Human Y-chromosome DNA haplogroup

In human genetics, a human Y-chromosome DNA haplogroup is a haplogroup defined by mutations in the non-recombining portions of DNA from the Y chromosome (called Y-DNA). Mutations that are shared by many people are called single-nucleotide polymorphisms (SNPs). The human Y-chromosome accumulates roughly two mutations per generation. Y-DNA haplogroups represent major branches of the Y-chromosome phylogenetic tree that share hundreds or even thousands of mutations unique to each haplogroup. The Y-chromosomal most recent common ancestor (Y-MRCA, informally known as Y-chromosomal Adam) is the most recent common ancestor (MRCA) from whom all currently living men are descended patrilineally. Y-chromosomal Adam is estimated to have lived roughly 236,000 years ago in Africa. By examining other bottlenecks most Eurasian men are descended from a man who lived 69,000 years ago. Other major bottlenecks occurred about 5,000 years ago and subsequently most Eurasian men can trace their ancestry back to a dozen ancestors who lived 5,000 years ago. 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): Haplogroup A is the NRY macrohaplogroup from which all modern paternal haplogroups descend. It is sparsely distributed in Africa, being concentrated among Khoisan populations in the southwest and Nilotic populations toward the northeast in the Nile Valley. BT is a subclade of haplogroup A; more precisely of the A1b clade (A2-T in Cruciani et al. 2011), as follows: The defining mutations separating CT (all haplogroups except for A and B) are M168 and M294. The site of origin is unknown but likely in either Asia or Africa approximately 88,000 years ago.