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