Haplogroup

A haplotype is a group of alleles in an organism that are inherited together from a single parent,^[1][2] and a haplogroup (haploid from the Greek: ἁπλοῦς, haploûs, "onefold, simple" and English: group) is a group of similar haplotypes that share a common ancestor with a single-nucleotide polymorphism mutation.^[3] More specifically, a haplotype is a combination of alleles at different chromosomal regions that are closely linked and that tend to be inherited together. As a haplogroup consists of similar haplotypes, it is usually possible to predict a haplogroup from haplotypes. Haplogroups pertain to a single line of descent. As such, membership of a haplogroup, by any individual, relies on a relatively small proportion of the genetic material possessed by that individual.

Each haplogroup originates from, and remains part of, a preceding single haplogroup (or paragroup). As such, any related group of haplogroups may be precisely modelled as a nested hierarchy, in which each set (haplogroup) is also a subset of a single broader set (as opposed, that is, to biparental models, such as human family trees). Haplogroups can be further divided into subclades.

Haplogroups are normally identified by an initial letter of the alphabet, and refinements consist of additional number and letter combinations, such as (for example) A → A1 → A1a. The alphabetical nomenclature was published in 2002 by the Y Chromosome Consortium.^[4]

In human genetics, the haplogroups most commonly studied are Y-chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups, each of which can be used to define genetic populations. Y-DNA is passed solely along the patrilineal line, from father to son, while mtDNA is passed down the matrilineal line, from mother to offspring of both sexes. Neither recombines, and thus Y-DNA and mtDNA change only by chance mutation at each generation with no intermixture between parents' genetic material.