Can a Genetic Relationship Be Proved by DNA Alone?
By Karen Stanbary, CG
The answer is a resounding No!
Sound genealogical conclusions rely on skillful analysis of sources, information, and evidence.1 Information extracted from sources becomes evidence only when applied to a well-formulated research question. DNA results must be considered with at least two other types of information to fully address a research question—proper interpretation of the amounts of shared DNA against known relationship estimates and the specific context of the familial composition.
Genetic evidence provides high quality information about biological relationships. Our DNA is akin to an original record providing information about our ancestral lines. When it’s possible and applicable to a question, incorporating analysis of DNA data into our proof arguments provides increased confidence that our conclusions are accurate.2
Genealogy Standards offers guidance on crafting effective research questions.3 The question “How is person A genetically related to person B?” is neither too broad nor too narrow. It allows us to impartially collect information from our sources without prejudgment. Consider a case where one source provides the information that A and B share 3587 centiMorgans (cM) of autosomal DNA (atDNA). Researchers use the centiMorgan—a measure of genetic linkage based on the probability of recombination—to predict a relationship between test takers.4 We consult a second source to learn that 3587 shared cM is predictive of a parent-child relationship.5 We combine those two pieces of evidence, yet we still do not know if person A is the parent or the child. We add a third item—documentary evidence on the ages of the individuals. This third evidentiary shard, when correlated with the other two, provides convincing evidence to answer the research question.6 We build a case and share our findings in writing.7
As researchers, we try as hard to disprove our hypothesis as we try to prove it. This process assures us our conclusion is accurate. For this example case, a few rare circumstances might provide an alternate conclusion—the parent is an identical twin or one individual in the studied pair had a stem cell transplant.8 Those alternatives must be considered and ruled out before we can say we have genealogical proof.
It is wise to create disciplined research habits. While the answer to the above scenario might seem obvious, answers about other genetic relationships are not. Rigorous analysis of DNA information and its application as evidence to well-crafted research questions become even more critical when looking at more distant genetic relationships. DNA evidence when skillfully combined with documentary evidence of the family context can provide the accurate genealogical conclusions we all seek.
1 Elizabeth Shown Mills, “Quick Lesson 17: The Evidence Analysis Process Map,” Evidence Explained: Historical Analysis, Citation & Source Usage (https://www.evidenceexplained.com/content/quicklesson-17-evidence-analysis-process-map). Also, Thomas W. Jones, Mastering Genealogical Proof (Arlington, Va.: National Genealogical Society, 2013), 1–22.
2 Board for Certification of Genealogists, Genealogy Standards (Nashville, Tenn.: Ancestry, 2014), 23–24 for Standard 38, “Source preference.”
3 Ibid., 10–11 for Standard 10, “Effective research questions.”
5 Genealogy Standards, 17 for Standard 24, “Understanding meanings.” For the specific source, see Blaine T. Bettinger, “August 2017 Update to the Shared cM Project,” blog post, 26 August 2017, The Genetic Genealogist (https://thegeneticgenealogist.com/2017/08/26/august-2017-update-to-the-shared-cm-project/). The average reported shared atDNA for a parent-child relationship is 3487 cM, with a range of 3330–3720 cM.
6 Genealogy Standards, 27 for Standard 47, “Evidence correlation.”
7 Ibid., 28–29 for Standard 50, “Assembling conclusions from evidence,” and 32–33 for Standard 53, “Selection of appropriate options.”
8 The author credits Blaine Bettinger, The Genetic Genealogist of Baldwinsville, New York, for the example involving identical twins. For the stem cell transplant example, the author credits Ruy Cardoso, CG, of Newtonville, Massachusetts.
The words Certified Genealogist and letters CG are registered certification marks, and the designations CGL and Certified Genealogical Lecturer are service marks of the Board for Certification of Genealogists®, used under license by board certificants after periodic evaluation.