Using Autosomal DNA To Help Establish Proof

DNA testing has become increasingly popular in the genealogical community over the past few years, and test results are now often used to help establish proof of relationships. In his case study, “Too Few Sources to Solve a Family Mystery? Some Greenfields in Central and Western New York,” which appeared in the June 2015 issue of the National Genealogical Society Quarterly, Thomas W. Jones, PhD, CG, CGL, FASG, FNGS, uses documentary and genetic evidence to prove two generations of Calista Jane Greenfield’s ancestors.

Calista was raised as a servant in the family of Isaac Huffman of Niagara County, New York. No records pertaining to Calista identify her parents, but family lore provides some details about their lives and the circumstances surrounding Calista’s early years. This context, along with indirect documentary evidence, points to Nathaniel Greenfield of Cayuga County, New York, as Calista’s father. Based on the little that is known about Nathaniel and his possible connection to Isaac Huffman, Jones hypothesizes that Nathaniel may be the son of Thomas Greenfield of Cayuga and Herkimer counties, New York. Documentary evidence connecting Calista to her father Nathaniel is convincing; however, little documentary evidence connecting Nathaniel to Thomas Greenfield exists, and DNA evidence helps to prove that connection.

About Autosomal DNA
Jones uses autosomal DNA test results as evidence to help support his overall proof argument. In order to understand the methodology in his case study, a basic knowledge of autosomal DNA is necessary.[1]

Everyone has two copies of each of their twenty-three chromosomes—one copy inherited from their mother, and one copy inherited from their father. Of those twenty-three pairs of chromosomes, one pair determines gender. The remaining twenty-two pairs are the autosomal chromosomes.

The autosomal DNA that each person receives from his or her parents shuffles around from generation to generation through a process called recombination. When a child is conceived, the father’s two copies of a chromosome mix with each other, and then a random half of that chromosome’s mix passes to the child, as one of their two copies. The same recombination happens with the mother, and the random half of her chromosome’s mix passes to the child as the other one of their two copies. This recombination happens for each of the twenty-two autosomal chromosomes. The result is that a child inherits autosomal DNA from numerous ancestors from all branches of his or her family tree.

Image: DNA Double Helix "DNA Double Helix" via Flickr Commons

Image: DNA Double Helix “DNA Double Helix” via Flickr Commons

When a person takes an autosomal DNA test, the genetic testing company searches for patterns within their DNA. If two people have an identical pattern on the same section of the same chromosome (called a “segment”), they are genetic matches to each other. This means they share a common ancestor, from whom they both inherited that segment of DNA.[2]

A matching segment between two people is measured in centimorgans. Some genetic testing sites provide testers with the sizes of segments of DNA they share with others (in centimorgans), and also identify on which chromosomes that DNA is shared. The predicted relationship between two individuals is based on the total amount of DNA they share, and other factors such as segment length.

Greenfield DNA
The autosomal DNA of Nathaniel Greenfield’s living descendants helps establish Thomas Greenfield as his father. Eleven of Nathaniel’s descendants took autosomal DNA tests, and the results of four of them (all Calista’s documented great-great-grandchildren) were presented in the case study. Additionally, five documented descendants of Thomas Greenfield, through his children Luther, Caleb, and Lovilla, took autosomal DNA tests. These individuals are all great-great-great-grandchildren of Thomas Greenfield. The four descendants of Calista who were tested would be fifth cousins once removed to the five descendants of Thomas who were tested, if Calista’s father, Nathaniel, was Thomas’s son.

Each of Thomas Greenfield’s five descendants matches at least one of Calista’s descendants. One of Thomas’s descendants, through daughter Lovilla, matches three of Calista’s four tested descendants. There are also two triangulations—when several people within a group match each other on the same segment of a chromosome—among the descendants. On chromosome 3, one of Calista’s descendants and two of Thomas’s descendants match on the same segment. On chromosome 18, three of Calista’s descendants and one of Thomas’s descendants match on the same segment.

Fifth cousins share, on average, 3.32 centimorgans (.0488 percent) of their DNA, inherited from a common ancestor. Some share more, some share less, and some share none. Thomas’s descendants share between 7.6 centimorgans and 14.9 centimorgans of DNA with Calista’s descendants. Additionally, all of the descendants’ kits were uploaded to GedMatch, a third-party website that allows for comparison of DNA test results from several companies. GedMatch provides estimates of the number of generations to the most recent common ancestor, based on mathematical calculations from DNA data. The average number of generations among all pairings within the two groups of descendants is 5.64, consistent with the hypothesized relationships of these individuals.

Jones also considers several other probable relationships between Nathaniel and Thomas, including first cousins, uncle/nephew, brothers, half-brothers, and parent/child. However, genetic and documentary evidence rule out these possibilities. He also recognizes and rules out the possibility that Calista’s mother was a Greenfield by descent, and the possibility that Calista’s descendants’ connection to the other Greenfield relatives was through Calista’s husband.

Dr. Jones’s methodology included testing the autosomal DNA of several of Calista’s descendants, testing a group of descendants of Thomas Greenfield (a candidate to be her grandfather), and comparing the test results for evidence of relationships between the descendants. Not all of Calista’s descendants were genetic matches to all of Thomas’s descendants; however, there were several pairings and two triangulations among the groups, which provided data consistent with the hypothesized relationships. Jones’s case study demonstrates how autosomal DNA test results can provide evidence necessary to supplement documentary sources and establish proof of relationships.

[1] See Debbie Parker Wayne’s article, “Using Autosomal DNA For Genealogy,” in NGS Magazine for more details.

[2] There are exceptions to this rule when small segments of shared DNA are identical by state (IBS) as opposed to identical by descent (IBD). See Identical By Descent.

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