A haplogroup (from the , haploûs, "onefold, single, simple" and ), is a group of individual organisms which, in terms of molecular evolution, share a common ancestor – a genetic line of descent demonstrated by a particular single nucleotide polymorphism (SNP) mutation. Because a haplogroup consists of similar haplotypes, it is usually possible to predict a haplogroup from haplotypes. Haplogroups pertain to a single line of descent, usually dating back thousands of years. 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).

Paragroup is a term used in population genetics to describe lineages within a haplogroup that are not defined by any additional unique markers. In human Y-chromosome DNA haplogroups, paragroups are typically represented by an asterisk (*) placed after the main haplogroup. The term "paragroup" is a portmanteau of the terms paraphyletic haplogroup indicating that paragroups form paraphyletic subclades. Apart from the mutations that define the parent haplogroup, paragroups may not possess any additional unique markers. Alternatively pargroups may possess unique markers that have not been discovered. If a unique marker is discovered within a paragroup, the specific lineage is given a unique name and is moved out of the paragroup to form an independent subclade.