Genetic genealogy
Genetic genealogy is the application of genetics to traditional genealogy.  Genetic genealogy involves the use of genealogical DNA testing to determine the level of genetic relationship between individuals.

# 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 (isonymy).  He arrived at a figure between 2.25% and 4.5% for cousin-marriage in the population of Great Britain, with the upper classes being on the high end and the general rural population on the low end. (His parents, Charles Darwin and Emma Wedgwood, were first cousins.) This simple study was innovative for its era. The next stimulus toward using genetics to study family history had to wait until the 1990s, when certain locations on the Y chromosome were identified as being useful for tracing male-to-male inheritance.
Dr. Karl Skorecki, a Canadian nephrologist of Ashkenazi parentage, noticed that a Sephardic fellow-congregant who was a Kohen like himself had completely different physical features. According to Jewish tradition, all Kohanim are descended from the priest Aaron, brother of Moses. Skorecki reasoned that if Kohanim were indeed the descendants of only one man, they should have a common set of genetic markers and should perhaps preserve some family resemblance to each other.
To test that hypothesis, he contacted Professor Michael Hammer of the University of Arizona, a researcher in molecular genetics and pioneer in Y chromosome research. Their report in the Nature in 1997 sent shock waves through the worlds of science and religion. A particular marker was indeed more likely to be present in Jewish men from the priestly tradition than in the general Jewish population. It was apparently true that a common descent had been strictly preserved for thousands of years. (See Y-chromosomal Aaron).  Moreover, the data showed that there were very few non-paternity events.
The first to test the new methodology in general surname research was Bryan Sykes, a molecular biologist at Oxford University. His study of the Sykes surname obtained valid results by looking at only four markers on the male chromosome. It pointed the way to genetics becoming a valuable assistant in the service of genealogy and history. In 2001, Sykes went on to write the popular book The Seven Daughters of Eve.
In the wake of that book's success, and the growing availability and affordability of genealogical DNA tests, genetic genealogy as a field began growing 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, had risen dramatically.
Another milestone in the acceptance of genetic genealogy is the Genographic Project.  The Genographic Project is a five-year research partnership launched by the National Geographic Society and IBM in 2005. Although its goals are primarily anthropological, not genealogical, the project's sale of over 225,000 testing kits (as of October 2007) of its public participation kits, which test the general public for either twelve STR markers on the Y chromosome or the HVR1 region of the mtDNA, has helped increase the visibility of genetic genealogy.
Annual sales of genetic genealogical tests for all companies, including the laboratories that support them, are estimated to be in the area of $60 million (2006).

# Interpretation
Since 2000 dozens of relevant academic papers have been published, and thousands of private test results organised by surname study groups have been made available on the internet.  The comparison of results may be complicated by the fact that some laboratories use different testing methods.  Apparently differing results from two sources may in fact be identical, and vice-versa.

# Uses

## Paternal and maternal lineages
The two most common types of genetic genealogy tests are Y-DNA (paternal line) and mtDNA (maternal line) genealogical DNA tests.
These tests involve the comparison of certain sequences of DNA pairs of individuals in order to estimate the probability that they share a common ancestor in a genealogical time frame and, through the use of a Bayesian model published by Bruce Walsh, to estimate the number of generations separating the two individuals from their most recent common ancestor.
Y-DNA research 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 reveals information on the strict paternal line.  These tests can provide insight in the recent (via STRs) and ancient (via SNPs) genetic ancestry.  A Y-chromosome STR test will reveal a haplotype, which should be similar among all male descendants of a male ancestor.  SNP tests are used to assign people to a paternal haplogroup, which defines a genetic population.
mtDNA research involves sequencing the HVR-1 region, HVR-2 region or both.  A mtDNA test can also be used to assign people to a maternal haplogroup.
Either Y-DNA or mtDNA test results can be compared to the results of others via private or public DNA databases.

## Biogeographical and ethnic origins
Additional DNA tests exist for determining biogeographical and ethnic origin, but these tests have less relevance for traditional genealogy.
Genetic genealogy has revealed astonishing links between the ancient Phoenician people and the present-day population of the island of Malta. Preliminary results from a study by Pierre Zalloua of the American University of Beirut and Spencer Wells, supported by a grant from National Geographic's Committee for Research and Exploration,
were published in the October 2004 issue of National Geographic. One of the conclusions is that
"more than half of the Y chromosome lineages that we see in today's Maltese population could have come in with the Phoenicians" 
See biogeographic ancestry, genealogical DNA test and population genetics (the study of the distribution of and change in allele frequencies).

## Human migration
Genealogical DNA testing methods are also being used on a longer time scale to trace human migratory patterns and determine, for example, when the first humans came to North America and how they got there.  For several years, a number of researchers and laboratories from around the world have been sampling indigenous populations from around the globe in an effort to map historical human migration patterns.  Recently, several projects have been created that are aimed at bringing this science to the public.  One example is the Genographic Project, which aims to map historical human migration patterns by collecting and analyzing DNA samples from over 100,000 people across five continents.

## Typical customers and interest groups
Male DNA testing customers most often start with a Y chromosome test to determine their father's paternal ancestry. Females generally begin with a mitochondrial test to trace their ancient maternal lineage, which males often have tested for the same purpose.
A common consumer goal in purchasing DNA testing services is to acquire quantified, scientific linkage to a specific ancestral group.  A compelling example of this motive is found in the expressed desires of some consumers to be proven to have Viking paternal ancestry.  In keeping with this marketplace demand, one British DNA testing service, Oxford Ancestors, offers a Y chromosome test purporting to measure whether given males are of "Viking stock."  Those whose DNA falls into the designated haplogroup are issued Viking Descendant certificates by the testing service.  The same DNA testing company (Oxford Ancestors) participated in producing a televised documentary, "The Blood of the Vikings," in conjunction with the BBC, which showed how DNA testing could reveal Viking ancestry.
The Rootsweb DNA-Genealogy Internet discussion group has a membership of 750 subscribers from around the world. Some subscribers have had various DNA tests performed and are seeking advice and guidance in interpreting their results, and the list also includes administrators of DNA projects that examine surnames (e.g., Stewart, Hurst, Wells, etc.), geographic regions (e.g., the Shetland Islands, Mexico, Scotland, etc.), or ethnic groups (e.g., the Pennsylvania Deutsch, African Americans, etc.).  The sophistication of subscribers ranges from expert to novice, and, in some cases, subscribers have been credited with making useful and novel contributions to knowledge in the field of genetic genealogy.

# Benefits
Genetic genealogy gives genealogists a means to check or supplement the historical record with information from genetic data.  A positive test match with another individual may:
- provide locations for further genealogical research
- help determine ancestral homeland
- discover living relatives
- validate existing research
- confirm or deny suspected connections between families
- prove or disprove theories regarding ancestry

# Drawbacks
People who resist testing may cite one of the following concerns:
- Cost
- Concerns over privacy issues
- Loss of ethnic identity
- Discovery of flaws in long-accepted lineages
Finally, Y-DNA and mtDNA tests each only trace a single lineage (one's father's father's father's etc. lineage or one's mother's mother's mother's etc. lineage).  At 10 generations back, an individual has up to 1024 unique ancestors (fewer if ancestor cousins interbred) and a Y-DNA or mtDNA test is only studying one of those ancestors, as well as their descendants and siblings (same sexed siblings for Y-DNA or all siblings for mtDNA).  However, most genealogists maintain contact with many cousins (1st, 2nd, 3rd, etc., with different surnames) whose Y-DNA and mtDNA are different, and thus can be encouraged to be tested to find additional ancestral DNA lineages.

# Expected growth
Genetic genealogy is a rapidly growing field.   As the cost of testing continues to drop, the number of people being tested continues to increase.  The probability of finding a genetic match among the DNA databases should continue to improve.  Laboratories and testing firms are engaging in active research and development that will allow for higher confidence intervals and better results interpretation, including historical interpretive reports and customized research.