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DNA Shared With Others

A Work In Progress by Herbert Holeman, PhD.

This draft workpaper was last modified .


• I share an ancestor in common with thirteen other DNA test-takers: Seven chosen by matched DNA; Six chosen by testing company evaluation.

• 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.


People with matching DNA likely share genetic ancestry. And, most significant is when matching DNA is Identical-by-descent (IBD).

My having shared DNA segments with another person is indicative of a blood relationship, meaning we have inherited our shared segments from a common ancestor. The common ancestor being a person from whom are descended. Moreover, if the segments we share are large, the more certain is our blood relationship.

Centimorgans (cMs) are units of genetic measurement. Two measures use cMs to describe how closely I am related to another person. One measure is the total length of matching DNA I share with another test-taker in cMs based on the Bettinger chart in Figure 30.

Figure 30
Relationships by Total DNA Shared

The other measure is also with matching DNA. It is the length of the longest DNA segment shared with another test-taker. 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. I use Brons' 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. Also, DNA segments will split up or lost as the number of generations grows more distant between me to a common ancestor.

Also worth noting is that segment length is not usually affected by endogamy. 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. Endogamy would be more likely in small, scattered villages before the transition to today's large, dense urban populations.

In general, Mercedes Brons reports segment matches greater than 45 centimorgans (cMs) in length offer a better chance 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.

But, segment matches greater than 45 centimorgans (cMs) guarantees a shared common ancestor. 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."

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

In my case, Figure 31 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).

Figure 31
DNA Shared 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.

A useful Bettinger chart in Figure 32, combines both total shared cMs and shared longest-segment to estimate a relationship.

Figure 32
Relationship by Shared DNA and Segments

Test company determination accounts for six other test-takers (70AN, 85AN, 86AN, 98AN, 116AN, 103AN) sharing a common ancestor with me. Yet each of their matching longest-segment lengths are less than the threshold of 45 cMs. The ancestry of one of them, 70AN, is 42% from Spain, 20% from Portugal, 3% from Mexico, and she has no Asian ancestry. The others are from the same family as 70AN and have similar ancestry. All of them reportedly share my great-great-grand parent as our common ancestor. He appears in my family tree as well as in the trees of two other genealogists.

Based on the Estes chart in Figure 33, I would share ~6.25% of my total DNA with this common ancestor, my great-great-grand parent.

Figure 33
Expected DNA Inherited

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 autosomal, mtDNA, and Y-DNA tests.

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