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Matching DNA of Other Test-Takers Reveal Ancestry

A Work In Progress by Herbert Holeman, PhD.

Draft Workpaper Posted.

Still another way of revealing ancestral admixture is by researching other test-takers with matching DNA. For discussion purposes, my DNA tests and findings are used as examples.

• I share an ancestor in common with twenty other DNA test-takers. Seven are chosen by matched DNA. Thirteen are chosen by testing companies from other test-takers family trees.

• I also share with them, admixture in Southeast Asia, southern Europe, and Mesoamerica.

• The findings from DNA Matching is consistent with results from autosomal, mtDNA, and Y-DNA tests.


My ancestry as determined by autosomal, mtDNA, and Y-DNA tests can be fine-tuned through matching my DNA with other DNA test-takers. This is based on Genetics being the study of inheritance. Therefore, my having shared DNA segments with another person is indicative of a shared blood relationship, meaning we have inherited our shared segments from a common ancestor.

Popular among DNA company offerings is the DNA Relatives Matching feature. People with matching DNA likely share genetic ancestry and admixture. And, most significant is when matching DNA is Identical-by-descent (IBD).

I should have both ancestors and ancestral admixture in common with other test-takers whose DNA matches mine that leads to a common ancestor, a person from whom we are descended. Moreover, if the DNA segments we share are large, the more certain is our blood relationship.

The amount and size of shared DNA between people, is measured in centimorgans cMs). As a unit of genetic measurement, one measure describes how closely I am related to another person by the total length of matching DNA shared with another test-taker based on the chart in Figure 28 created by Blaine Bettinger. He also offers an easy online calculator for this purpose as well.

Figure 28
Relationships by Total DNA Shared

However, the increase in shared DNA in the offspring of cousin marriages skews, evaluating actual relationships. More than 80% of all marriages were estimated to be cousin marriages, as recently as just three centuries ago. One explanation for this may be when these marriages took place. Endogam would be more likely in small, scattered villages before the transition to today's large, dense urban populations.

Matches based on size of Longest-Segment

Another measure using matching DNA measured in cMs is with the length of the longest DNA segment shared with another test-taker. The length of such a segment is used as a statistical yardstick to determine relatedness. A common ancestor having lived more recently would have longer length segments. Mercedes Brons' says two people related within the last 10 generations or so, would be expected to have shared DNA across relatively large stretches of DNA - that is, more than would be expected by chance. This is her rule of thumb to estimate how small or large a chance a long segment size has of signifying a common ancestor relationship.

• Less than 12 cMs = small

• Between 12-50 cMs = medium

• Between than 50-100 cMs = large

• Longer than 100 cMs = very large

In essence, the longer the matching DNA segment, the closer the relationship between persons and the more recent their relationship. Likewise, a shorter, matching long-segment suggests the relation is a more distant common ancestor. Long-segments become shorter and even diminish as they pass from one generation to the next. DNA segments will split up or lost as the number of generations grows more distant between me to a common ancestor.

In general, Brons reports segment matches greater than 45 centimorgans (cMs) guarantees a shared common ancestor. That is the DNA segment is IBD. It leads to my sharing a common ancestor with the person I match. Brons says these matches have an almost statistically insignificant chance of being just coincidence. Brons adds, "one other great thing about these long segments, is the fact that you can just about bet the farm on them and that the common ancestor is not very many generations removed."Also worth noting is that segment length is not usually affected by endogamy.

Aside from my nuclear family, seven test-takers share a segment-length of ~46 or more cMs with me.

In my case, Figure 29 reports those DNA test-takers who meet the criterion for my likely sharing a common ancestor based on the length of the longest DNA segment. In general, for the same amount of cMs shared, longer segments give a closer predicted relationship than a greater number of shorter segments (Num.Seg). The chart in Figure 29 also reports the number of segments shared, but and other companies do not use the number of shared segments in predicting relationship and only use the total amount of shared centimorgans.

Figure 30
Longest Segment DNA > 15cMs
Shared by me with Test-takers

One test-taker matches me on a very large segment-length:

• This person is identified as 10AN and is also evaluated by testing company analysis as my 1st or 2nd cousin. However, she is 25 years younger than me. She is of 100% Asian ancestry (the Philippines) and is related to other test-takers identified as 05AN, 09AN, 10AN, and 11AN.

Six test-takers match me on a large segment length:

• Two of them, 05AN and 09AN, are also evaluated by the testing company as being in the range of close family or 1st cousins Both are related to each other and are of split ancestry -- Asian-European (the Philippines and NW Europe) with no ancestry in the Americas.

• One of them, 12MH, is likely related to 05AN, 09AN, 10AN, and 11AN, and also likely possessing Asian (Philippines) ancestry. The DNA testing company evaluates him as my 1st cousin.

• Another one, 11AN, is 100% Asian ancestry (the Philippines). She is related to test-takers, 05AN, 09AN, and 10AN. The DNA testing company evaluates her as my 1st cousin.

• Two of them, 45AN and 46AN, differ from the other four in their ancestry. These two have no ancestry in Asia, but do have in Mexico and a lesser amount in Europe.

To consider how these matches relate to me, a useful Bettinger chart in Figure 31, combines both total shared cMs and shared longest-segment to estimate a relationship.

Figure 31
Relationship by Shared DNA
and Longest Segment

Matches based on shared Common Ancestor(s)

The companies, Ancestry, FTDNA, and MyHeritage accomodate test-takers in uploading their family trees to the company. It allows DNA findings and family trees to be used by companies to identify genetic ancestors test-takers share in common. That said, thirteen test-takers are identified in Figure 32 who share a common ancestor with me. However, as Kitty Cooper points out "a relative who shares a DNA segment from a common ancestor with you will match you along the chromosome you got from the parent who is descended from that ancestor.

To date, data are collected on five these test-takers (70AN, 85AN, 86AN, 98AN, 116AN, 103AN). Yet the size of each of their longest-segments is less than the Brons' threshold (45 cMs), which offers the better chance the matching DNA segment is IBD.

Figure 32
Common Ancestors
Shared by me with Test-takers

As for their ancestry, the ancestry of 70AN, is reported as being 42% from Spain, 20% from Portugal, 3% from Mexico, and having no Asian ancestry. All five are from related families, with similar ancestry, and share my Great-great-grandparent as our common ancestor. He appears in their family trees and mine. Figure 33 provides useful way of viewing relationships (particularly for common ancestors with in terms of consagnguinity (degree of kinship). It depicts relationships with direct ancestors, descendants, siblings, and cousins in terms of percent DNA shared and amount of DNA.

Figure 33
Consagnguinity Relationships

This verbatim quote from an article in the September 2020 of Nature speaks to the concept of common ancestors: "Combining DNA samples with family trees is the core of forensic genetic genealogy. The process rests on the simple statistical rules of genetics. A parent and child, or two siblings, share 50% of their DNA. Grandparents and grandchildren share 25%. Even distant relatives share small portions of DNA. This allows consumer genetic-testing companies to estimate relationships between two individuals who have submitted samples, as far out as fourth cousins (who share a pair of great-great-great grandparents)."

Accordingly, based on the Estes chart in Figure 34, I would share ~6.25% of my total DNA with the common ancestor identified by the testing company as my Great-great-grandparent. The website of Legacy Tree Genealogists offers ways to explore Probable Relationship Levels Based on Total Shared Centimorgans.

Figure 34
Expected Inherited DNA

Overall, DNA Matching on segment length reveals the test-takers and I share most of our admixture in the present-day countries of the Philippines, Spain, and Mexico in that order. As a mere hundred years ago, these countries were part of the Spanish Empire, it may account for our admixture.

In any case, the admixture findings here are consistent with results from my autosomal, mtDNA, and Y-DNA tests as reported in sthe following work pages.

Copyright © 2018 Herbert P. Holeman, Ph.D. All Rights Reserved.