Amount of DNA vs Number of Matches

A Segment-ology TIDBIT

There are two concepts at work here – each is different – keep them straight and separate. Each in it’s own way can provide insights.

1. Amount of DNA from Ancestors. Although DNA from your Ancestors is random, there is some uniformity. You get exactly 1/2 of your atDNA from each parent; pretty close to 1/4 from each grandparent; about 1/8 from each great grandparent. By the time you get to your 128 5G grandparents, the average will be 1/128 from each one of them (roughly 1 percent), but with each generation past your parents, the deviation from the average increases. Still – the total of all ancestors at any generation will sum to 100 percent. On average you got about 15cM from each 5G grandparent, which may be in one segment or spread over several segments. The companies report that, in general, you will not share enough DNA to match most of your 6th cousins; but we know that you have so many 6th cousins that you will share with many of them – experience shows you will share from 0-21cM with some your 6th cousins. [see The Shared cM Project 2.0 at https://isogg.org/wiki/Autosomal_DNA_statistics ]

2.Number of Matches from Ancestors. This depends on several factors: Number of cousins, who tests, endogamy, etc.

A. The number of cousins you have from each Ancestor is directly related to the size of their families leading to the number of their living descendants. Larger families => more cousins => more Matches.

B. Similarly the number of cousins you have from each Ancestor is directly related to how far back that Ancestor is. Distant Ancestors => more cousins => more Matches.

C. Who tests. This may be most pronounced with recent immigration. The more recent an Ancestor immigrates, the fewer number of living descendants in the US, where the highest percentage of atDNA test takers live. Recent immigration => fewer cousins tested => fewer Matches.

D. Endogamy. With endogamy, the amount of shared DNA increases, resulting in many more Matches who otherwise might not have matched. The classic case is Ashkenazi Jews, who tend to get many times the number of Matches others get, and the Matches are in fact somewhat more distant than indicated. This also applies to other endogamous populations. Endogamy => more cousins => more Matches.

Some of my experience and observations

My maternal grandmother was from immigrants to US in 1850s. Roughly 1/4 of my atDNA is from her, and 1/4 of my Chromosome Map has TGs from her line. But, I get relatively few Matches on this 1/4 of my Ancestry, and some of the “gaps” in my Map have no Matches in them at all. When I get only 1 or 2 Matches in a TG on my maternal side, it’s pretty certain they are from my grandmother’s line. The other maternal TGs are from my mother’s paternal side with deep Colonial Virginia roots – they tend to have many Matches.

As the Matches pour in, and are added to my TGs and Chromosome Map, I can see where some TGs have many Matches and must come from distant Ancestors – perhaps at the limits of my Tree, or beyond – so I focus on the closer MRCAs (Most Recent Common Ancestors) which provide pointers. In some cases I’ve found the Ancestor had a Plantation and raised many children.

Some of the other TGs have small to medium numbers of Matches, and where I can find an MRCA, they tend to be smaller Colonial families – folks who moved around a lot; died earlier in life; and/or had few known children who survived.

So as you form TGs, and look at the resulting Chromosome Map, they start to paint a picture…

 

[22K] Segment-ology: Amount of DNA vs Number of Matches TIDBIT; by Jim Bartlett 20170517

12 thoughts on “Amount of DNA vs Number of Matches

  1. You raise many of the ideas I have pondered about atDNA matches. Whenever I see a high number of in-common matches I presume the CA is someone with ancestry in colonial America, When I see a match with a low amount of dna spread over several segments I wonder if we have multiple CA’s, probably tracing to colonial America. A match with a high-ish amount of dna and few shared matches, those I get excited about since it may lead to new information.

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    • Barb, There are many possibilities, and the DNA is random. So it’s hard to form rock-hard rules. But you and I are in agreement that the number of Matches is an indicator that may help.

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  2. I just started reading this blog and find the information very valuable, but am confused as to what the acronym TG stands for in this most recent post? And in the first comment is CA the same as MRCA?

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    • Eileen, My apologies. TG means Triangulated Group. Usually I type the full phrase, and then use the TG abbreviation, but I slipped up this time. CA means Common Ancestor. Each TG has a Common Ancestor who first formed the DNA segment defined by the TG. Matches who are closer closer cousins on an Ancestor closer that the CA, will have an MRCA (Most Recent Common Ancestor). It is possible to have closer cousins on different MRCAs out to the CA of the TG segment.

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  3. Thanks for that- you appear in a little TG with one of my McIntyre’s on Chromosome 21 I have about 4 Bartletts on FTDNA. Love to solve that one, we are still in Scotland.

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    • Valerie – well you sure have your share of kits at GEDmatch – and I only match one. Our TG is about 15cM and includes 22 Matches (including my maternal uncle), but no other leads on the Common Ancestor. Send me an email off line about the BARTLETTs – I am the Admin for the BARTLETT Y-DNA Project, and have a lot of info about all BARTLETT lines.

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