A clade represents all the branches - and thus descendants - of an taxonomic (or ancestral) tree that share a single common ancestor.Wikipedia
Clade I represents one of the twenty major branches, or haplogroups, of the Y chromosome as defined by the Y Chromosome Consortium (YCC2008).
Clade I represents one of two major European Y chromosome haplogroups (the other major haplogroup is Clade R). Unlike R, clade I is widespread in Europe and is virtually absent elsewhere. Subclade I1 is found mostly in Northern Europe, while subclade I2 is the most frequent haplogroup in Eastern Europe and the Balkans.[PMID 18385274]
Clade I is now defined by six mutations: