All of these molecules are part of the ancestral haplogroup. At some time, a mutation occurred in the ancestral molecule, mutation A, which produced a new lineage. This is haplogroup A, defined by mutation A. Later, a new mutation, mutation B, occurred in a person carrying haplogroup A. Mutation B defined haplogroup B. Haplogroup B is a subgroup of haplogroup A. Both haplogroups A and B are subgroups of the ancestral haplogroup
Haplogroups show deep ancestral origins dating back thousands of years.
In human genetics, the haplogroups usually studied are Y-chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups. Both can be used to define genetic populations. Y-DNA is passed only from father to son, while mtDNA is passed only from mother to children. Neither recombines, and thus Y-DNA and mtDNA change only by chance mutations with no intermixture between parents' genetic material.