23 May 2018

DNA Standards - Pedigree Analysis (Tree Analysis)

20 June 2018: Updated to link to new page for the survey on BCG website.

This is part of a series on Genealogy Standards for using DNA. This series represents the opinions and interpretation of the proposed standards by this author and does not necessarily reflect BCG’s official position. The proposed standards are not being addressed in numerical order, but all articles will be linked. For other parts in the series see

You can participate in a survey and provide your opinion on the Proposed DNA Standards through a Google Docs survey linked from https://bcgcertification.org/proposed-dna-standards-for-public-comment/. Please leave comments by 23 July 2018 explaining your agreement or disagreement with the proposed standards. Comments will be used to modify the standards as needed before acceptance and publication.

Proposed new DNA standard (proposed standard numbers may change before acceptance and publication) #3 is:

What does this all mean? Standards are written formally and most of us understand informal language better. Breaking down each segment makes the meaning more clear.

The first thing many researchers who share DNA do is compare pedigrees (family trees) or surname lists looking for a common ancestor (or an ancestral couple; the term common ancestor will be used for simplicity even when an ancestral couple may be the source of a DNA segment). The first common surname, person, or couple found is often assumed to be the source of the shared DNA segments. That assumption may be right or wrong. More evidence is needed to determine which is more likely.

Genealogists must also consider that two test takers who share DNA may not have inherited all of that DNA from one common ancestral couple. If two test takers are related in more than one way (such as through pedigree collapse or endogamy) this can be difficult to determine except with thorough research and correlation of the DNA and documentary evidence.

© 2018, Debbie Parker Wayne

When analyzing pedigrees there are three critical concepts. Some common things to review when analyzing pedigrees are listed here.

  1. Accuracy of the pedigree: a pedigree either has the correct ancestors linked for each generation or it does not. If the pedigree of any DNA test-taker under analysis is inaccurate then the common ancestors may never be identified.

    Accurate pedigrees are the result of research that meets the Genealogical Proof Standard (See "Useful References" below; the GPS summarized and paraphrased is): A focused research question, thorough research, correctly cited sources, thorough and competent analysis and correlation of all evidence that is pertinent to the question, resolution of any conflicting evidence, and a sound written conclusion).

    Researchers can analyze the accuracy of pedigrees by confirming the consistency of assertions (no children born when a parent would be too young, too old, deceased for more than nine months, in a different location at the time of conception, etc.) and that the most credible sources support each assertion.

    See “Accuracy” at the bottom left of the pedigree image.

  2. Depth of the pedigree: ideally, each DNA test taker’s pedigree chart should be complete back to the level of the hypothesized common ancestor, and preferably a few generations further back. If two DNA test takers are predicted to be third cousins, then both pedigrees should be complete at least back to the second-great-grandparents (the hypothesized common ancestral level). An extra generation or three in each tree helps if the test takers inherited more than the statistical average amount of DNA; in that case they may actually be fourth or fifth cousins instead of the predicted third cousins.

    See “Depth” at the top left of the image. In this example, all names are complete up to the great-grandparent level that would be shared with second cousins. However, all of the missing information on the birth, marriage, and death of many of these ancestors indicates this tree is not deep enough or verifiably accurate enough even at this level.

  3. Gaps in the pedigree: ideally, each pedigree will be complete with no gaps. In the real world many researchers have brick walls on some lines or just have not had time to research every possible line yet. Add to that the fact that every time a new ancestor is identified the next step is to identify that ancestor’s parents making genealogy truly a never-ending search.

    See “Gaps” at the top right of the image. Those gaps in the tree may be hiding the common ancestor or perhaps a second (and third, fourth, and so on) common ancestral line shared by two DNA test takers. Our conclusion may be easily overturned if we do not consider those other possible shared ancestors. We can address the gaps by one or more of the following

    • Doing further documentary research to fill in the gaps—we would want to do this eventually as we work on our pedigree, but a specific DNA match may focus our research on a specific line now

    • Target test more cousins, or find more test takers in our match list who share the same ancestor, to gain more DNA evidence to support the conclusion—in some cases (like burned counties) there may be little to no documentary evidence to be found. DNA evidence may help answer the question, but more than two or three DNA test takers will be needed to credibly support most conclusions

    • Clear explanations may justify a conclusion that a gap is irrelevant to the research question—perhaps the pedigree gap is in a line that originated or resided in a locale that is irrelevant to the focus question, or it is a line with a different biogeographical origin, or the gap is so far back in the pedigree it is not relevant based on the DNA evidence, and there are other possibilities

    • Segment triangulation does not work in every situation, but when it exists it can be strong evidence—all cousins will not share every triangulated segment, but groups of cousins may share one triangulated segment, while some of those cousins may also share segments with cousins in a different group—showing how each of the groups overlaps may support a conclusion

    • Clustering and genetic networks work in a similar way to triangulated segment groups. Many names are used for clusters or networks: shared matches, in common with groups, DNA circles, matches who share DNA with both of two kits, and more—for example, a group of cousins share DNA with each other, a second group of cousins share DNA, and there may be some cousins who are in both groups providing a link to the common ancestor

Useful References:

Board for Certification of Genealogists, "Ethics and Standards," scroll down to "Genealogical Proof Standard (GPS)" (https://bcgcertification.org/ethics/ethics-standards/).

Board for Certification of Genealogists, Genealogy Standards, 50th anniversary ed. (Nashville, Tennessee: Turner Publ., 2014; https://bcgcertification.org/product/bcg-genealogy-standards/).

Genetic Genealogy Standards Committee, Genetic Genealogy Standards, http://geneticgenealogystandards.com/.

Full disclosure:

I have held Certified Genealogist® credentials from BCG since September 2010. I helped form the BCG Genetic Genealogy Committee to discuss DNA standards. I resigned from the committee due to personal commitments, but have continued to participate as an adviser, reviewer, and in other ways.

I support the adoption of standards to be used when incorporating DNA analysis into a genealogical conclusion. I support BCG seeking input on the proposed standards from the greater genealogical community using DNA. I see this as a positive step to ensure newly adopted standards will meet the needs of the entire research community. No matter what is adopted, updates will certainly be needed just as research methodology and documentary research standards have evolved over the decades.

All statements made in this blog are the opinion of the post author. This blog is not sponsored by any entity other than Debbie Parker Wayne nor is it supported through free or reduced price access to items discussed unless so indicated in the blog post. Hot links to other sites are provided as a courtesy to the reader and are not an endorsement of the other entities except as clearly stated in the narrative.

To cite this blog post:
Debbie Parker Wayne, "DNA Standards - Pedigree Analysis (Tree Analysis)," Deb's Delvings, 23 May 2018 (http://debsdelvings.blogspot.com/ : accessed [date]).

© 2018, Debbie Parker Wayne, Certified Genealogist®, All Rights Reserved

No comments:

Post a Comment