Ancestry Magazine

For the family historian, DNA is one more type of evidence. Used together with the names and associated dates and places of events obtained from traditional sources, it can help us identify an individual uniquely, and also distinguish a person from other people with similar names or ages who were in the same place at the same time.

Science

The most significant parts of the DNA strand—genes—contain the codes for our human and individual characteristics, including hair and eye color, facial features, even health predispositions. While genes are important, however, they’re not the part of the DNA strand used for genealogy.

Strands of  chromosomal  DNA are made up of base units (designated A, G, C, and T) that comprise the genetic code. The strands come in pairs—either A with T or C with G. The strands are twisted into the familiar double helix or screw-shaped structure. Since each strand mirrors the other, only one needs to be tested. There are two types of DNA found in each of our cells; for family history we look at parts of the strands that are inherited but don’t have any known effect on us. One type of DNA, chromosomal DNA, is a very long strand with part of its pattern inherited from each of our parents. It is divided among the 46 chromosomes found in the central nucleus of each cell.

The other type of DNA is a much smaller strand held in the mitocondria of each cell. This mitochondrial DNA, or mtDNA, is inherited entirely from our mother, and is traditionally most useful in determining ancient ancestal origin rather than finding distant cousins, although there are exceptions (see “Mother’s Eternal Influence,” page 56, for more information on mtDNA.)

Connection

Currently, the most useful DNA for family history is the small part of the strand found in the Y chromosome. It occurs only in males and is inherited intact from fathers, so Y chromosome DNA can be used to identify all-male surname lines. This small part of the strand is about 58 million base-pairs long (for comparison, the entire nuclear strand is about 3 billion base-pairs long). Y chromosome DNA tests can help tell how closely two people are related through their paternal lines. If they compare their results, called markers, and find an exact match of all their markers, they have a common ancestor in the not-too-distant past. Close but not exact matches show a common ancestor in the more distant past. A non-match rules out a common biological ancestor.

Results

DNA can’t replace good old-fashioned record research. But for the genealogist who wants additional evidence for an ancestral line, DNA testing is an excellent source of scientific data available at reasonable cost.

Donn Devine, CG, CGI, is an attorney and archivist and has been a board member of the National Genealogical Society and the Board for Certification of Genealogists.