Basics of DNA

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Revision as of 16:12, 20 December 2012

Basics of Family History Research

This article is part of a series.
Overview of Family History Research
Home Sources
Family History Collaboration
Basics of Family History and Technology
Basics of DNA
Basic Record-keeping
Evaluation and Goal Setting
Family History in Time and Place
Family History Etiquette, Ethics, Legalities
List of Useful Resources for Beginners
Topics

This article originally appeared in "The Foundations of Family History Research" by Sandra Hargreaves Luebking, FUGA, and Loretto Dennis Szucs, FUGA in The Source: A Guidebook to American Genealogy This article was written by Donn Devine, CG, CGI.

One of the newest developments in genealogy is the use of DNA (deoxyribonucleic acid) as a source of genealogical information. DNA is the substance within every living cell that carries the code for passing on its exact makeup to new cells, and although DNA is uniquely different for each individual, it is similar in cells of related individuals. As applied to genealogical research, distinctive DNA patterns can be used to determine whether and how closely individuals are related to other individuals whose DNA patterns are known.

Genealogical DNA testing looks at the non-coding portions of the DNA strand (sometimes misleadingly called junk DNA) that have no known function. For the most part, these stretches of DNA remain unchanged from generation to generation. However, chance changes, called mutations or polymorphisms, do occur at infrequent intervals, and it is these changes that let us distinguish different lines of descent and determine how closely people may be related to each other from the closeness of their DNA matches. A DNA sequence that is passed on unchanged from one parent to a child is called a haplotype, and these are the distinctive patterns we use to establish genealogical links.

Contents

Three Types of DNA

Y chromosome DNA is found only in males and is the type most frequently used in genealogy because almost all of it passes as a single haplotype from father to son, essentially unchanged except for chance mutations. This type of DNA is used to identify a common male ancestor in all-male genealogical lines.

mtDNA is a haplotype that children inherit only from their mothers and can be used to identify all-female genealogical lines. Two people who share the same mtDNA haplotype have a common female ancestor in their all-female maternal lines. But, because mtDNA mutates much more slowly than Y-DNA, she may be too many generations back to identify or be of genealogical significance.

Autosomal DNA is found in our 22 chromosomes numbered one to twenty-two. Autosomal DNA is a fairly recent and better way of determining ancestry because it provides concrete evidence of a cousin relationship. Sharing DNA segments which are identical by descent (IBD) indicates common ancestry. A higher percentage of DNA shared indicates a closer ancestor. For example, a person might share 50.23% with a child; 25.88% with a grandparent, and 1.5% with a 3rd cousin. Cousins identified through autosomal testing can share ancestry notes and insightful genealogical clues. It is not uncommon to find cousins who share DNA, living on distant continents.

Genealogical Uses for DNA Tests

Additional Identity Item

For those ancestors at the head of an ancestral line, for whom we may know little more than a name and event date or place, a DNA sample from an appropriate descendant will provide the same pattern present in the ancestor, in the absence of any chance mutation along the way. For many family historians, a test of their own DNA is often their first step, providing a genetic signature for a distant paternal-line or maternal-line ancestor. Matching samples from two descendants through different lines provides assurance that the common ancestor’s DNA sequence descended unchanged, with no mutation in either line.

Verifying Probable or Suspected Relationships

Verifying relationships is perhaps the most frequent use being made of DNA, as tests can quickly determine whether any two men descend from a common ancestor through their all-male surname line or whether any two people of either sex are related through their all-female maternal lines to a common female ancestor. However, the number of generations to the common ancestor, if not known from other sources, can be only estimated. A widely publicized example of this application was the Jefferson-Hemings study. There were no sons from President Thomas Jefferson’s marriage, but DNA tests showed that a male-line descendant of his slave Sally Hemings shared the same DNA as descendants in two male lines from the president’s Jefferson grandfather, proving that a Jefferson fathered at least one of Hemmings’s children.[1]

Sorting Family Lines

People with the same surname frequently come from very different ancestral origins. DNA can show which share a common heritage, can show which are unrelated, and, with enough samples associated with ancestral localities of origin, can point modern descendants to their family’s geographic origin. For example, there were four families named Smolenyak living near each other in the tiny Slovak village of Osturma, but DNA tests on male Smolenyak descendants from each of the four families showed they were unrelated through the surname line.[2]

Family Reconstruction

Family and surname associations use DNA to confirm links in lines where records are ambiguous or less than convincing. Associations are also establishing previously unknown links of some members’ lines to known founder-ancestors. The Stidham Family Association sought proof that two lines, with problematic record links, truly descended from a seventeenth-century ancestor. DNA provided the assurance, but also revealed that another line, with clear documentary evidence of descent, was not biologically connected to the ancestor.[3]

Future Promise

The DNA, called autosomal DNA, is widely used for forensic identification and for verifying paternity because individuals receive a full copy of DNA from each of their parents, which then pairs to form the individual’s DNA. During production of an individual's egg or sperm, the paired DNA is randomly shuffled and recombined to create a combined version of a person's parent's DNA. This shuffled recombined copy is passed fully to the child. In each following generation, the genetic code is further randomly shuffled and recombined as DNA passes to a new generation. Small sequences of genes that pass unchanged over many generations are called haplotypes. A haplotype can occur when all the grandparents share an identical sequence of genes within a chromosome, so quite naturally no shuffling can occur on that particular segment for the resulting grandchild, because all the original combining segments were identical.

Most sections of one's autosomal DNA represent a fully randomized mixture of unidentifiable DNA from your ancestors. The human genome consists of just over 3 billion DNA base pairs. But the shuffling process is very imperfect and oftentimes perfect, unshuffled, duplicate copies of DNA pass from a grandparent to parent to child. Over time homogenized groups of people who are relatively isolated also can come to share duplicated haplotype DNA sequences among the related population. In Tibet, for example, it was recently discovered that the Tibetan population shares a common, duplicated gene sequence that gives them resistance to high altitude cold weather. This haplotype sequence, unique to Tibetan's, is believed to have occurred fairly recently.

Small haplotype sequences can be inherited from a relatively small number of unknown ancestors among the thousands we had tens of generations back. But newer haplotype sequences can also be inherited from more recent ancestors. Large, stable populations tend to result in a diversity of haplotype gene sequences. Autosomal DNA is likely to find more uses in genealogy as a result of research now underway to identify inheritance patterns for haplotype segments in the DNA of the recombining chromosomes. The Sorenson Molecular Genealogy Foundation is testing sample donors from all over the world, comparing inherited DNA sequences on all their chromosomes with genealogies submitted by the donors (visit http://smgf.org for more information.)

Another worldwide research project, the National Geographic Society’s Genographic Project, is also searching for DNA markers that can be matched with geographic areas of ancestral origins.

Other laboratories are working on direct to consumer applications of data from one's autosomal chromosomes, using comparisons to known reference populations or to scientific studies. These genetic consumer applications using autosomal DNA can help illuminate deep roots. The results of genealogical significance are given in percentages, with rather wide confidence limits, and indicate how much of one's genetic heritage compares to reference populations that originally lived in Sub-Saharan Africa, Europe (including western Asia and the Mediterranean fringe), East Asia, and the Americas. These consumer test results may suggest avenues of research that might otherwise have been overlooked.

Autosomal consumer testing enables genetic cousins to discover each other and directly share ancestry information, through sites such as 23andMe.com, FTDNA.com and decodeme.com. Autosomal testing can also be a clever method for adopted children to find their blood relatives, and learn more about their ancestry or when a brick wall exists in one's tree, because an ancestor's parents are not known.

References

  1. E. A. Foster, et al., “Jefferson Fathered Slave’s Last Child,” Nature 396 (5 November 1998): 27–28.
  2. Family Tree DNA, “Spotlight: Smolenyak DNA Project,” Facts and Genes 2 (11 August 2003), downloaded 30 May 2004 from http://www.familytreedna.com/facts_genes.asp?act=show&nk=2.>.
  3. Richard L. Steadham, “The Saga of How Our Project Evolved,” with link to “Current Results of the Stidham DNA Study,” updated 24 February 2004, downloaded 4 June 2004 from http://homepages.rootsweb.com/~tstiddem/Pages/dna.html.

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