroup I1a except in Fennoscandia, which suggests that it was probably harbored by at least one of the Paleolithic refuge populations that also harbored Haplogroup I1a; the lack of correlation between the distributions of I1a and I1b2 in Fennoscandia may be a result of Haplogroup I1b2's being more strongly affected in the earliest settlement of this region by founder effects and genetic drift due to its rarity, as Haplogroup I1b2 comprises less than 10% of the total Y-chromosome diversity of all populations outside of Lower Saxony. Haplogroup I1b2 has been found in over 4% of the population only in Germany, the Netherlands, Belgium, Denmark, England (not including Cornwall), Scotland, and the southern tips of Sweden and Norway in Northwest Europe; the provinces of Normandy, Maine, Anjou, and Perche in northwestern France; the province of Provence in southeastern France; the regions of Tuscany, Umbria, and Latium in Italy; and Moldavia and the area around Russia's Ryazan Oblast and Republic of Mordovia in Eastern Europe. One subclade of Haplogroup I1b2, namely I1b2a1 (M284), has been found almost exclusively among the population of Great Britain, which has been taken to suggest that the clade may have a very long history in that island. It is notable, however, that the distributions of Haplogroup I1a and Haplogroup I1b2 seem to correlate fairly well with the extent of historical influence of Germanic peoples, although the punctual presence of both haplogroups at a low frequency in the area of the historical regions of Bithynia and Galatia in Turkey rather suggests a connection with the ancient Gauls of Thrace, several tribes of which are recorded to have immigrated to those parts of Anatolia at the invitation of Nicomedes I of Bithynia.
Haplogroup I1b2 also occurs among approximately 1% of Sardinians.