Genetic genealogy

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George Darwin, son of Charles Darwin, was the first to estimate the frequency of first-cousin marriages

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.

Contents

[edit] 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).[1] 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”.[2]

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, which described the seven major haplogroups of European ancestors.

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.[3]

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 by October 2007 of over 225,000 of its public participation testing 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.[4]

More state-of-the-art commercial laboratories now recommend testing at least 25 markers, since the more markers tested, the more discriminating and powerful the results will be. A 12-marker STR test is usually not discriminating enough to provide conclusive results for a common surname. Genetic laboratories such as Genebase and Family Tree DNA give the option of testing 67 Y-DNA Markers.[5]

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).[6]

[edit] Interpretation

Since the year 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.

[edit] Uses

[edit] 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.

[edit] 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 peoples. For instance, it has shown that the ancient Phoenician people were ancestors of much of 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."[7]

See biogeographic ancestry, genealogical DNA test and population genetics (the study of the distribution of and change in allele frequencies).

[edit] Human migration

Main article: Human migration
mtDNA-based chart of large human migrations.

Genealogical DNA testing methods are also being used on a longer time scale to trace human migratory patterns. For example, they have been used to determine when the first humans came to North America and what path they followed.

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 National Geographic Society's Genographic Project, which aims to map historical human migration patterns by collecting and analyzing DNA samples from over 100,000 people across five continents. Another example is the DNA Clans Genetic Ancestry Analysis, which measures a person's precise genetic connections to indigenous ethnic groups from around the world.[8]

[edit] 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 assess 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 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. The list also includes administrators of DNA projects that examine surnames, geographic regions, or ethnic groups. The sophistication of subscribers ranges from expert to novice. In some cases, subscribers have been credited with making useful and novel contributions to knowledge in the field of genetic genealogy.[citation needed]

[edit] Paternal and maternal lineages

     Ancestral Haplogroup      Haplogroup A (Hg A)     Haplogroup B (Hg B) All of these molecules are part of the ancestral haplogroup, but at some point in the past a mutation occurred in the ancestral molecule, mutation A, which produced a new lineage; this is haplogroup A and is defined by mutation A. At some more recent point in the past, a new mutation, mutation B, occurred in a person carrying haplogroup A; mutation B defined haplogroup B. Haplogroup B is a subgroup, or subclade of haplogroup A; both haplogrups A and B are subclades of the ancestral haplogroup.

Mitochondria are small organelles that lie in the cytoplasm of eucaryotic cells, such as those of humans. Their primary purpose is to provide energy to the cell. Mitochondria are thought to be the vestigial remains of symbiotic bacteria that were once free living. One indication that mitochondria were once free living is that they contain a relatively small circular segment of DNA, called mitochondrial DNA (mtDNA). The overwhelming majority of a human's DNA is contained in chromosomes in the nucleus of the cell, but mtDNA is an exception. An individual inherits their cytoplasm and the organelles it contains exclusively from their mother, as these are derived from the ovum (egg cell), sperm only carry chromosomal DNA due to the necessity of maintaining motility. When a mutation arises in mtDNA molecule, the mutation is therefore passed in a direct female line of descent. These mutations are derived from copying mistakes, when the DNA is copied it is possible that a single mistake occurs in the DNA sequence, these single mistakes are called single nucleotide polymorphisms (SNPs).

Human Y chromosomes are male-specific sex chromosomes; nearly all humans that possess a Y chromosome will be morphologically male. Y chromosomes are therefore passed from father to son; although Y chromosomes are situated in the cell nucleus, they only recombine with the X chromosome at the ends of the Y chromosome; the vast majority of the Y chromosome (95%) does not recombine. When mutations (SNPs) arise in the Y chromosome, they are passed on directly from father to son in a direct male line of descent. The Y chromosome and mtDNA therefore share certain properties.

Humans are all linked to one man in reference to their Y chromosome, scientists named him [Adam][1]. He was alive about 60,000 years ago near Kenya, Africa. Everyone shares his DNA, which means we are all descendants of him. Humans are also linked to one female also found near Kenya, Africa. [Mitochondrial Eve][2] lived 140,000 years ago, much older than her counterpart Adam. Adam and Eve are considered our([MRCA][3]) [Most Recent Common Ancestor][4].

Other chromosomes, autosomes and X chromosomes in women, share their genetic material (called crossing over leading to recombination) during meiosis (a special type of cell division that occurs for the purposes of sexual reproduction). Effectively this means that the genetic material from these chromosomes gets mixed up in every generation, and so any new mutations are passed down randomly from parents to offspring.

The special feature that both Y chromosomes and mtDNA display is that mutations can accrue along a certain segment of both molecules and these mutations remain fixed in place on the DNA. Furthermore the historical sequence of these mutations can also be inferred. For example, if a set of ten Y chromosomes (derived from ten different men) contains a mutation, A, but only five of these chromosomes contain a second mutation, B, it must be the case that mutation B occurred after mutation A. Furthermore all ten men who carry the chromosome with mutation A are the direct male line descendants of the same man who was the first person to carry this mutation. The first man to carry mutation B was also a direct male line descendant of this man, but is also the direct male line ancestor of all men carrying mutation B. Series of mutations such as this form molecular lineages. Furthermore each mutation defines a set of specific Y chromosomes called a haplogroup. All men carrying mutation A form a single haplogroup, all men carrying mutation B are part of this haplogroup, but mutation B also defines a more recent haplogroup (which is a subgroup or subclade) of its own which men carrying only mutation A do not belong to. Both mtDNA and Y chromosomes are grouped into lineages and haplogroups; these are often presented as tree like diagrams.

[edit] 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

[edit] Drawbacks

People who resist testing may cite one of the following concerns:

  • Cost
  • Quality of testing
  • Concerns over privacy issues
  • Loss of ethnic identity

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.

[edit] 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.

[edit] Genetic similarity among relatives

Where the genogram or family tree of individuals is known, it can be used to determine the genetic identity between individuals. It is often described as percentage of genetic identity, referring to the fraction of genome inherited from common ancestors, and not actual genomic identity, which is always approximately 99.9%[9] identical from one human to another.

One method of calculating this genetic similarity is to do an inbreeding calculation by the path or tabular method and then multiply by 2, because any progeny would have a 1 in 2 risk of actually inheriting the identical alleles from both parents. For instance, a brother/sister relation gives 25% risk for two alleles to be identical by descent.

Degree of relation Genetic identity (%) Relation examples
100% Identical twins
1st 50%
  • parents
  • siblings
  • children
  • fraternal twins
2nd 25%
  • grandparents
  • grandchildren
  • aunts
  • uncles
  • nieces
  • nephews
  • half-siblings
  • Double first cousins
  • identical twin cousins (children of identical twins)
3rd 12.5%
  • first cousins
  • great grandparents
  • great grandchildren
  • great aunts
  • great uncles
  • grandnieces
  • grandnephews
  • half-aunts
  • half-uncles
  • half-nephews
  • half-nieces
4th 6.25%
  • Great great grandparents
  • great great grandchildren
  • great great aunts
  • great great uncles
  • great grandnieces
  • great grandnephews
  • first cousin once removed
  • great half-aunts
  • great half-uncles
  • half-grandnephews
  • half-grandnieces
5th 3.13%
  • great great great grandparents
  • great great great grandchildren
  • second cousins
  • first cousin twice removed
  • great great great uncles
  • great great great aunts
  • great great grandnieces
  • great great grandnephews
  • great great half-aunts
  • great great half-uncles
  • great half-grandnephews
  • great half-grandnieces
Unless else specified in table, then source is: [10]

[edit] References

  1. ^ George H. Darwin, "Note on the Marriages of First Cousins", Journal of the Statistical Society of London 38:3 (Sep., 1875), pp. 344-348. DOI
  2. ^ Steve Olson, "Who’s Your Daddy?", The Atlantic, Jul-Aug 2007, accessed 19 Feb 2009
  3. ^ Guido Deboeck, "Genetic Genealogy Becomes Mainstream", BellaOnline, accessed 19 Feb 2009
  4. ^ "The Genographic Project: A Landmark Study of the Human Journey", National Geographic, accessed 19 Feb 2009
  5. ^ Genebase, Genetic Genealogy, accessed 19 Feb 2009
  6. ^ "How Big Is the Genetic Genealogy Market?", The Genetic Genealogist, accessed 19 Feb 2009
  7. ^ Cassandra Franklin-Barbajosa, "In the Wake of the Phoenicians: DNA study reveals a Phoenician-Maltese link", National Geographic Online, Oct 2004, accessed 19 Feb 2009
  8. ^ "DNA Clans (Y-Clan)", DNA Ancestry Analysis, Genebase, accessed 19 Feb 2009
  9. ^ AMNH > Our genetic identity Retrieved on 21 Mars, 2009
  10. ^ Ancestry.com > Degrees of Relation and Number of Genes Shared Retrieved on 21 Mars, 2009

Jobling, Mark; Chris Tyler-Smith (August 2003). "The human Y chromosome: an evolutionary marker comes of age" ([dead link] – Scholar search). Nature Reviews Genetics (Nature Publishing Group) 4: 599–612. doi:10.1038/nrg1124. http://www.gs.washington.edu/courses/king/46506/YChromosome_NatRevGen2003.pdf. 

[edit] See also

Main article: List of genetic genealogy topics

[edit] Recommended readings

  • Terrence Carmichael and Alexander Kuklin (2000). How to DNA Test Our Family Relationships. DNA Press. Early (and still unique) book on adoptions, paternity and other relationship testing. Carmichael is a founder of GeneTree.
  • L. Cavalli-Sforza et al (1994). The History and Geography of Human Genes. Princeton: Princeton University Press. Dense but very comprehensive.
  • Luigi-Luca and Francesco Cavalli-Sforza (1998). The Great Human Diasporas, translated from the Italian by Sarah Thorne. Reading, Mass. : Perseus Books. More readable than the Stanford professor’s other books.
  • Colleen Fitzpatrick and Andrew Yeiser (2005). DNA and Genealogy. Rice Book Press. Highly regarded commentary on news stories about DNA and how-to introduction for laymen.
  • Clive Gamble (1993). Timewalkers: The Prehistory of Global Colonization. Stroud: Sutton. Popular account of human prehistory by British anthropologist/archeologist. Article from American Scientist.
  • Cyndi Howells (n.d.). Netting Your Ancestors – Genealogical Research on the Internet. Baltimore: Genealogical Publishing Company. Guide to electronic sources by author of Cyndi’s List website.
  • M. Jobling (2003). Human Evolutionary Genetics. Standard college and graduate school level textbook by leading expert.
  • Steve Olson (2002). Mapping Human History. Boston: Houghton Mifflin Company. Survey of major populations.
  • Stephen Oppenheimer (2003). The Real Eve. Modern Man’s Journey out of Africa. Carroll & Graf. Champions the “beachcomber route” theory with much technical detail.
  • PBS (2003). The Journey of Man DVD. Broadcast aired in January 2003, Spencer Wells, host.
  • Donald Panther-Yates and Elizabeth Caldwell Hirschman (2006). “DNA Haplotyping and Diversity: An Anthropogenealogical Method for Researching Lineages and Family Ethnicity,” International Journal of the Humanities 2:2043-55. Guide to finding matches in world databanks and interpreting genetic information in terms of history and recent emigration studies.
  • Chris Pomery (2004) DNA and Family History: How Genetic Testing Can Advance Your Genealogical Research. London: National Archives. Early guide for do-it-yourself genealogists. Now updated (2007) as Family History in the Genes: Trace Your DNA and Grow Your Family Tree.
  • Alan Savin (2003). DNA for Family Historians. Maidenhead: Genetic Genealogy Guides. Slim paperback first published in 2000, now available also in German.
  • Thomas H. Shawker (2004). Unlocking Your Genetic History: A Step-by-Step Guide to Discovering Your Family's Medical and Genetic Heritage (National Genealogical Society Guide, 6). Guide to the difficult subject of family medical history and genetic diseases.
  • Megan Smolenyak and Ann Turner (2005). Trace Your Roots with DNA: Using Genetic Tests to Explore Your Family Tree. Rodale Books. Recent tool for amateur genealogists by seminar speaker and DNA listserv moderator.
  • Linda Tagliaferro (1999). The Complete Idiot’s Guide to Decoding Your Genes. Alpha Books. Uses everyday language to explain the role genes play in shaping who we are.
  • Spencer Wells (2004). The Journey of Man. New York: Random House.

[edit] External links and resources

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[edit] Maps

[edit] News

  • MSNBC — Genetic Genealogy Front Page

[edit] Research facilities/institutions and organizations

[edit] Informational websites

[edit] Haplogroup and Surname Projects

[edit] DNA databases

[edit] Y chromosome (Y-DNA) testing

[edit] Mitochondrial DNA (mtDNA) testing

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