In the study of molecular evolution, a haplogroup is a large group of haplotypes, which are series of alleles at specific locations on a chromosome.
In human genetics, the haplogroups mostly commonly studied are Y-chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups, both of which can be used to define genetic populations. Y-DNA has the advantage of being passed solely along the patrilineal line, while mtDNA is passed solely on the matrilineal line.
Classifications of human haplogroups of either sort based on genetic markers has been rapidly evolving over the past several years as new markers are found.
Haplogroup population genetics
It is usually assumed that there is little natural selection for or against a particular haplotype mutation which has survived to the present day, so apart from mutation rates (which may vary from one marker to another) the main driver of population genetics affecting the proportions of haplotypes in a population is genetic drift - random fluctuation caused by the sampling randomness of which members of the population happen to pass their DNA on to members of the next generation of the appropriate sex. This causes the prevalence of a particular marker in a population to continue to fluctuate, until it either hits 100%, or falls out of the population entirely. In a large population with efficient mixing the rate of genetic drift for common alleles? is very slow; however, in a very small interbreeding population the proportions can change much more quickly. The marked geographical variations and concentrations of particular haplotypes and groups of haplotypes therefore witness the distinctive effects of repeated population bottlenecks or founder events followed by population separations and increases.
The lineages which can be traced back from the present will not reflect the full genetic variation of the older population: genetic drift means that some of the variants will have died out. With a full mtDNA sequence currently costing over $1000 commercially as of 2006, few data are yet available. Haplotype coalescence times and current geographical prevalences both carry considerable error uncertainties.
Human Y-chromosome DNA haplogroups
Human Y chromosome DNA (Y-DNA) haplogroups are lettered A through R, and are further subdivided using numbers and lower case letters. Y chromosome haplogroup designations are established by the Y Chromosome Consortium. For a graphical depiction of haplogroup branching, see the 2005 Y-chromosome Phylogenetic Tree.
Y-chromosomal Adam is the name given by researchers to the male who is the most recent common patrilineal (male-lineage) ancestor of all living humans.
Major Y-chromosome haplogroups
Groups without mutation M168
Groups with mutation M168
Groups descended from Haplogroup F*
Groups descended from Haplogroup K
Human Mitochondrial DNA haplogroups
Human Mitochondrial DNA (mtDNA) haplogroups are lettered A, B, C, D, E, F, G, H, I, J, K, L, L1, L2, L3, M, N, T, U, V, W, X, and Z. For a graphical depiction of haplogroup branching, see the mtDNA haplogroup skeleton.
Mitochondrial Eve is the name given by researchers to the woman who is the most recent common matrilineal (female-lineage) ancestor of all living humans.
Haplogroups can be used to define genetic populations and are often geographically oriented. For example, the following are common divisions for mtDNA haplogroups:
- West Eurasian: H, T, U, V, X, K, N, I, J
- Sub-Saharan African: L, L1, L2, L3, L3*
- East Asian: A, B, C, D, E, F, G (note: C, D, E, and G belong to macro-haplogroup M)
- Native American: A, B, C, D, and sometimes X
mtDNA haplogroup skeleton