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Genetic genealogy is the use of DNA testing in combination with traditional genealogical methods to infer relationships between individuals and find ancestors. Genetic genealogy involves the use of genealogical DNA testing to determine the level and type of the genetic relationship between individuals. This application of genetics became popular with family historians in the 21st century, as tests became affordable. The tests have been promoted by amateur groups, such as surname study groups, or regional genealogical groups, as well as research projects such as the genographic project. As of 2018, 12 million people had been tested. As this field has developed, the aims of practitioners broadened, with many seeking knowledge of their ancestry beyond the recent centuries for which traditional pedigrees can be constructed.

History
The investigation of surnames in genetics can be said to go back to George Darwin, a son of Charles Darwin. In 1875, George Darwin used surnames to estimate the frequency of first-cousin marriages and calculated the expected incidence of marriage between people of the same surname. He arrived at a figure between 2.25% and 4.5% for cousin-marriage in the population of Great Britain, higher among the upper classes and lower among the general rural population.

Surname studies
One famous study examined the lineage of descendants of Thomas Jefferson’s paternal line and male lineage descendants of the freed slave, Sally Hemmings.

Bryan Sykes, a molecular biologist at Oxford University tested the new methodology in general surname research. His study of the Sykes surname obtained results by looking at four STR markers on the male chromosome. It pointed the way to genetics becoming a valuable assistant in the service of genealogy and history.

Direct to consumer DNA testing
The first company to provide direct-to-consumer genetic DNA testing was the now defunct GeneTree. However, it did not offer multi-generational genealogy tests. In fall 2001, GeneTree sold its assets to Salt Lake City-based Sorenson Molecular Genealogy Foundation (SMGF) which originated in 1999. While in operation, SMGF provided free Y-Chromosome and mitochondrial DNA tests to thousands. Later, GeneTree returned to genetic testing for genealogy in conjunction with the Sorenson parent company and eventually was part of the assets acquired in the Ancestry.com buyout of SMGF.

In 2000, Family Tree DNA, founded by Bennett Greenspan and Max Blankfeld, was the first company dedicated to direct-to-consumer testing for genealogy research. They initially offered eleven marker Y-Chromosome STR tests and HVR1 mitochondrial DNA tests. They originally tested in partnership with the University of Arizona.

In 2007, 23andMe was the first company to offer a saliva-based direct-to-consumer genetic testing. It was also the first to implement using autosomal DNA for ancestry testing, which all other major companies now use.

By 2018, over 12 million people had had their DNA tested for genealogical purposes - most of whom were American.

The genetic genealogy revolution
The publication of The Seven Daughters of Eve by Sykes in 2001, which described the seven major haplogroups of European ancestors, helped push personal ancestry testing through DNA tests into wide public notice. With the growing availability and affordability of genealogical DNA testing, genetic genealogy as a field grew rapidly. By 2003, the field of DNA testing of surnames was declared officially to have “arrived” in an article by Jobling and Tyler-Smith in Nature Reviews Genetics. The number of firms offering tests, and the number of consumers ordering them, rose dramatically.

The Genographic Project
The original Genographic Project was a five-year research study launched in 2005 by the National Geographic Society and IBM, in partnership with the University of Arizona and Family Tree DNA. Its goals were primarily anthropological. The project announced that by April 2010 it had sold more than 350,000 of its public participation testing kits, which test the general public for either twelve STR markers on the Y-Chromosome or mutations on the HVR1 region of the mtDNA.

In 2007, annual sales of genetic genealogical tests for all companies, including the laboratories that support them, were estimated to be in the area of $60 million (2006).

Typical customers and interest groups
Genetic genealogy has enabled groups of people to trace their ancestry even though they are not able to use conventional genealogical techniques. This may be because they do not know one or both of their birth parents or because conventional genealogical records have been lost, destroyed or never existed. These groups include adoptees, foundlings, Holocaust survivors, GI babies, child migrants, descendants of children from orphan trains and people with slave ancestry.

The earliest test takers were customers most often those who started with a Y-Chromosome test to determine their father's paternal ancestry. These men often took part in surname projects. The first phase of the Genographic project brought new participants into genetic genealogy. Those who tested were as likely to be interested in direct maternal heritage as their paternal. The number of those taking mtDNA tests increased. The introduction of autosomal SNP tests based on microarray chip technology changed the demographics. Women were as likely as men to test themselves.

Citizen science and ISOGG
Members of the growing genetic genealogy community have been credited with making useful contributions to knowledge in the field.

One of the earliest interest groups to emerge was the International Society of Genetic Genealogy (ISOGG). Their stated goal is to promote DNA testing for genealogy. Members advocate the use of genetics in genealogical research and the group facilitates networking among genetic genealogists. Since 2006 ISOGG has maintained the regularly updated ISOGG Y-chromosome phylogenetic tree. ISOGG aims to keep the tree as up-to-date as possible, incorporating new SNPs. However, the tree has been described by academics as not completely academically verified, phylogenetic trees of Y chromosome haplogroups.

Autosomal DNA 2007-present
In 2007, 23andMe was the first major company to begin offering a test of the autosome. This is the DNA excluding the Y-chromosomes and mitochondria. It is inherited from all ancestors in recent generations and so can be used to match with other testers who may be related. Later on, companies were also able to use this data to estimate how much of each ethnicity a customer has. FamilyTreeDNA entered this market in 2010, and AncestryDNA in 2012. Since then the number of DNA tests has expanded rapidly. By 2017, the combined totals of customers at the four largest companies was nearly 10 million. Autosomal testing is now the dominant type of genealogical DNA test, and for many companies the only test they offer.

Direct maternal lineages
mtDNA testing involves sequencing at least part of the mitochondria. The mitochondria is inherited from mother to child, and so can reveal information about the direct maternal line. When two individuals have matching or near mitochondria, is can be projected that they share a common maternal-line ancestor at some point in the recent past.

Direct paternal lineages
Y-Chromosome DNA (Y-DNA) testing involves short tandem repeat (STR) and, sometimes, single nucleotide polymorphism (SNP) testing of the Y-Chromosome. The Y-Chromosome is present only in males and only reveals information on the strict-paternal line. As with the mitochondria, close matches with individuals indicate a recent common ancestor. Because surnames in many cultures are transmitted down the paternal line, this testing is often used by [Surname DNA Project]s.

Ancestral origins
A common component of many autosomal tests is a prediction of biogeographical origin. The company offering the test uses computer algorithms and calculations to make a prediction of what percentage of an individual's DNA comes from particular ancestral groups. A typical number of populations is at least 20. Despite this aspect of the tests being heavily promoted and advertised, many genetic genealogists have warned consumers that the results may be inaccurate, and at best are only approximate.

Modern DNA sequencing has identified various ancestral components in contemporary populations. A number of these genetic elements have West Eurasian origins. They include the following ancestral components, with their geographical hubs and main associated populations:

Human migration
Genealogical DNA testing methods are in use on a longer time scale to trace humans. For example, they determined when the first humans came to North America and what path they followed.

For several years, researchers and laboratories from around the world sampled indigenous populations from around the globe in an effort to map historical human migration patterns. The National Geographic Society's Genographic Project aims to map historical human migration patterns by collecting and analyzing DNA samples from over 100,000 people across five continents. The DNA Clans Genetic Ancestry Analysis measures a person's precise genetic connections to indigenous ethnic groups from around the world.