I 

The I, I1, and I1a lineages are nearly completely restricted to northwestern Europe. These would most likely have been common within Viking populations. One lineage of this group extends down into central Europe.

I1a1

This line was derived within Viking / Scandinavian populations in northwest Europe and has since spread down into southern Europe where it is present at low frequencies.

 I1b

This subgroup of Haplogroup I is found within the Balkans countries at it's greatest frequency and diversity. These countries probably harbored this subset of Haplogroup I as a refuge during the Last Glacial Maximum.

Description borrowed from Family Tree DNA