YDNA

In human genetics, a human Y-chromosome DNA haplogroup is a haplogroup defined by differences in the non-recombining portions of DNA from the Y chromosome (called Y-DNA). It represents human genetic diversity based on single-nucleotide polymorphisms (SNPs) on the Y chromosome.

Y-DNA haplogroups represent major branches of the Y-chromosome phylogenetic tree. Y-chromosomal Adam is the name given by researchers to the patrilineal most recent common ancestor of all living humans at the root of this tree. Estimates of the date when Y-chromosomal Adam lived have varied significantly in different studies. Archaeological and genetic data suggest that the source populations of Paleolithic humans survived the glacial maxima (including the LGM) and human Y-DNA haplogroups emerged in sparsely wooded refugia, and dispersed through areas of high primary productivity while avoiding dense forest cover.

Y-DNA haplogroups are defined by the presence of a series of Y-DNA SNP markers. Subclades are defined by a terminal SNP, the SNP furthest down in the Y-chromosome phylogenetic tree. The Y Chromosome Consortium (YCC) developed a system of naming major Y-DNA haplogroups with the capital letters A through T, with further subclades named using numbers and lower case letters (YCC longhand nomenclature). YCC shorthand nomenclature names Y-DNA haplogroups and their subclades with the first letter of the major Y-DNA haplogroup followed by a dash and the name of the defining terminal SNP.

Y-DNA haplogroup nomenclature is changing over time to accommodate the increasing number of SNPs being discovered and tested, and the resulting expansion of the Y-chromosome phylogenetic tree. This change in nomenclature has resulted in inconsistent nomenclature being used in different sources. This inconsistency, and increasingly cumbersome longhand nomenclature, has prompted a move towards using the simpler shorthand nomenclature. In September 2012, Family Tree DNA provided the following explanation of its changing Y-DNA haplogroup nomenclature to individual customers on their Y-DNA results pages (note that the haplogroup mentioned below relates to a specific individual):

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Wikipedia