Percent DNA vs Percent Matches

In a perfect world, our DNA would be 1/2 from each parent,1/4 from each grandparent, 1/8 from each Great grandparent, etc. Also in a perfect world, 1/2 of our DNA Matches would come through a parent, 1/4 through a grandparent, 1/8 through a Great grandparent, etc. But, as we all know, the world is not perfect.

The only constant in the above is that 1/2 of our atDNA (44 Chromosomes) comes from each parent.

Percent DNA

So, let’s look at the DNA side of this issue first, addressing only the 44 atDNA Chromosomes – 50% from each parent. Because of the random DNA recombination from our grandparents, the grandparents don’t have to divide in equal proportions – and in fact they almost never will, exactly. Often, we will get one whole Chromosome from a grandparent (and none of that Chromosome from the other grandparent.)

However, over all of our 22 Chromosomes from each side, the amount per grandparent tends to roughly even out. But – a big BUT – the total of two paternal or two paternal grandparents must add up to 50% of our DNA that each parent gave us. For example, our four grandparents (reading across a Tree from paternal to maternal Ancestors) may have percentages like: 24%, 26%, 27%, 23%. Note that the first two percentages (on the paternal side) total 50% and the second two percentages total 50% (on the maternal side). This process will hold exactly for each succeeding generation: The two percentages for a male/female couple will have to total to the percentage for their child. So the percentages from the Great grandparent level may look like: 13%, 11%, 12%, 14%, 13%, 14%, 10%, 13%. The total is 100%; the total for each couple equals the amount for their child. This continues at each generation, and eventually some ancestors drop out; but the totals for each couple must add up to the child’s % (even if one of Ancestors in a couple is 0%).

If you do segment Triangulation and/or chromosome mapping, you can actually calculate these percentages – in cM or SNPs or Mbp. The sum of the parts must equal the whole at each generation.

Percent Matches

Now we’ll look at the percent (or number) of Matches for each Ancestor in a generation. All other things being equal, we would expect the percentage of our Matches to mirror the percentage of DNA. But many things are not equal…  First on my list is size of the ancestral branch. Some of my Ancestors had 10-15 children; some had 1-2 children. Clearly, we can say the more living descendants from an Ancestor, the more Matches we’ll see from that Ancestor. Large families result in more Matches. A close second on my list is: who tests? For a variety of reasons (interest, geography, income, etc.), some lines tend to have more descendants testing than others.

It’s interesting to note (at least for me) that as we move back with each generation, each Ancestor will have more descendants, but the size of a Shared DNA Segment will decrease. On the one hand, we have more cousins, on the other hand, they tend to share less DNA. In fact, at the 4th cousin level, about half of them will not show up as a DNA Match; and the percentage of cousins who do match, falls off quickly with each additional generation. This is partially offset by the increasing number of cousins. Roughly 10% of my Matches at Ancestry are 4C, the rest are 5C-8C or more (I cannot believe the valid cousins just stop at the 5C or 6C level – they are all on some kind of distribution curve). If our genealogy is deep enough, and if we are willing to build out our Matches’ Trees, I think we can find and document many more of our cousins among our DNA Matches.

Do genealogy records and documentation play a role? I’ll go out on a limb and say no! Our DNA doesn’t depend on any records – each Ancestor (and their descendants) is just as valid with or without records. At each generation, every box on our Ancestral Tree is essentially equivalent. We needed every one of them for us to be here. The records make it easier to determine how we are related to our Matches, but the amount of records wouldn’t change the number of Matches.

Summary

DNA is roughly evenly divided among our closer Ancestors at each generation, but the DNA may drift into “unevenness” in later generations. Our results will vary. Eventually some Ancestors fall out as contributors to our DNA.

We should get DNA Matches from almost all of our Ancestors (in a genealogical timeframe). The number of Matches will depend a lot on the number of descendants of each Ancestor.

An Outcome

An outcome when looking beyond brick walls and/or for bio-Ancestors:

We should find a number of Matches that tie back to every box (Ancestor) in our Tree, generally commensurate with the number of descendants from that box. If we are testing a surname for brickwall/bio-Ancestor, we should get Match-Trees that build a family, with the largest concentration of Matches being close to Ancestor we are looking for. We should get roughly the same experience with this “prospective” Ancestor, that we would get with a proven Ancestor, at the same generational level and number of descendants. In other words: the Goldilocks result. Clearly this is not a hard rule, or formula – it’s an overall feeling of what is reasonable. It’s also an expectation that we have Matches for our Brickwall and bio-Ancestors (and their Ancestors), and that those Matches should combine to build a Tree for us.

[22AW] Segment-ology: Percent DNA vs Percent Matches – A TIDBIT by Jim Bartlett 20210504

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